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Vlsi design

  1. 1. VLSI Design Introduction to the subject Lecture 0 Rajesh Ghongade VIIT
  2. 2. • Teaching Scheme – Lectures: 3 Hrs/week – Practical: 2 Hrs/Week • Examination Scheme – Paper: 100 Marks – Practical: 50 Marks – Oral: 25 Marks
  3. 3. Unit-I VHDL Modeling and Design Flow Introduction to VLSI: complete VLSI design flow (with reference to an EDA tool). Sequential, Data flow, and Structural Modeling. Functions. Procedures, attributes. Test benches, Synthesizable, and non synthesizable statements; packages and configurations Modeling in VHDL with examples of circuits such as counters, shift registers, bidirectional bus, etc.
  4. 4. Unit 2 FSM And Sequential Logic Principles Sequential Circuits, Meta-stability Synchronization, Design of Finite State Machines, and State minimization, FSM CASE STUDIES - Traffic Light control. Lift Control and UART STA and DTA
  5. 5. Unit 3 Programmable Logic Devices Introduction to the CPLDs, Study of architecture of CPLD. and Study of the Architecture of FPGA
  6. 6. Unit 4 System On Chip One, two phase clock, Clock distribution. Power distribution. Power optimization, SRC and DRC, Design validation, Global routing, Switch box routing. Off chip connections, I/O Architectures, Wire parasitics, EMI immune design. Study of memory-Basics of memory includes types of memory cells and memory architectures. Types of memory, based on architecture specific and application specific viz. SRAM, DRAM, SDRAM, FLASH, FIFO.
  7. 7. Unit 5 CMOS VLSI CMOS parasitics, equivalent circuit, body effect, Technology Scaling, A. parameter. Detail study of Inverter Characteristics, power dissipation, power delay product, CMOS combinational logic design and W/L calculations. Transmission gates, Introduction to CMOS layout.
  8. 8. Unit 6 Testability Need of Design for testability, Introduction to Fault Coverage, Testability. Design- for- Testability, Controllability and Observability, Stuck-at Fault Model. Stuck-Open and Stuck-Short faults. Boundary Scan check. JTAG technology; TAP Controller and TAP Controller State Diagram. Scan path. Full and Partial scan. BIST
  9. 9. Text Books 1. John F. Wakerly, "Digital Design, Principles and Practices", Prentice Hall Publication 2. Neil H. E Weste and Kamran Eshraghian, "Principles of CMOS VLSI Design". 3. Wayne Wolf, "Modern VLSI Design" 4. Sudhkar Yalamachalli, "Introductory VHDL from simulation to Synthesis"
  10. 10. Reference Books 1. Perry "VHDL" 2. Charles Roth, "Digital System Design using VHDL". McGraw hill. 3. Xilinx Data Manual "The Programmable Logic Data Book". 4. Stephen Brown and Zvonko Vranesic, "Fundamentals of Digital Logic with VHDL Design", Second Edition, McGraw-Hill, 2005. 5. Michael John Sebastian Smith, "Application-Specific Integrated Circuits", Addison Wesley. 6. Wayne Wolf, "FPGA-Based System Design", Prentice Hall, 7. Miron Abramovici, "Digital Systems Testing and Testable Design", Jaico Publishing. 8. Sung-Mo (Steve) kang, Yusuf Leblebici, " CMOS Digital Integrated Circuit", Tata McGraw-Hill Publication.
  11. 11. LIST OF EXPERIMENTS Any 8 assignments out of the following: Simulation, Synthesis, and Implementation of: 1. 8: 1 Multiplexer, 2:4 Decoder, Comparator and Adder. 2. Flip Flop, Shift Register and Counter 3. Lift Controller /Traffic Light Controller/ UART. Anyone of the three. 4. Parity generator and Checker. 5. Implementation of RAM/FIFO. 6. Ramp waveform generator using DAC 7. Bi-directional buffer 8. Temperature sensing using ADC, Displaying on 7-Segment display and threshold setting using keyboard 9. Implementation of 4-bit RISC processor
  12. 12. EDA Tools • Active-HDL 6.3 sp1 – (VHDL compiling and simulation) • Synpilfy Pro 8.2 – Synthesis • Xilinx Webpack 8.X – Implementation
  13. 13. Alternate EDA Tools • Libero Gold 6.2 – (VHDL compiling & implementation) • Modelsim 6.0 – Simulation • Synpilfy 8.2 – Synthesis
  14. 14. Hardware available • Actel ProASIC PLUS evaluation board with APA300-PQ208 device • Xilinx CPLD Dedicated Trainer XC 9572 PC 84 • Xilinx FPGA Dedicated Trainer XC 2S50 – TQ 144(Spartan II) • Universal Trainer kit (BASE UNIT) (ADD on Modules) – Xilinx CPLD Module • XC 95108 – PLCC 84 – Xilinx FPGA Module • XC V100E – PQ 240 XC (Virtex) – Altera Flex 10K Series FPGA – Atmel CPLD Device ATF 150 4AS / ATF 1508 AS PC 84 – Cypress CPLD Module Device CY37064V PC84
  15. 15. Lecture 1 Introduction to VLSI R.B.Ghongade
  16. 16. Microelectronics Is the art, science and technology of designing and fabricating integrated circuits with small-dimension electronic devices Areas of Microelectronics are : • VLSI Design • VLSI CAD Tools • Technology & Fabrication • Physics • Modeling and Simulation • Characterization • Testing Nearly all the advances in the modern day electronic systems and devices are a direct outcome of VLSI technology
  17. 17. Some keywords! • Very-large-scale-integration (VLSI) is defined as a technology that allows the construction and interconnection of large numbers (millions) of transistors on a single integrated circuit. • Integrated circuit is a collection of one or more gates fabricated on a single silicon chip. • Wafer is a thin slice of semiconductor material on which semiconductor devices are made. Also called a slice or substrate. • Chip is a small piece of semiconductor material upon which miniaturized electronic circuits can be built. • Die is an individual circuit or subsystem that is one of several identical chips that are produced after dicing up a wafer. If you use these key-words often, people will think that you are an expert VLSI engineer!!!
  18. 18. • The origin of this terminology can be traced as the logical extension to the integration techniques namely the Small Scale Integration, SSI (the ICs which functioned as logic gates, flip-flops), the Medium Scale Integration, MSI (multiplexers, decoders)., the Large Scale Integration LSI (early microprocessors, small memories, PAL, GAL Technology Number of gates/transistors* per chip Examples Year SSI 1 to 20 74XX series, 4xxx series 60’s MSI 100 to 1000 74XXX series, 45XX series 70’s LSI 1000 to 10,000/100 to 100,000* 8085, 80’s VLSI 10,000 to 100,000/1,000,000 * CPLD, FPGA, advanced μC, SoC 90’s Dividing line between LSI and VLSI is somewhat fuzzy hence number of transistors provides a good criterion
  19. 19. The advances in the integration techniques can be attributed directly to : •Advances in photolithography techniques •New designs of semiconductor devices •Newer methods of metallization The development of integration technology has followed the famous Moore’s Law. It was stated by Gordon Moore, co-founder of Intel, in the year 1965, that “the number of transistors per chip would grow exponentially (double every 18 months)”. In fact the doubling period has shortened to a mere 12 months!
  20. 20. Increasing transistor density
  21. 21. The number of transistors/gates that can fit in to the semiconductor die dictates the complexity of the functionality that the device can perform. The important factors that fuel the research in VLSI technology can be summarized as below: • Increased functionality • Higher reliability • Small footprint • Very low power consumption • Increased speed of operation • Re-programmability( except ASIC devices) • Mass production • Low cost
  22. 22. VLSI is thus a technology that can be harnessed for various applications covering analog, digital and mixed signal electronics. The current trend is to reduce the entire system design to a single chip solution called as system on chip (SoC)
  23. 23. Building blocks of VLSI system on chip Digital MCU/MPU Memory MEMS CCD sensors microtransformers microresonators RF/Analog Frequency generation mixers filters VCO LNA RF power amplifiers Opamps Mixed-Signal DSP Audio ,Video circuits MPEG engine Power Management converter regulator on-chip power supply Applications multimedia computing communications biomedical ...
  24. 24. VLSI Design Process VLSI technology thus provides a platform for developing systems for various applications The integrated circuits so developed can be further classified as :
  25. 25. ASIC • An Application Specific Integrated Circuit (ASIC) is a semiconductor device designed especially for a particular customer (versus a Standard Product, which is designed for general use by any customer) • The three major categories of ASIC Technology are : – Gate Array-Based – Standard Cell-Based – Full custom
  26. 26. Gate Arrays • There are two types of gate arrays: – a channeled gate array – channel-less gate array • A channeled gate-array is manufactured with single or double rows of basic cells across the silicon • A basic cell consists of a number of transistors • The channels between the rows of cells are used for interconnecting the basic cells during the final customization process • A channel-less gate array is manufactured with a “sea” of basic cells across the silicon and there are no dedicated channels for interconnection • Gate arrays contain from a few thousand equivalent gates to hundreds of thousand of equivalent gates • Due to the limited routing space on channeled gate arrays, typically only 70% to 90% of the total number of available gates can be used
  27. 27. • The library of cells provided by a gate array vendor will contain: – primitive logic gates – registers, – hard-macros – soft-macros Hard-macros and soft-macros are usually of MSI and LSI complexity, such as multiplexers, comparators and counters. Hard macros are defined by the manufacturer in terms of cell primitives Soft-macros are characterized by the designer, for example, specifying the width of a particular counter
  28. 28. Standard Cell • Standard cell devices do not have the concept of a basic cell and no components are prefabricated on the silicon chip • The manufacturer creates custom masks for every stage of the device’s process which leads to a more efficient utilization of silicon as compared to gate arrays • Manufacturers supply hard-macro and soft-macro libraries containing elements of LSI and VLSI complexity, such as controllers, ALUs and microprocessors. • Additionally, soft-macro libraries contain RAM functions that cannot be implemented efficiently in gate array devices; ROM functions are more efficiently implemented in cell primitives
  29. 29. The Characteristics of ASICs The remarks that follow further discuss some trade-offs of ASICs with respect to the following categories: • Complexity • Silicon Efficiency • Design Risks • Prototype Turnaround • NRE • CAD / CAE Support • Performance
  30. 30. Complexity • Complexity here means the number of transistors (or the amount of logic and/or memory) per given amount of area, plus the associated interconnect capability • Current Array-Based and Cell-Based chips accommodate as many as 20,000,000 usable logic gates on a single die • Array-Based designs -especially in a Channel-Free Array technology - are capable of realizing functions that represent actual system building blocks and incorporate system memory functions on the same die • The Array-Based memories do tend to be about 5 times less dense than Cell-Based memories because they are constructed out of the gates on the master slice. And full custom memories would provide much higher densities than do Array-Based memories • But in fact many designers who are using the Array-Based technologies to get fast turn around tend to be using very small “scratch pad” or “cache” types of memories which fit very well into the ASIC concept
  31. 31. Silicon Efficiency • Array-Based technologies focus on fast implementation of logic integration onto a single chip, rather than on absolute highest density. • Cell-Based designs allow you to get more logic onto a chip in a given area. • Cell-Based designs feature transistors and routing tracks whose gradations of size are finer than those in Array-Based products. Thus Cell- Based designs use silicon more efficiently than Array-Based designs
  32. 32. NRE • NRE (“Non-Recurring Engineering”) charges are the costs associated with developing an ASIC • NRE is based on a number of factors like: – the complexity of the design, – the technology chosen (# of masks required) – the amount of work to be done by the customer and by the silicon vendor – the need for special cells or procedures – the type of package required – the schedule the number of layers of metal – …
  33. 33. • The more work the silicon vendor does and the more special the requirements, the higher will be the NRE . The more work the customer does, the lower the NRE ! • Array-Based designs require the fewest number of design-specific masks and therefore offer the lowest NRE to prototypes. • Cell- Based designs require all masks to be generated for the chosen process and therefore the NRE charge will be higher for a Cell-Based design than for an Array-Based design
  34. 34. Design Risks • The penalty for discovering a design error is higher for a Cell-Based ASIC than for an Array-Based ASIC • Mistakes after prototype fabrication in Array-Based designs usually only require that the metal mask layers be redone. On the other hand, design changes for a Cell- Based design may require that all masks be redone !
  35. 35. Prototype Turnaround Time (TAT) • Designs that require a complete mask set (Cell- Based) will always require more time to manufacture than designs which use a basic set of diffusion masks and only require customization at the metal layers (Array-Based) • This difference in time could be anywhere from one week to 4 weeks depending on how fast the silicon vendor can get masks from the mask shop and depending on how long the FAB cycle is for a given process
  36. 36. CAD / CAE Support The use of EDA tools ensure: • Clean documentation • Reusable data • Functional verification • Easy modification • Automated rule check • Back-annotation (synchronization between schematic and layout) • Bill of material
  37. 37. Performance • The two most critical parameters that have been used to measure the worth of new technologies have been speed and power • High power circuits are normally fast, but the increased power requires larger power supplies and tends to heat up the junctions on silicon chips which slows the devices. • In today's most dominant ASIC technology - CMOS - high power can cause accelerated junction temperatures which can slow down speed • One way to reduce the power and still maintain speed is to develop circuits such as differential pairs that do not switch from voltage rail to voltage rail
  40. 40. Programmable logic device (PLD) • It is an integrated circuit able to implement combinational and/or sequential digital functions defined by the designer and programmed into this circuit • There are a wide variety of ICs that can have their logic function “programmed” into them after they are manufactured. Most of these devices use technology that also allows the function to be reprogrammed
  41. 41. • Historically, programmable logic arrays (PLAs) were the first programmable logic devices • PLAs contained a two-level structure of AND and OR gates with user-programmable connections • Using this structure, a designer could accommodate any logic function up to a certain level of complexity using the well-known theory of logic synthesis and minimization • PLA structure was enhanced and PLA costs were reduced with the introduction of programmable array logic (PAL) devices • Today, such devices are generically called programmable logic devices (PLDs), and are the “MSI” of the programmable logic industry • The ever-increasing capacity of integrated circuits created an opportunity for IC manufacturers to design larger PLDs for larger digital-design applications • However, the basic two-level AND-OR structure of PLDs could not be scaled to larger sizes. Instead, IC manufacturers devised complex PLD (CPLD) architectures to achieve the required scale
  42. 42. • A typical CPLD is merely a collection of multiple PLDs and an interconnection structure, all on the same chip. In addition to the individual PLDs, the on-chip interconnection structure is also programmable, providing a rich variety of design possibilities • CPLDs can be scaled to larger sizes by increasing the number of individual PLDs and the richness of the interconnection structure on the CPLD chip • At about the same time that CPLDs were being invented, other IC manufacturers took a different approach to scaling the size of programmable logic chips. • Compared to a CPLD, a field-programmable gate arrays (FPGA) contains a much larger number of smaller individual logic blocks, and provides a large, distributed interconnection structure that dominates the entire chip
  43. 43. CPLD and FPGA
  44. 44. Top-Down design methodology • Means describing a complete system at an abstract level using hardware description language(HDL) and the use of EDA tools like partitioners and synthesizers • More time is spent on designing HDL models, considering different architectures and considering system test & testability issues. Practically no time is spent on designing at gate level To consider a concept without thinking of a specific example; consider abstractly or theoretically. ABSTRACT
  45. 45. Levels of behavioural abstractionINCREASINGBEHAVIOURALABSTRACTION INCREASINGDETAILEDREALIZATION&COMPLEXITY TRANSISTOR / SWITCH GATE RTL / DATAFLOW ALGORITHM SYSTEM CONCEPT The process of formulating general concepts by abstracting common properties of instances
  46. 46. • System Level: All the specifications (input and output) are described at this level. This level completely ignores the hardware structure. However HDLs are not useful at this stage. It simply treats the design like a black box. • Algorithmic (also called behavioural) level: This is the highest level of abstraction provided by most HDLs. A module can be implemented in terms of the desired deign algorithm without the concern of the hardware implementation details. Design at this level is very similar to a conventional high level language programming like C. • RTL (Register Transfer Level) (also called dataflow): At this level the module is designed by specifying the data flow between the registers. The designer is aware of how data flows between hardware registers and how the data is processed in the design. • Gate Level: The module is implemented in terms of logic gates and interconnections between these gates. Design at this level is similar to describing a design in terms of gate-level logic diagram. • Transistor (also called Switch) Level: This is the lowest level of abstraction. A module can be implemented in terms of switches, storage nodes, and the interconnections between them. Design at this level requires knowledge of switch-level implementation details.
  47. 47. Lecture 2 Introduction to VHDL R.B.Ghongade
  48. 48. PLD based design flow •A decision has to be arrived at regarding the selection of the type of a PLD since we have two options the CPLD and the FPGA •The selected device is then called the target device • Specifications • Design Entry • Compilation • Functional Simulation/Verification • Synthesis • Post-synthesis simulation • Implementation • Timing Simulation • Hardware Implementation Steps involved:
  50. 50. Idea Design Entry (Schematic) VHDL Template Model Generator Target Device Library Synthesis (Synthesizer Tool) Design Entry (Text Editor) Netlist (EDIF,XNF,DSL...) Functional Simulation (VHDL Simulator) Algorithmic Simulation (VHDL) Test Vectors (Input Stimuli & Output Expected) Post- Simulation (VHDL Simulator) Structural VHDL (VITAL primitives) Timing Analysis VITAL Primitive Library SDF (Delay Information) Program Data (Fuse Map: JED, HEX...) Device Programming JTAG-ISP Chip Placement & Routing (Device Vendor's Tool) Back-annotation TO PLACE & ROUTE NETLIST Detailed HDL based design flow
  51. 51. • It may again include the input, output and ambient specifications • Target device may be finalized • Choice of target device as CPLD or FPGA depends on various factors • Specific type of device may be selected by comparing the specifications provided by the manufacturer in the datasheet and the actual design requirements 6 3 31 5 5 15 11 24 Xilinx Actel Cypress Altera Lattice AMD AT&T Others • XILINX • LATTICE • LUCENT • ALTERA • ACTEL • CYPRESS • AT&T • AMD Specifications
  52. 52. Design Entry • This is essentially the design entry point in an EDA tool • It can be done by the following means: • Block Diagram/Schematic capture • State Diagram entry • HDL code entry
  53. 53. Block Diagram/Schematic capture • A schematic circuit is literally “drawn” in an appropriate graphical editor • The EDA tool associated with this task is called Schematic Capture Tool • An electrical rule check (ERC) is usually run • The main job of the ERC tool is to check for incorrect electrical connections for example if a VCC pin of an IC is accidentally shorted to ground, then the ERC tool will point out such a discrepancy • For this tool to be effective the IC pins have to be earlier declared as power, ground, input, output, bidirectional etc. • After removing the ERC errors a netlist is generated by the editor • A netlist is a text file showing the nets i.e. a set of components connected together • It is also possible to generate VHDL netlist
  54. 54. Block Diagram/Schematic capture
  55. 55. State Diagram entry • Many designs are most effectively described /designed by state diagram approach. • Effective for sequential designs • The EDA tools provide a graphical interface so that the designer can directly make an entry of the state diagram and generate the netlist. • This method is preferred since it is a fast way of creating the design
  56. 56. State Diagram entry
  57. 57. HDL code entry • A designer can enter his /her design using a hardware description language (HDL) • The HDLs prominent in the industry are “VHDL” and “Verilog” • There is another language that is recently making ground called as “System C” • Being similar to C language gives it an advantage to be more user friendly and comfortable to designers familiar with C • Using the code entry method is the most preferred one since it offers: – Design flexibility – Code re-use – Easy modification – Tighter control over resources • A netlist is again created by compiling the HDL code
  58. 58. HDL code entry
  59. 59. Compilation • At this stage the design is said to be at the Register Transfer Level (RTL) • All the data manipulation is done here at the register of the host CPU or we can say that the design is not in terms of the logic gates but “internal” to the environment • After successful compilation of the design using any one of the three methods a netlist is generated which is called the gate-level netlist • The design now is translated in terms of the logic gates and modular entities like multiplexers, decoders. Thus we now have the design at Gate-level
  60. 60. Functional Simulation/Verification • There are two very different tasks in digital system development – Verification is the process of determining whether a design meets the specification and performance goals – It concerns the correctness of the initial design as well as the refinement processes – Testing is the process of detecting the physical defects of a die or a package that occurred during manufacturing
  61. 61. Functional Simulation/Verification • A functional test is done by simulating the gate-level design using logic simulators that may be available as a built-in feature of the EDA tool. There are two ways of functional verification: – Interactive mode • In the interactive mode the designer can change the input signals and observe the corresponding output behaviour of the design. This method is becomes cumbersome for designs involving large number of inputs and outputs. – Batch mode • Batch mode uses the concept of test-benches (also a piece of VHDL code) that generates test patterns and checks the validity of the output. This mode is attractive for larger designs. • If any undesirable behaviour is observed, the designer can correct the flaw by going back to the design entry level • It is important here to note that none of the timing aspects have been considered during this simulation. Functional verification can thus be compared to the algorithm testing in conventional programming languages
  62. 62. Synthesis • Synthesis means to properly put together so as to make the whole complex design • At this stage it is necessary to specify the target device, since the synthesis tool (again dedicated software) requires knowing the resources available with the target device • Synthesis optimally maps the design functionality (at the gate-level) in to the actual devices available with the target device • For example if the design uses a four- input AND gate but since this is not available with the target device, the synthesis tool can break down the four- input AND gate into two two-input AND gates and map correspondingly • Optimization is very important otherwise the design may get “blown-up” and the target device may prove too small for the design • Synthesis tools have built-in proprietary algorithms for optimization
  63. 63. Post-synthesis simulation • After synthesis the design needs to be re- checked to confirm its functionality • Simulation at this level ensures that the mapped design behaves correctly as desired • A possibility may exist wherein, the synthesis tool may incorrectly recompose the design while mapping • Again timing parameters are ignored here
  64. 64. Implementation • This is the process of physical placing of the design into the target device. • Though it is a physical placement, it still takes place in the virtual environment provided by the EDA tool • A physical map of the target device is loaded into the environment and the components are virtually fitted into the target device. • Again this process may have two phases: – Physical synthesis • Physical synthesis means optimal relocation of the design into the target device. Proprietary software tools are available for this task and may be quite costly. This phase is however optional. – Place and route • Place and route is the phase where the tool completes the task of virtually placing the components of the design in to the target device and then wiring the individual modules to complete the design.
  65. 65. Timing Simulation • One most important change the design undergoes is after the implementation. • The modules in the design now may be physically placed apart from each other. • This factor introduces the delay aspect in the signal propagation. • Many synchronous circuits will fail if the timing aspects are ignored, even though they appear to be functionally perfect! • Hence a simulation is necessary again to test the timing behaviour of the design. • This provides the designer with a better view of the design functionality. • In fact the real-world behaviour of the device can be very accurately studied by the simulation with timing aspects
  66. 66. Hardware Implementation • The final step in design is to “download” the functionality into the actual hardware i.e. the target device • The synthesis tool generates the output in terms of “bit-stream” that is used to configure the target device • Vendor specific tools can be used for downloading the bit-stream into the physical device
  67. 67. HDL • A hardware description language (HDL) is a software coding language used to model the intended operation of a piece of hardware • There are two aspects to the description of hardware that an HDL facilitates: – true abstract behaviour modeling – hardware structure modeling • Abstract behaviour modeling – A hardware description language is declarative in order to facilitate the abstract description of hardware behaviour for specification purpose. This behaviour is not influenced by structural or design aspects of the hardware intent. • Hardware structure modeling – Hardware structure is capable of being modeled in a hardware description language irrespective of the design’s behaviour.
  68. 68. VHDL • What is VHDL? – Digital system design using Hardware Description Language is an established methodology in EDA – VHDL stands for “VERY HIGH SPEED INTEGRATED CIRCUITS HARDWARE DESCRIPTION LANUAGE” EDA stands for “ELECTRONIC DESIGN AUTOMATION”
  69. 69. FEATURES • VHDL is an amalgamation of following languages – Concurrent language – Sequential Language – Timing Specification – Simulation Language – Test Language
  70. 70. • VHDL has got powerful language constructs – {if…else}, {with…select} etc • Design hierarchies to create modular designs • Supports Design Libraries • Facilitates device independent design and portability
  71. 71. Concurrent Language • Concurrent statements execute at the same time in parallel as in hardware Z <= C + X ; X <= A + B ; + + A B C X Z
  72. 72. Sequential Language • Sequential statements execute one at a time in sequence • As the case with any conventional programming language the sequence of statements is important Z <= C + X ; X <= A + B; X <= A + B; Z <= C + X ;≠ Sequential statements are required to design sequential circuits
  73. 73. Timing Specification • Providing timing attributes in a sequential digital design is of prime importance since the operations are synchronized to a common clock • Example: process begin clk <= ‘0’ ; wait for 20 ns ; clk <= ‘1’ ; wait for 12 ns ; end process ; 0 20 32 52 64 ns 84 Timing can be specified in a process only
  74. 74. Simulation language • For analyzing a digital design it is important the design be simulated • Simulation has different flavours – Functional simulation – Post-synthesis simulation – Post- layout simulation • Any HDL should thus be equipped with simulation capability for verification and troubleshooting purposes
  75. 75. Test Language • Testbench – It is a part of a VHDL module that generates a set of test vectors (test inputs) and sends them to the module being tested – It collects the responses generated by the module under test and compares them against a specification of correct results – Thus testbench is required to ensure that the design is correct and that the module is operating as desired Equivalent to checking of logical errors in any conventional programming language
  76. 76. Testbench use Test tst_a tst_b tst_c MODULE UNDER TEST ABC.vhdABC_testbench.vhd Equivalent to mechanical test jigs used for testing functionality of mass produced pcbs as in TV sets or motherboards
  77. 77. Design Hierarchy • Hierarchy can be represented using VHDL • Example – A full adder which is the top level module being composed of three lower level modules that are; half adder and OR gate HALF ADDER HALF ADDER OR A B Cin SUM CARRY Design hierarchy simplifies the design procedure and manageability in case of complex designs
  78. 78. Design Libraries • Design Unit – It is any block of VHDL code or collection of VHDL codes that may be independently analyzed and inserted into a design library • Design Library – It is a storage facility in which analysed VHDL descriptions are stored for repeated uses 1 3 2 54 Design Library Simulator Analyze DESIGN UNIT
  79. 79. Logic systems • Need for multi-valued logic system – Conventional logic systems have only three values i.e. ‘0’, ‘1’, ‘Z’ • Example – Consider the truth-table for AND gate A B Y 0 0 0 0 1 0 1 0 0 1 1 1 0 Z ? HOW TO RESOLVE THIS CONDITION ?
  80. 80. • For this problem a 9-valued logic system or package was developed that is called “STD_LOGIC_1164” and it is accepted as IEEE STD 1164-1993 • Multi-valued logic – Unknown: value was known but not anymore – Un-initialized: value was never known – High impedance: net has no driver – Drive strengths: handle different output drivers – Don’t care: value is immaterial
  81. 81. Levels of abstraction • Different styles are adopted for writing VHDL code • Abstraction defines how much detail about the design is specified in a particular description • Four levels are: – Layout level – Logic level – Register Transfer level – Behavioral level LOGIC LAYOUT RTL BEHAVIORAL
  82. 82. Layout Level • This is the lowest level and describes the CMOS layout level design on silicon
  83. 83. Logic Level • Design has information about – Function – Architecture – Technology – Detailed timings • Layout information and analog effects are ignored
  84. 84. Register Transfer Level • Using HDL every register in the design and the logic in between is defined • Design contains: – Architecture information – No details of technology – No specification of absolute timing delays
  85. 85. Behavioral Level • Describing function of a design using HDL without specifying the architecture of registers • Contains timing information required to represent a function
  86. 86. Basic building blocks of VHDL code • A VHDL design is composed of following blocks: – Library declarations – Entity – Architecture – Configuration
  88. 88. Detailed anatomy of VHDL code Functions Procedures Types Constants Components Package Library EntityGenerics Ports Architecture (Style I) DATAFLOW Architecture (Style II) BEHAVIOURAL Architecture (Style III) STRUCTURAL Concurrent Statements Concurrent Statements Process Sequential Statements
  89. 89. Elements of VHDL R.B.Ghongade Lecture 3
  91. 91. Overview
  92. 92. Library • It is a collection of compiled VHDL units • It enables sharing of compiled designs and hides the source code from the users • Commonly used functions, procedures and user data types can be compiled into a user defined library for use with all designs • Library should be declared before each entity declaration even if it is in the same VHDL file
  93. 93. Library • To declare a library (i.e. to make it visible to the design) two lines of code are needed , one containing name of the library, the other a use clause • A library structure can be as follows: FUNCTIONS PROCEDURES TYPES CONSTANTS COMPONENTS LIBRARY PACKAGE
  94. 94. Library syntax LIBRARY library_name ; USE library_name.package_name.package_parts ; LIBRARY IEEE ; -- semicolon indicates USE IEEE.std_logic_1164.all ; -- end of statement or -- declaration LIBRARY work ; -- double dash (--) -- indicates a comment USE work.all ; Example
  95. 95. Library details IEEE.MATH_COMPLEX.all This package defines a standard for designers to use in describing VHDL models that make use of common COMPLEX constants and common COMPLEX mathematical functions and operators. IEEE.MATH_REAL.all This package defines a standard for designers to use in describing VHDL models that make use of common REAL constants and common REAL elementary mathematical functions. IEEE.NUMERIC_BIT.all This package defines numeric types and arithmetic functions for use with synthesis tools. Two numeric types are defined: -- UNSIGNED: represents an UNSIGNED number in vector form -- SIGNED: represents a SIGNED number in vector form The base element type is type BIT.
  96. 96. Library details IEEE.NUMERIC_STD.alll This package defines numeric types and arithmetic functions for use with synthesis tools. Two numeric types are defined: -- UNSIGNED: represents UNSIGNED number in vector form -- SIGNED: represents a SIGNED number in vector form -- The base element type is type STD_LOGIC. IEEE.STD_LOGIC_1164.all This package defines a standard for designers to use in describing the interconnection data types used in VHDL modeling. Defines multi-value logic types and related functions. IEEE.STD_LOGIC_ARITH.all This package defines a set of arithmetic, conversion, and comparison functions for SIGNED, UNSIGNED, SMALL_INT, INTEGER, STD_ULOGIC, STD_LOGIC, and STD_LOGIC_VECTOR.
  97. 97. Library details IEEE.STD_LOGIC_MISC.alll This package defines supplemental types, subtypes, constants, and functions for the Std_logic_1164 Package. IEEE.STD_LOGIC_SIGNED.all This package defines a set of signed arithmetic, conversion, and comparison functions for STD_LOGIC_VECTOR. IEEE.STD_LOGIC_TEXTIO.all This package overloads the standard TEXTIO procedures READ and WRITE. IEEE.STD_LOGIC_UNSIGNED.all This package defines a set of unsigned arithmetic, conversion and comparison functions for STD_LOGIC_VECTOR.
  98. 98. Entity – It is the design’s interface to the external circuitry – Equivalent to pinout /package of an IC – VHDL design must include one and only one entity per module – It can be used as a component in other entities after being compiled into a library
  99. 99. Entity declaration • Defines the input and output ports of the design • Name of the entity can be anything other than the reserved VHDL word • Each port in the port list must be allotted: – a name ( should be self-explanatory that provides information about its function – data flow direction or mode – a type • Ports should be well documented with comments at the end of line providing additional information about the signal
  100. 100. Entity syntax entity entity_name is port ( port_name : signal_mode signal_type ; port_name : signal_mode signal_type ; port_name : signal_mode signal_type ) ; end entity_name ;
  101. 101. Modes • Ports in the portlist have modes which indicate the driver direction • Mode also indicates whether or not the port can be read from within the entity • Four modes are available: – Mode IN – Mode OUT – Mode INOUT – Mode BUFFER
  102. 102. • Mode IN Value can be read from but not assigned to (by the entity) ENTITY Port signal A Drivers reside outside the entity port ( A : in std_logic ) ;
  103. 103. • Mode OUT Value can be assigned to but not read from (by the entity) port ( B : out std_logic ) ; ENTITY Port signal B Drivers reside inside the entity
  104. 104. • Mode INOUT Bi-directional , value can be assigned to as well as read from (by the entity) port ( C : inout std_logic ) ; ENTITY Port signal C Drivers reside both inside and outside the entity
  105. 105. • Mode BUFFER Output port with internal read capability port ( D : buffer std_logic ) ; ENTITY Port signal D Drivers reside inside the entity Signal can be read inside the entityDO NOT USE UNLESS REQUIRED
  106. 106. Entity example entity and_gate is port ( 1A , 2A , 3A, 4A : in std_logic ; 1B , 2B , 3B, 4B : in std_logic ; 1Y , 2Y , 3Y, 4Y : out std_logic ) ; end and_gate ; 1A 1 1B 2 1Y 3 2A 4 2B 5 2Y 6 GND 7 3Y 8 3A 9 3B 10 4Y 11 4A 12 4B 13 VCC 14
  107. 107. Entity example entity ALU is port ( In1 : in std_logic_vector ( 3 downto 0) ; -- 1st operand In2 ; in std_logic_vector ( 3 downto 0) ; -- 2nd operand Opsel : in std_logic_vector ( 3 downto 0) ; -- opn select Cin : in std_logic ; Mode : in std_logic ; Result : out std_logic_vector ( 3 downto 0 ) ; Cout : out std_logic ; Equal : out std_logic ) ; end ALU ; ALU In1 In2 Op sel Result Cout Mode Cin Equal
  108. 108. Architecture It specifies • Behaviour • Function • Relationship between inputs and outputs of an entity Syntax architecture achitecture_name of entity_name is [declarations] -- optional begin code -- concurrent statements only end achitecture_name ;
  109. 109. • Architecture can contain only concurrent statements • A design can be described in an architecture using various levels of abstraction • An entity can have more than one architectures since a function can be implemented in a number of ways • There can be no architecture without an entity
  110. 110. Architectural bodies • Behavioural – It is the high-level description – It contains a set of assignment statements to represent behaviour – No need to focus on the gate-level implementation of a design Example: architecture behave of and_gate is begin process ( a, b ) if a=‘1’ and b=‘1’ then c <= ‘1’ ; else c <=‘0’ ; end if ; end process ; end behave ;
  111. 111. • Dataflow – It uses concurrent signal assignment statements Example: architecture dataflow of and_gate is begin c<= a and b ; end dataflow ;
  112. 112. • Structural – Components from libraries are connected together – Designs are hierarchical – each component can be individually simulated – it makes use of component instantiation HALF ADDER HALF ADDER OR A B Cin SUM CARRY Functional units Top level design
  113. 113. Configuration • Since a number of architectures can exist for an entity , using configuration statement we can bind a particular architecture to the entity configuration CONFIGURATION_NAME of ENTITY_NAME is for ARCHITECTURE_NAME end for; end CONFIGURATION_NAME; Syntax
  114. 114. Language elements Next class
  115. 115. Language Elements I R.B.Ghongade Lecture 4
  116. 116. • VHDL is a strongly typed language – Designers have to declare the type before using it • VHDL is not case sensitive ( but avoid mixed cases as a good programming practice) • VHDL supports a wide variety of data types and operators – OBJECTS – OPERATORS – AGGREGATES
  117. 117. Objects • They are used to represent and store the data in the design being described • Object contains a value of specific type Class Object Data type SIGNAL COUNT : INTEGER This results in an object called COUNT that holds INTEGER value that belongs to class SIGNAL • The name given to the object is called as identifier • Do not use reserved words as identifiers
  118. 118. • Each object has a data type and class • Class indicates how the object is used in the module and what can be done with that object • Type indicates what type of data the object contains • Each object belongs to one of the following class: – CONSTANT – SIGNAL – VARIABLE – FILES CONSTANT SIGNAL VARIABLE CLASS FILES
  119. 119. Data Types • In order to write VHDL code efficiently it is necessary to study the specification and use of data types • Following are the categories of data types: – Pre-defined – User defined – Subtypes – Arrays – Port arrays – Records – Signed and unsigned
  120. 120. Pre-defined data types • Specified by IEEE 1076 and IEEE 1164 Package Library Type/Functions standard std BIT, BOOLEAN, INTEGER, REAL std_logic_1164 ieee STD_LOGIC, STD_ULOGIC std_logic_arith ieee SIGNED, UNSIGNED / data conversion functions std_logic_signed ieee Functions that allow operations with STD_LOGIC_VECTOR std_logic_unsigned ieee
  121. 121. BIT (and BIT_VECTOR): 2 level logic (‘0’, ‘1’) Examples: SIGNAL X : BIT ; X is declared as a one- digit SIGNAL of type BIT SIGNAL Y : BIT_VECTOR (3 downto 0); Y is 4-bit vector, leftmost bit is MSB SIGNAL W : BIT_VECTOR (0 to 7); W is 8-bit vector, rightmost bit is MSB
  122. 122. • To assign a value to the signal use the operator “ < = ” • Assignment examples: X <= ‘1’ ; X is a single bit SIGNAL whose value is ‘1’ Note that single quotes are used for a single bit Y <= “0111” ; Y is a 4- bit SIGNAL whose value is “0111” . Note that double quotes are used for vectors W <= “01110001” ; W is an 8- bit SIGNAL whose value is “0111001” . MSB is ‘1’
  123. 123. STD_LOGIC (and STD_LOGIC_VECTOR): 8 valued logic (introduced in IEEE 1164 standard) Symbol Description Remark ‘X’ Forcing unknown Synthesizable unknown ‘0’ Forcing low Synthesizable logic ‘0’ ‘1’ Forcing high Synthesizable logic ‘1’ ‘Z’ High impedance Synthesizable tri-state buffer ‘W’ Weak unknown ‘L’ Weak low ‘H’ Weak high ‘-’ Don’t care
  124. 124. Examples: SIGNAL X : STD_LOGIC ; X is declared as a one-digit (scalar) SIGNAL of type STD_LOGIC SIGNAL Y : STD_LOGIC_VECTOR (3 downto 0); Y is 4-bit vector, leftmost bit is MSB SIGNAL Y : STD_LOGIC_VECTOR (3 downto 0) : = “0001” for (optional) initial value use “ := ” Most of std_logic levels are intended for simulation only. However ‘0’, ‘1’ and ‘Z’ are synthesizable with no restrictions
  125. 125. • With respect to “weak” values, they are resolved in favour of the “forcing” values in multiple-driven nodes. If any two std_logic signals are connected to the same node, then conflicting logic levels are resolved by using the shown table X 0 1 Z W L H - X X X X X X X X X 0 X 0 X 0 0 0 0 X 1 X X 1 1 1 1 1 X Z X 0 1 Z W L H Z W X 0 1 W W W W X L X 0 1 L W L W X H X 0 1 H W W H X - X X X X X X X X •The STD_ULOGIC has 9 valued logic levels : additional value is ‘U’ for “Un-resolved or ”Un-initialized”
  126. 126. • Other types BOOLEAN TRUE, FALSE INTEGER 32-bit integers (from - 2,147,483,647 to + 2,147,483,647 NATURAL Non-negative numbers (from 0 to 2,147,483,647 REAL Real numbers (from -1.0E-38 to +1.0E38) Physical literals Used to inform physical quantities like , time, voltage etc. Useful for simulation but not synthesizable Character literals Single ASCII character or a string of such characters. Not synthesizable SIGNED, UNSIGNED They have appearance of STD_LOGIC_VECTOR, but accept arithmetic operations which are typical of INTEGER data type
  127. 127. User defined data types • VHDL allows user defined data types • Two categories of this data type are: – Integer – Enumerated • User defined integer type TYPE my_integer IS RANGE -32 to +32 ; TYPE student_grade IS RANGE 0 to 100 ;
  128. 128. • User defined enumerated type TYPE my_logic IS (‘0’, ‘1’, ‘Z’ ); TYPE my_state IS ( idle, forward, backward, stop) ; An enumerated type, typically used in state machines • The encoding of enumerated types is done sequentially and automatically • Since here there are 4 states only two bits are required hence “00” is assigned to first state ( idle), “01” to second state (forward) and so on.
  129. 129. Subtypes • A SUBTYPE is a TYPE with a constraint • Though operations between data of different types are not allowed, they are allowed between the subtype and its corresponding base type SUBTYPE sub_state IS my_state RANGE idle to backward ; This means that the subtype sub_state =(idle, forward, backward)
  130. 130. Arrays • Arrays are collections of objects of same type • Can be 1-dimensional, 2-dimensional or 1D X 1D • Higher dimensional arrays are possible but not synthesizable 0 0 1 0 0 1 0 1 0 1 1 0 0 0 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 1 0 0 1 0 1 0 Scalar 1D 1D x 1D 2D data array
  131. 131. Array syntax TYPE type_name IS ARRAY (specification) OF data_type ; To specify an array : SIGNAL signal_name : type_name [:= initial_value] To use an array :
  132. 132. Example : 1D x 1D array – We want to build an array containing 4 vectors, each of size 8 bits – we will call each vector as row and the complete array as matrix TYPE row IS ARRAY (7 downto 0 ) OF STD_LOGIC ; TYPE matrix IS ARRAY (3 downto 0 ) OF row ; SIGNAL X : matrix ; 1D x 1D SIGNAL
  133. 133. Example : 2D array – This array will be created with scalars only TYPE matrix2D IS ARRAY (0 TO 3, 7 DOWNTO 0 ) OF STD_LOGIC ; L M L M ROWS COLUMNS
  134. 134. Port Arrays • In the specification of the input or output pins (PORTS) of a circuit (which is made in the ENTITY), we might need to specify the ports as arrays of vectors. • Since TYPE declarations are not allowed in an ENTITY, the solution is to declare user-defined data types in a PACKAGE, which will then be visible to the whole design (thus including the ENTITY)
  135. 135. ------- Package: -------------------------- LIBRARY ieee; USE ieee.std_logic_1164.all; ---------------------------- PACKAGE my_data_types IS TYPE vector_array IS ARRAY (NATURAL RANGE <>) OF STD_LOGIC_VECTOR(7 DOWNTO 0); END my_data_types; -------------------------------------------- ------- Main code: ------------------------- LIBRARY ieee; USE ieee.std_logic_1164.all; USE work.my_data_types.all; -- user-defined package --------------------------- ENTITY mux IS PORT (inp: IN VECTOR_ARRAY (0 TO 3); ... ); END mux; ... ; --------------------------------------------
  136. 136. • As can be seen in the example above, a user- defined data type, called vector_array,was created, which can contain an indefinite number of vectors of size eight bits each (NATURAL RANGE <> signifies that the range is not fixed, with the only restriction that it must fall within the NATURAL range, which goes from 0 to +2,147,483,647) • The data type was saved in a PACKAGE called my_data_types, and later used in an ENTITY to specify a PORT called inp • Notice in the main code the inclusion of an additional USE clause to make the user-defined package my_data_types visible to the design.
  137. 137. Records • Records are similar to arrays, with the only difference that they contain objects of different types. TYPE birthday IS RECORD day: INTEGER RANGE 1 TO 31; month: month_name; END RECORD;
  138. 138. Signed and Unsigned data types • These types are defined in the std_logic_arith package of the ieee library Examples: SIGNAL x: SIGNED (7 DOWNTO 0); SIGNAL y: UNSIGNED (0 TO 3);
  139. 139. • An UNSIGNED value is a number never lower than zero. For example, ‘‘0101’’ represents the decimal 5, while ‘‘1101’’ signifies 13. • If type SIGNED is used instead, the value can be positive or negative (in two’s complement format). Therefore,‘‘0101’’ would represent the decimal 5, while ‘‘1101’’ would mean 3. • To use SIGNED or UNSIGNED data types, the std_logic_arith package, of the ieee library, must be declared
  140. 140. Next class Language Elements II
  141. 141. Language Elements II R.B.Ghongade Lecture 5
  142. 142. Operators • VHDL provides several kinds of pre- defined operators – Assignment operators – Logical operators – Arithmetic operators – Relational operators – Shift operators – Concatenation operators
  143. 143. Assignment operators • Are used to assign values to signals, variables, and constants. <= Used to assign a value to a SIGNAL := Used to assign a value to a VARIABLE, CONSTANT, or GENERIC Used also for establishing initial values => Used to assign values to individual vector elements or with OTHERS
  144. 144. SIGNAL x : STD_LOGIC; VARIABLE y : STD_LOGIC_VECTOR(3 DOWNTO 0); SIGNAL w: STD_LOGIC_VECTOR(0 TO 7); x <= '1'; -- '1' is assigned to SIGNAL x using "<=" y := "0000"; -- "0000" is assigned to VARIABLE y using --":=" w <= "10000000"; -- LSB is '1', the others are '0' w <= (0 =>'1', OTHERS =>'0'); -- LSB is '1', the others -- are '0' Then the following assignments are legal:
  145. 145. Logical operators • Used to perform logical operations. • The data must be of type: – BIT, STD_LOGIC – STD_ULOGIC – BIT_VECTOR – STD_LOGIC_VECTOR – STD_ULOGIC_VECTOR
  146. 146. • The logical operators are: – NOT – AND – OR – NAND – NOR – XOR – XNOR The NOT operator has precedence over the others Examples: y <= NOT a AND b; -- (a'.b) y <= NOT (a AND b); -- (a.b)' y <= a NAND b; -- (a.b)'
  147. 147. Arithmetic operators • Used to perform arithmetic operations. The data can be of type INTEGER, SIGNED, UNSIGNED, or REAL (the last cannot be synthesized directly). • Also, if the std_logic_signed or the std_logic_unsigned package of the ieee library is used, then STD_LOGIC_VECTOR can also be employed directly in addition and subtraction operations
  148. 148. + ( Addition) - (Subtraction) * (Multiplication) / (Division) ** (Exponentiation) MOD ( Modulus) REM ( Remainder) ABS ( Absolute value) •There are no synthesis restrictions regarding addition and subtraction, and the same is generally true for multiplication •For division, only power of two dividers (shift operation) are allowed •For exponentiation, only static values of base and exponent are accepted •Regarding the mod and rem operators, y mod x returns the remainder of y/x with the signal of x, while y rem x returns the remainder of y/x with the signal of y •Finally, abs returns the absolute value For mod, rem, abs , there generally is little or no synthesis support
  149. 149. Comparison operators = Equal to /= Not equal to < Less than > Greater than <= Less than or equal to >= Greater than or equal to Also called RELATIONAL operators
  150. 150. Shift operators sll shift left logical srl shift right logical sla shift left arithmetic sra shift right arithmetic ror rotate left logical rol rotate right logical 0 0 LOGICAL SHIFTING ARITHMETIC SHIFTING (retains sign bit) ROTATE • LOGICAL • ARITHMETIC • ROTATE
  151. 151. Concatenation operator • Operands can be one-dimensional array type or element type • “ &” works on vectors only & Concatenation Example: SIGNAL a : STD_LOGIC_VECTOR ( 5 DOWNTO 0 ) ; SIGNAL b,c,d : STD_LOGIC_VECTOR ( 2 DOWNTO 0 ) ; BEGIN b <= ‘0’ & c(1) & d(2) ; a <= c & d ; . . .
  152. 152. Operator summary Operator type Operators Data types Logical NOT, AND, AND OR, NOR, XOR, XNOR BIT, BIT_VECTOR, STD_LOGIC, STD_LOGIC_VECTOR, STD_ULOGIC, STD_ULOGIC_VECTOR Arithmetic +, -,*,/,** (mod, rem , abs) INTEGER, SIGNED, UNSIGNED Comparison =, /=, <, >, <=, >= All above Shift sll, srl, sla, sra, rol, ror BIT_VECTOR Concatenation &, ( , , , ) Same as for logical operators, plus SIGNED and UNSIGNED
  153. 153. Operator overloading • Operators can be user-defined • Let us consider the pre-defined arithmetic operators seen earlier (+,- , *, /, etc.). They specify arithmetic operations between data of certain types (INTEGER, for example) • For instance, the pre-defined ‘‘+’’ operator does not allow addition between data of type BIT. • We can define our own operators, using the same name as the pre-defined ones
  154. 154. • For example, we could use ‘‘+’’ to indicate a new kind of addition, this time between values of type BIT_VECTOR. This technique is called operator overloading • Example: Consider that we want to add an integer to a binary 1-bit number. Then the following FUNCTION could be used FUNCTION "+" (a: INTEGER, b: BIT) RETURN INTEGER IS BEGIN IF (b='1') THEN RETURN a+1; ELSE RETURN a; END IF; END "+";
  155. 155. A call to the function above could thus be the following: SIGNAL inp1, outp: INTEGER RANGE 0 TO 15; SIGNAL inp2: BIT; (...) outp <= 3 + inp1 + inp2; (...) • In ‘‘outp<=3+inp1+inp2;’’, the first ‘‘+’’ is the pre- defined addition operator (adds two integers), while the second is the overloaded user-defined addition operator (adds an integer and a bit).
  156. 156. Aggregates • It assigns values to elements of an array a <= (OTHERS => ‘0’ ) ; a <= “0000” ; • We can assign values to some bits in a vector and use “OTHERS” clause to assign default values to remaining bits a <= (0 => ‘1’, 2 => ‘1’, OTHERS => ‘0’ ) ; is equivalent to a <= “00101” ; Useful when we are dealing with large vectors
  157. 157. • Each object has a data type and class • Class indicates how the object is used in the module and what can be done with that object • Type indicates what type of data the object contains • Each object belongs to one of the following class: – CONSTANT – SIGNAL – VARIABLE CONSTANT SIGNAL VARIABLE CLASS Classes re-visited
  158. 158. Constants • These are identifiers with fixed values • The value is assigned only once when declared • Values cannot be changed during simulation CONSTANT bus_width : INTEGER :=16 ; CONSTANT CLK_PERIOD : TIME :=15 ns ; • Constants make the design description more readable • Design changed at later time becomes easy
  159. 159. Signals Equivalent to wires within a circuit Example: architecture and_gate of myand is signal TEMP : STD_LOGIC ; begin U1 : AND2 portmap ( a, b, TEMP ) ; U2 : AND2 portmap (TEMP, c , d ) ; end and_gate ; a b AND2 TEMP c AND2 d
  160. 160. • Thus signals are used : – to connect design entities together and communicate changes in values within a design – instead of INOUT mode • Each signal has a history of values i.e. they hold a list of values which include current value of the signal and a set of possible future values that can appear on the signal • Computed value is assigned to signal after specified delay called DELTA DELAY
  161. 161. Variables • These are objects with single current value • They are used to store the intermediate values between the sequential statements • Variable assignment occurs immediately • Variables can be declared and used inside the process statement only. But they retain their value throughout the entire simulation
  162. 162. process ( a ) variable count : INTEGER : = 1 ; begin count : = count+ 1 ; end process ; Example : count contains the total number of events that occurred on signal a
  163. 163. Language elements III Next class
  164. 164. Language Elements III R.B.Ghongade Lecture 6
  165. 165. Attributes • An attribute is data that are attached to VHDL objects or predefined data about VHDL objects • Examples are the current drive capability of a buffer or the maximum operating temperature of the device • Types are – Data Attributes – Signal Attributes – User-defined Attributes
  166. 166. Data Attributes The pre-defined, synthesizable data attributes are the following: • d’LOW : Returns lower array index • d’HIGH : Returns upper array index • d’LEFT : Returns leftmost array index • d’RIGHT : Returns rightmost array index • d’LENGTH : Returns vector size • d’RANGE : Returns vector range • d’REVERSE_RANGE: Returns vector range in reverse order
  167. 167. Example Consider the following signal: SIGNAL d : STD_LOGIC_VECTOR (7 DOWNTO 0); Then: d'LOW=0, d'HIGH=7, d'LEFT=7, d'RIGHT=0, d'LENGTH=8, d'RANGE=(7 downto 0), d'REVERSE_RANGE=(0 to 7)
  168. 168. If the signal is of enumerated type, then: • d’VAL(pos) : Returns value in the position specified • d’POS(value) : Returns position of the value specified • d’LEFTOF(value) : Returns value in the position to the left of the value specified • d’VAL(row, column) : Returns value in the position specified; etc There is little or no synthesis support for enumerated data type attributes
  169. 169. Signal Attributes Let us consider a signal s Then: • s’EVENT : Returns true when an event occurs on s • s’STABLE : Returns true if no event has occurred on s • s’ACTIVE : Returns true if s = ‘1’ • s’QUIET <time> : Returns true if no event has occurred during the time specified • s’LAST_EVENT : Returns the time elapsed since last event • s’LAST_ACTIVE: Returns the time elapsed since last s=‘1’ • s’LAST_VALUE : Returns the value of s before the last event; etc.
  170. 170. Example All four assignments shown below are synthesizable and equivalent. They return TRUE when an event (a change) occurs on clk, AND if such event is upward (in other words, when a rising edge occurs on clk) IF (clk'EVENT AND clk='1')... -- EVENT attribute -- used with IF IF (NOT clk'STABLE AND clk='1')... -- STABLE --attribute used -- with IF WAIT UNTIL (clk'EVENT AND clk='1'); -- EVENT --attribute used -- with WAIT IF RISING_EDGE(clk)... -- call to a function
  171. 171. User-defined Attributes • VHDL also allows the construction of user-defined attributes • To employ a user-defined attribute, it must be declared and specified Attribute Declaration: ATTRIBUTE attribute_name: attribute_type ; Attribute Specification: ATTRIBUTE attribute_name OF target_name: class IS value; where: attribute_type: any data type (BIT, INTEGER, STD_LOGIC_VECTOR, etc.) class: TYPE, SIGNAL, FUNCTION, etc. value: ‘0’, 27, ‘‘00 11 10 01’’, etc.
  172. 172. Example ATTRIBUTE number_of_inputs: INTEGER; ATTRIBUTE number_of_inputs OF nand3: SIGNAL IS 3; ... inputs <= nand3'number_of_inputs; -- attribute call, -- returns 3
  173. 173. Generics • As the name suggests, GENERIC is a way of specifying a generic parameter • a static parameter that can be easily modified and adapted to different applications • The purpose is to make the code more flexible and reusable • must be declared in the ENTITY • More than one GENERIC parameter can be specified in an ENTITY
  174. 174. Syntax GENERIC (parameter_name : parameter_type := parameter_value); The GENERIC statement below specifies a parameter called n, of type INTEGER, whose default value is 8. Therefore, whenever n is found in the ENTITY itself or in the ARCHITECTURE (one or more) that follows, its value will be assumed to be 8 ENTITY my_entity IS GENERIC (n : INTEGER := 8; vector: BIT_VECTOR := "00001111"); PORT (...); END my_entity; ARCHITECTURE my_architecture OF my_entity IS ... END my_architecture; Example
  175. 175. ARCHITECTURE generic_decoder OF decoder IS BEGIN PROCESS (ena, sel) VARIABLE temp1 : STD_LOGIC_VECTOR (x'HIGH DOWNTO 0); VARIABLE temp2 : INTEGER RANGE 0 TO x'HIGH; .... Example
  176. 176. Delays in VHDL • In VHDL, there are three types of delay that are encountered – Inertial delay – Transport delay – Delta delay
  177. 177. Inertial Delay • Inertial delay is the default in VHDL • Behaves similarly to the actual device • Output signal of the device has inertia, which must be overcome for the signal to change value • The inertial delay model is by far the most commonly used in all currently available simulators
  178. 178. Inertial delay prevents prolific propagation of spikes throughout the circuit LIBRARY IEEE; USE IEEE.std_logic_1164.ALL; ENTITY buf IS PORT ( a : IN std_logic; PORT ( b : OUT std_logic); END buf; ARCHITECTURE buf OF buf IS BEGIN b <= a AFTER 20 ns; END buf; Due to inertial delay pulse is swallowed up
  179. 179. Transport Delay • It represents a wire delay in which any pulse, no matter how small, is propagated to the output signal delayed by the delay value specified • Especially useful for modeling delay line devices, wire delays on a PCB, and path delays on an ASIC
  180. 180. LIBRARY IEEE; USE IEEE.std_logic_1164.ALL; ENTITY delay_line IS PORT ( a : IN std_logic; PORT ( b : OUT std_logic); END delay_line; ARCHITECTURE delay_line OF delay_line IS BEGIN b <= TRANSPORT a AFTER 20 ns; END delay_line; Pulse is simply delayed
  181. 181. Delta delay • These are used since the PC that processes and simulates a concurrent phenomenon is basically a sequential machine • The simulation program mimics concurrency by scheduling events in some order • Simulation deltas are used to order some types of events during a simulation • Specifically, zero delay events must be ordered to produce consistent results
  182. 182. Circuit that shows the difference! CLK D E Q' Q DFF CLK A B C F D Zero delay components CLK=‘1’ A=‘1’ Assumptions
  183. 183. Problem when no delta delay concept is used CLK D E Q' Q DFF CLK A B C F CLK D E Q' Q DFF CLK A B C F CLK D E Q' Q DFF CLK A B C F 1) A becomes 0 2) Evaluate inverter 3) B <= 1 4) Evaluate AND with C=1 5) D<=1 6) Evaluate NAND 7) C<=0CLK D E Q' Q DFF CLK A B C F 8) Evaluate AND 9) D<=0 CLK D E Q' Q DFF CLK A B C F D Unwanted D assertion DD
  184. 184. Problem when no delta delay concept is used CLK D E Q' Q DFF CLK A B C F DD 1) A becomes 0 2) Evaluate inverter 3) B <= 1 4) Evaluate NAND 5) C<=0 6) Evaluate AND 7) D<=0 CLK D E Q' Q DFF CLK A B C F CLK D E Q' Q DFF CLK A B C F D CLK D E Q' Q DFF CLK A B C F
  185. 185. Delta delay use Delta 1 Delta 2 Delta 3 Delta 4 10 ns 11 ns A<=0 Evaluate inverter B<=0 Evaluate AND Evaluate NAND D<= 1 C<=0 Evaluate AND D<= 0 To summarize, simulation deltas are an infinitesimal amount of time used as a synchronization mechanism when 0 delay events are present.
  186. 186. Concurrent Statements and Constructs
  187. 187. Combinational vs. Sequential Logic The output of the circuit depends solely on the current inputs Output does depend on previous inputs hence storage elements are required Combinational Logic outputinput Storage Elements Present State Next State Combinational Logic outputinput
  188. 188. Concurrent Code • Consider the following statement: X = X + Y ; In conventional software X and Y are register locations hence contents of X and Y are added and stored in X Register X Register Y +
  189. 189. Difference in VHDL • In VHDL the same statement will mean a feedback in a purely combinational logic which is invalid + X Y
  190. 190. • VHDL code is inherently concurrent (parallel) • Only statements placed inside a PROCESS, FUNCTION, or PROCEDURE are sequential • Concurrent code is also called dataflow code • Order does not matter • We can only build combinational logic circuits with concurrent code • Concurrent assignment produces one driver for each assignment statement z <= a; a z
  191. 191. Multiple driver assignment architecture ABC of XYZ is signal z,a,b,c,d : std_logic ; begin z <= a and b; z <= c and d; . . . ? a b c d z Care has to be taken to handle such conditions with a hi-impedance state
  192. 192. Concurrent constructs Next Class
  193. 193. Concurrent Constructs R.B.Ghongade Lecture 7
  194. 194. Types of concurrent constructs • when … else • with … select These constructs need not be in the process
  195. 195. when…else • A concurrent statement which assigns one of several expressions to a signal, depending on the values of Boolean conditions which are tested in sequence • Equivalent to a process containing an if statement Syntax [Label:] Target <= [Options] Expression [after TimeExpression] when Condition else Expression [after TimeExpression] when Condition else ... Expression [after TimeExpression] [when Condition];
  196. 196. Where to use ? architecture – begin – HERE - end block – begin – HERE - end generate – begin – HERE - end Rules: • The reserved word guarded may only appear in a signal assignment within a guarded block. A guarded assignment only executes when the guard expression on the surrounding block is true • An Expression on the right hand side may be replaced by the reserved word “unaffected”
  197. 197. Synthesis • Conditional signal assignments are synthesized to combinational logic • The Expressions on the right hand side are multiplexed onto the Target signal • The resulting logic will be priority encoded, because the conditions are tested in sequence Remarks: • Conditional and selected signal assignments are a concise way to describe combinational logic in Register Transfer Level descriptions, although processes can be easier to read and maintain in some cases • A conditional assignment is a neat way to convert from a Boolean condition to the type Std_logic
  198. 198. Example z <= a when s1=‘1’ else b when s2=‘1’ else c ; c b a s2 s1 z MUX21 MUX21
  199. 199. Example (Tri-state Buffer) architecture tri_buff of tri_buff_part is begin out1 <= in1 when control=‘1’ else ‘z’; end tri_buff ; in1 out1 control
  200. 200. with…select • A concurrent statement which assigns one of several expressions to a signal, depending on the value of the expression at the top. • Equivalent to a process containing a case statement Syntax [Label:] with Expression select Target <= [Options] Expression [after TimeExpression] when Choices, Expression [after TimeExpression] when Choices, Expression when others;
  201. 201. Where to use ? architecture – begin – HERE – end block – begin – HERE – end generate – begin – HERE – end Rules: • Every case of the Expression at the top must be covered once and only once by the choices • An Expression on the right hand side may be replaced by the reserved word “unaffected” • All possible choices must be enumerated • “others” clause is important since we have 9- valued logic
  202. 202. Synthesis • Selected signal assignments are synthesized to combinational logic • The Expressions on the right hand side are multiplexed onto the Target signal Remarks: • Conditional and selected signal assignments are a good way to describe combinational logic in Register Transfer Level descriptions
  203. 203. Example (Multiplexer) architecture mux41 of mux is -- Assumptions begin -- a,b,c,d,z are with control select -- std_logic z <= a when “00” , -- control is b when “01” ,-- std_logic_vector(1 downto 0) c when “10” , d when “11” , ‘Z’ when others ; end mux41 ; control a b c d z MUX41
  204. 204. Block • There are two types of blocks – Simple – Guarded
  205. 205. Simple block • The BLOCK statement, in its simple form, represents only a way of locally partitioning the code • It allows a set of concurrent statements to be clustered into a BLOCK, with the purpose of turning the overall code more readable and more manageable (which might be helpful when dealing with long codes)
  206. 206. Syntax label: BLOCK [declarative part] BEGIN (concurrent statements) END BLOCK label; ARCHITECTURE example ... BEGIN ... block1: BLOCK BEGIN ... END BLOCK block1 ; ... block2: BLOCK BEGIN ... END BLOCK block2 ; ... END example ; General form of architecture using block for partitioning
  207. 207. • Block can be nested inside another block Syntax label1: BLOCK [declarative part of top block] BEGIN [concurrent statements of top block] label2: BLOCK [declarative part nested block] BEGIN (concurrent statements of nested block) END BLOCK label2; [more concurrent statements of top block] END BLOCK label1;
  208. 208. Guarded block • A guarded BLOCK is a special kind of BLOCK, which includes an additional expression, called guard expression • A guarded statement in a guarded BLOCK is executed only when the guard expression is TRUE Syntax label: BLOCK (guard expression) [declarative part] BEGIN (concurrent guarded and unguarded statements) END BLOCK label;
  209. 209. • Even though only concurrent statements can be written within a BLOCK, with a guarded BLOCK even sequential circuits can be constructed LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY latch IS PORT (d, clk: IN STD_LOGIC; q: OUT STD_LOGIC); END latch; ARCHITECTURE latch OF latch IS BEGIN b1: BLOCK (clk='1') BEGIN q <= GUARDED d; END BLOCK b1; END latch; Latch
  210. 210. LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY DFF IS PORT (d, clk, rst: IN STD_LOGIC; q: OUT STD_LOGIC); END DFF; ARCHITECTURE DFF OF DFF IS BEGIN b1: BLOCK (clk’EVENT AND clk='1') BEGIN q <= GUARDED ‘0’ WHEN rst=‘1’ ELSE d; END BLOCK b1; END DFF; Here, a positive-edge sensitive D-type flip-flop, with synchronous reset, is designed. In it, clk'EVENT AND clk='1' is the guard expression, while q <= GUARDED '0‘ WHEN rst='1' is a guarded statement. Therefore, q<='0' will occur when the guard expression is true and rst is ‘1’ DFF
  211. 211. Homework Problems 1)Generic encoder 2) 8- bit ALU
  212. 212. For ALU in problem 2 sel Operation Function Unit 0000 y <= a Transfer a Arithmetic 0001 y <= a+1 Increment a 0010 y <= a-1 Decrement a 0011 y <= b Transfer b 0100 y <= b+1 Increment b 0101 y <= b-1 Decrement b 0110 y <= a + b Add a and b 0111 y <= a + b + cin Add a and b with carry 1000 y <= NOT a Complement a Logic 1001 y <= NOT b Complement b 1010 y <= a AND b AND 1011 y <= a OR b OR 1100 y <= a NAND b NAND 1101 y <= a NOR b NOR 1110 y <= a XOR b XOR 1111 y <= a XNOR b XNOR
  213. 213. 3) Priority Encoder The circuit must encode the address of the input bit of highest order that is active. ‘‘000’’ should indicate that there is no request at the input (no bit active)
  214. 214. Expected waveform for Problem 3
  215. 215. Component Instantiation Next Class DO NOT MISS IN ANY CASE !
  216. 216. Component Instantiation R.B.Ghongade Lecture 8,9,10
  217. 217. Component • A component is analogous to a chip socket; it gives an indirect way to use one hierarchical block within another • A component is instantiated within an architecture, and is associated with a (lower level) entity and architecture during elaboration using information from a configuration. • A component declaration is similar in form to an entity declaration, in that it includes the required ports and parameters of the component • The difference is that it refers to a design described in a separate VHDL file • The ports and parameters in the component declaration may be a subset of those in the component file, but they must have the same names
  218. 218. Component can be declared in the main code itself
  219. 219. Component can be declared in a package
  220. 220. Syntax : COMPONENT component_name GENERIC ( parameter_name : string := default_value ; parameter_name : integer := default_value); PORT (input_name, input_name : IN STD_LOGIC; bidir_name, bidir_name : INOUT STD_LOGIC; output_name, output_name : OUT STD_LOGIC); END COMPONENT; Where : package - <HERE> - end architecture - is - <HERE> - begin - end block - <HERE> - begin - end generate - <HERE> - begin - end
  221. 221. Rules: • For default configuration, the component name must match the name of the corresponding entity to be used in its place, and generics and ports must also match in name, mode and type Synthesis: • A component without a corresponding design entity is synthesized as a black box In VHDL'93, components are not necessary. It is possible instead to directly instantiate an entity within an architecture.
  222. 222. Example component Counter generic (N: INTEGER); port (Clock, Reset, Enable: in Std_logic; Q: buffer Std_logic_vector (N-1 downto 0)); end component ;
  223. 223. Instantiation • A concurrent statement used to define the design hierarchy by making a copy of a lower level design entity within an architecture • In VHDL'93, a direct instantiation of an entity bypasses the component and configuration
  224. 224. Syntax: InstanceLabel: [component] ComponentName [GenericMap] [PortMap]; InstanceLabel: entity EntityName[(ArchitectureName)] [GenericMap] [PortMap]; InstanceLabel: configuration ConfigurationName [GenericMap] [PortMap]; Where: architecture – begin - <HERE> - end block – begin - <HERE> - end generate – begin - <HERE> - end
  225. 225. Rules: • An entity, architecture or configuration must be compiled into a library before the corresponding instance can be compiled • However, an instance of a component can be compiled before the corresponding design entity has even been written Example : G1: NAND2 generic map (1.2 ns) port map (N1, N2, N3); G2: entity WORK.Counter(RTL) port map (Clk, Rst, Count);
  226. 226. Generic Map • Used to define the values of generics • Usually given in an Instance, but may also appear in a configuration Syntax generic map ([Formal =>] Actual, ...) Formal = {either} Name FunctionCall Actual = Expression
  227. 227. Where : Label : ComponentName <HERE> port map (…); for - use - <HERE> port map (…) block – generic (…); <HERE> ; port – begin - end Rules : The two forms of syntax (ordered list or explicitly named choices) can be mixed, but the ordered list must come before the named choices A generic map does not end with a semicolon!
  228. 228. Example: architecture Structure of Ent is component NAND2 generic (TPLH, TPHL: TIME := 0 NS); port (A, B: in STD_LOGIC; F : out STD_LOGIC); end component; begin G1: NAND2 generic map (1.9 NS, 2.8 NS) port map (N1, N2, N3); G2: NAND2 generic map (TPLH => 2 NS, TPHL => 3 NS) port map (N4, N5, N6); end Structure;
  229. 229. Port Map • A port map is typically used to define the interconnection between instances in a structural description (or netlist) • A port map maps signals in an architecture to ports on an instance within that architecture • Port maps can also appear in a configuration or a block
  230. 230. Syntax: port map ([Formal =>] Actual, ...); Formal = {either} Name FunctionCall Actual = {either} Name FunctionCall open Where: Label : ComponentName generic map (…) <HERE>; for - use - generic map (…) <HERE>; block - port (…) ; <HERE>; - begin - end
  231. 231. Rules: • The two forms of syntax (ordered list or explicitly named ports) can be mixed, but the ordered list must come before the named ports • Within an instance, the formals are ports on the component or entity being instanced, the actuals are signals visible in the architecture containing the instance • Within a configuration, the formals are ports on the entity, the actuals are ports on the component • If the actual is a conversion function, this is called implicitly as values are passed in • If the formal is a conversion function, this is called implicitly as values are passed out Use the port names rather than order to improve readability and reduce the risk of making connection errors
  232. 232. Example: component COUNTER port (CLK, RESET: in Std_logic; UpDown: in Std_logic := '0';-- default value Q: out Std_logic_vector(3 downto 0)); end component; ... -- Positional association... G1: COUNTER port map (Clk32MHz, RST, open, Count); -- Named association (order doesn't matter)... G2: COUNTER port map ( RESET => RST, CLK => Clk32MHz, Q(3) => Q2MHz, Q(2) => open, -- unconnected Q(1 downto 0) => Cnt2, UpDown => open);
  233. 233. Top Level Entity and Lower Level Entity TOP LEVEL ENTITY Clk32MHz Q2MHz Cnt2 RST COUNT COUNTER (LOWER LEVEL ENTITY) CLK RESET Q Updown
  234. 234. TOP LEVEL ENTITY Clk32MHz Q2MHz Cnt2 RST COUNT G1 CLK RESET Q Updown G2 CLK RESET Q(3) Updown Q(0) CLK => Clk32MHz Q(3) => Q2MHz UpDown => open RESET => RST
  235. 235. A still simpler example entity ND4 is port (in1,in2,in3,in4 : in std_logic ; z : out std_logic); end ND4; architecture ND4_CI of ND4 is component ND2 port (a , b : in std_logic; c : out std_logic); end component ; signal temp1, temp2 : std_logic; begin U1 : ND2 port map (a => in1 , b => in2 , c => temp1); U2 : ND2 port map (a => in3 , b => in4 , c => temp2); U3 : ND2 port map (a => temp1 , b => temp2 , c => z); end ND4_CI ;
  236. 236. infers… ND4 U1 U3 U2 IN1 IN2 IN3 IN4 Z a a b b a b c c c c => temp1 for U1 a => temp1 for U3 c => temp2 for U2 b => temp2 for U3
  237. 237. Generate statement • A concurrent statement used to create regular structures or conditional structures during elaboration • Used to create multiple copies of components , processes or blocks • It provides a compact description of regular structures such as memories , registers and counters
  238. 238. • Two flavours of generate statement are: – for … generate • Number of copies is determined by a discrete range – if … generate • Zero or one copy is made conditionally • Range must be a computable integer in any of the following forms: – integer_expression to integer_expression – integer_expression downto integer_expression – Each integer_expression evaluates to an integer
  239. 239. Syntax : Label: for ParameterName in Range generate [Declarations... begin] ConcurrentStatements... end generate [Label]; Label: if Condition generate [Declarations... begin] ConcurrentStatements... end generate [Label];
  240. 240. Where: architecture – begin - <HERE> - end block – begin - <HERE> - end generate – begin - <HERE> - end Rules : • The Range and Condition must both be static, i.e. they cannot include signals • The Label at the beginning of the generate statement cannot be omitted Synthesis: • Synthesis is straightforward, but not all synthesis tools support generate!
  241. 241. Example: architecture ABC of full_add4 is component full_add port (PA , PB , PC : in std_logic ; PCOUT , PSUM : out std_logic) ; end component ; signal c: std_logic_vector(4 downto 0); begin c(0) <= cin ; -- cin is declared in entity GK : for k in 3 downto 0 generate FA :full_add port map(A(k),B(k),C(k),C(k+1),SUM(k); end generate GK ; cout <= c(4) ; -- cout is declared in entity end ABC ;
  242. 242. infers… FA2 A(2) B(2) C(2) SUM(2) FA3 A(3) B(3) Cout SUM(3) FA0 A(0) B(0) Cin C(1) SUM(0) FA1 A(1) B(1) SUM(1) C(3)
  243. 243. architecture SHIFTER_ARCH of SHIFTER is component DFF port (D , CLK : in std_logic ; Q : out std_logic) ; end component ; begin GK : for k in 0 to 3 generate GK0 : IF k=0 generate DFILPFLOP : DFF port map (count , clock , Q(k)); end generate GK0 ; GK1_3 : if k > 0 generate DFILPFLOP : DFF port map (Q(k-1), clock , Q(k)); end generate GK1_3 ; end generate GK ; end SHIFTER_ARCH ; Another example
  244. 244. infers… DF1DF0 DF3DF2 CLOCK COUNT Q(0) Q(1) Q(2) Q(3)
  245. 245. Ways to describe a circuit! • Three types of descriptions possible with VHDL – Structural – Dataflow – Behavioral
  246. 246. Structural Method • At the structural level, which is the lowest level, you have to first manually design the circuit. • Use VHDL to specify the components and gates that are needed by the circuit and how they are connected together by following your circuit exactly • Synthesizing a structural VHDL description of a circuit will produce a netlist that is exactly like your original circuit • The advantage of working at the structural level is that you have full control as to what components are used and how they are connected. • But you need to first come up with the circuit and so the full capabilities of the synthesizer are not utilized
  247. 247. Dataflow Method • At the dataflow level, you use the built-in logical functions of VHDL in signal assignment statements to describe a circuit, which again you have to first design manually • Boolean functions that describe a circuit can be easily converted to signal assignment statements using the built-in logical functions • The only drawback is that the built-in logical functions such as the AND and OR function only take two operands. This is like having only 2-input gates to work with ! All the statements use in the structural and dataflow level are executed concurrently
  248. 248. Behavioral Method • Describing a circuit at the behavioral level is most similar to writing a computer program • You have all the standard high-level programming constructs such as the FOR LOOP, WHILE LOOP, IF THEN ELSE, CASE, and variable assignments • The statements are enclosed in a process block and are executed sequentially
  249. 249. Example BCD to 7- segment display decoder a b c d e f g BCD to 7-segment display decoder I3 I2 I1 I0 segs(6) {seg 'a'} segs(5) {seg 'b'} segs(4) {seg 'c'} segs(3) {seg 'd'} segs(2) {seg 'e'} segs(1) {seg 'f'} segs(0) {seg 'g'}
  250. 250. Truth-table
  251. 251. Logic Equations 2 1 0 3 1 2 0 ' ' 2 1 0 ' ' ' ' ' 1 0 2 0 2 1 2 1 0 ' ' ' 1 0 2 0 ' ' ' ' 3 2 1 2 0 1 0 ' 3 2 1 1 0 ( ) ( ) ( ) a I I I I b I I I c I I I d I I I I I I I I I e I I I I f I I I I I I I g I I I I I = + + = + = + + = + + + = + = + + + = + ⊕ +
  252. 252. Logic gates
  253. 253. Structural VHDL description ENTITY myxnor2 IS PORT(i1, i2: IN BIT;o: OUT BIT); END myxnor2; ARCHITECTURE Dataflow OF myxnor2 IS BEGIN o <= not(i1 XOR i2); END Dataflow; ENTITY myxor2 IS PORT(i1, i2: IN BIT;o: OUT BIT); END myxor2; ARCHITECTURE Dataflow OF myxor2 IS BEGIN o <= i1 XOR i2; END Dataflow; ENTITY myand2 IS PORT(i1, i2: IN BIT;o: OUT BIT); END myand2; ARCHITECTURE Dataflow OF myand2 IS BEGIN o <= i1 AND i2; END Dataflow;
  254. 254. ENTITY myand3 IS PORT(i1, i2, i3: IN BIT; o: OUT BIT); END myand3; ARCHITECTURE Dataflow OF myand3 IS BEGIN o <= (i1 AND i2 AND i3); END Dataflow; ENTITY myor2 IS PORT(i1, i2: IN BIT;o: OUT BIT); END myor2; ARCHITECTURE Dataflow OF myor2 IS BEGIN o <= i1 OR i2; END Dataflow; ENTITY myor3 IS PORT(i1, i2, i3: IN BIT;o: OUT BIT); END myor3; ARCHITECTURE Dataflow OF myor3 IS BEGIN o <= i1 OR i2 OR i3; END Dataflow;
  255. 255. ENTITY myor4 IS PORT(i1, i2, i3, i4: IN BIT; o: OUT BIT); END myor4; ARCHITECTURE Dataflow OF myor4 IS BEGIN o <= i1 OR i2 OR i3 OR i4; END Dataflow; ENTITY inv IS PORT (i: IN BIT; o: OUT BIT); END inv; ARCHITECTURE Dataflow OF inv IS BEGIN o <= not i; END Dataflow;
  256. 256. LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY bcd IS PORT(i0, i1, i2, i3: IN BIT; a, b, c, d, e, f, g: OUT BIT); END bcd; ARCHITECTURE Structural OF bcd IS COMPONENT inv PORT (i: IN BIT ;o: OUT BIT); END COMPONENT; COMPONENT myand2 PORT(i1, i2: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myand3 PORT(i1, i2, i3: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myor2 PORT(i1, i2: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myor3 PORT(i1, i2, i3: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myor4 PORT(i1, i2, i3, i4: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myxnor2 PORT(i1, i2: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myxor2 PORT(i1, i2: IN BIT;o: OUT BIT); END COMPONENT;
  257. 257. SIGNAL j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z: BIT; BEGIN U1: INV port map(i2,j); U2: INV port map(i1,k); U3: INV port map(i0,l); U4: myXNOR2 port map(i2, i0, z); U5: myOR3 port map(i3, i1, z, a); U6: myXNOR2 port map(i1, i0, y); U7: myOR2 port map(j, y, b); U8: myOR3 port map(i2, k, i0, c); U9: myAND2 port map(i1, l, x); U10: myAND2 port map(j, l, w); U11: myAND2 port map(j, i1, v); U12: myAND3 port map(i2, k, i0, t); U13: myOR4 port map(x, w, v, t, d); U14: myAND2 port map(i1, l, s); U15: myAND2 port map(j, l, r); U16: myOR2 port map(s, r, e); U17: myAND2 port map(i2, k, q); U18: myAND2 port map(i2, l, p); U19: myAND2 port map(k, l, o); U20: myOR4 port map(i3, q, p, o, f); U21: myXOR2 port map(i2, i1, n); U22: myAND2 port map(i1, l, m); U23: myOR3 port map(i3, n, m, g); END Structural;
  258. 258. Dataflow VHDL description LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY bcd IS PORT ( I: IN STD_LOGIC_VECTOR (3 DOWNTO 0); Segs: OUT std_logic_vector (1 TO 7)); END bcd; ARCHITECTURE Dataflow OF bcd IS BEGIN Segs(1) <= I(3) OR I(1) OR NOT (I(2) XOR I(0)); -- seg a Segs(2) <= (NOT I(2)) OR NOT (I(1) XOR I(0)); -- seg b Segs(3) <= I(2) OR (NOT I(1)) OR I(0); -- seg c Segs(4) <= (I(1) AND NOT I(0)) OR (NOT I(2) AND NOT I(0)) -- seg d OR (NOT I(2) AND I(1)) OR (I(2) AND NOT I(1) AND I(0)); Segs(5) <= (I(1) AND NOT I(0)) OR (NOT I(2) AND NOT I(0)); -- seg e Segs(6) <= I(3) OR (I(2) AND NOT I(1)) -- seg f OR (I(2) AND NOT I(0)) OR (NOT I(1) AND NOT I(0)); Segs(7) <= I(3) OR (I(2) XOR I(1)) OR (I(1) AND NOT I(0)); -- seg g END Dataflow;
  259. 259. Behavioral VHDL description library IEEE; use IEEE.STD_LOGIC_1164.all; entity BCD is port( I : in STD_LOGIC_VECTOR(3 downto 0); segs : out STD_LOGIC_VECTOR(6 downto 0) ); end BCD; architecture Behavioral of BCD is begin with I select Segs <= "1111110" when "0000", "0110000" when "0001", "1101001" when "0010", "1111001" when "0011", "0110011" when "0100", "1011011" when "0101", "1011111" when "0110", "1110000" when "0111", "1111111" when "1000", "1110011" when "1001", "0000000" when others; end Behavioral;
  260. 260. Output
  261. 261. Equations for carry_generate(G) and carry_propagate(P) for ALU 74181 Carry Lookahead Logic xi yi xi-1 x0 yi-1 y0 c0 ci hsi si gi= xi . yi pi=xi + yi ci+1= gi + pi . ci Assignment No 3
  262. 262. c1= g0 + p0 . c0 c2= g1 + p1 . g0 + p1.p0.c0 c3= g2 + p2 . g1 + p2.p1.g0+p2.p1.p0.c0 c4= g3 + p3 . g2 + p3.p2.g1+p3.p2.p1.g0+p3.p2.p1.p0.c0 Additional Information
  263. 263. G_L= (g3+p3.g2+p3.p2.g1+p3.p2.p1.g0)’ P_L=(p3.p2.p1.p0)’ Implement the Carry_Generate and Carry_Propagate outputs also to complete the ALU assignment Equations for implementation of G_L , P_L outputs
  264. 264. Sequential Statements Next Class
  265. 265. ‘generate’d doubt ! fulladder GK.0.FA fulladder GK.3.FA fulladder GK.2.FA fulladder GK.1.FA COUT [4] SUM[3:0] [3:0] Cin B[3:0] [3:0] A[3:0] [3:0] [0] PA [0] PB PC [1] PCOUT [0] PSUM [3] PA [3] PB [3] PC [4] PCOUT [3] PSUM [2] PA [2] PB [2] PC [3] PCOUT [2] PSUM [1] PA [1] PB [1] PC [2] PCOUT [1] PSUM fulladder GK.3.FAfulladder GK.0.FA fulladder GK.1.FA fulladder GK.2.FA COUT [4] SUM[3:0] [3:0] Cin B[3:0] [3:0] A[3:0] [3:0] [3] PA [3] PB [3] PC [4] PCOUT [3] PSUM [0] PA [0] PB PC [1] PCOUT [0] PSUM [1] PA [1] PB [1] PC [2] PCOUT [1] PSUM [2] PA [2] PB [2] PC [3] PCOUT [2] PSUM GK : for k in 3 downto 0 generate GK : for k in 0 to 3 generate=
  266. 266. Sequential Statements R.B.Ghongade Lecture 12
  267. 267. Sequential Statements • VHDL code is inherently concurrent • Sections of code that are executed sequentially are : – PROCESS – FUNCTION – PROCEDURE • One important aspect of sequential code is that it is not limited to sequential logic • We can build sequential circuits as well as combinational circuits • Sequential code is also called behavioral code • Thus a PROCESS is a concurrent statement which describes behaviour
  268. 268. • Sequential statements are allowed only inside PROCESSES, FUNCTIONS, or PROCEDURES • Sequential statements are: – IF – WAIT – CASE – LOOP • VARIABLES are also restricted to be used in sequential code only VARIABLE can never be global, so its value can not be passed out directly
  269. 269. SIGNALS and VARIABLES revisited ! • VHDL has two ways of passing non-static values around: by means of a SIGNAL or by means of a VARIABLE • A SIGNAL can be declared in a PACKAGE, ENTITY or ARCHITECTURE (in its declarative part), while a VARIABLE can only be declared inside a piece of sequential code • SIGNAL is global while VARIABLE is local • The value of a VARIABLE can never be passed out of the PROCESS directly; if necessary, then it must be assigned to a SIGNAL • Update of VARIABLE is immediate whereas new value for SIGNAL is generally only guaranteed to be available after the conclusion of the present run of the PROCESS • Assignment operator for SIGNAL is “<= “ while that for VARIABLE is “ : = “
  270. 270. Process • A PROCESS is a sequential section of VHDL code • It is characterized by the presence of IF, WAIT, CASE, or LOOP, and by a sensitivity list (except when WAIT is used) • A PROCESS must be installed in the main code, and is executed every time a signal in the sensitivity list changes (or the condition related to WAIT is fulfilled)
  271. 271. Syntax [label:] [postponed] PROCESS (sensitivity list) [VARIABLE name type [range] [:= initial_value;]] BEGIN (sequential code) END [postponed] PROCESS [label]; entity - begin - <HERE> - end architecture - begin - <HERE> - end block - begin - <HERE> - end generate - begin - <HERE> - end Where “POSTPONED” is a reserved VHDL word
  272. 272. Rules • A process must contain either a sensitivity list or wait statements, but not both • Every process executes once during initialization, before simulation starts • A postponed process is not executed until the final simulation cycle of a particular simulation time, and thus sees the stable values of signals and variables A process with neither a sensitivity list nor a wait will loop forever !
  273. 273. • To construct a synchronous circuit, monitoring a signal (clock, for example) is necessary • A common way of detecting a signal change is by means of the EVENT attribute • For instance, if clk is a signal to be monitored, then clk ’ EVENT returns TRUE when a change on clk occurs (rising or falling edge) Using EVENT attribute
  274. 274. IF construct • A sequential statement which executes one branch from a set of branches dependent upon the conditions, which are tested in sequence Syntax [Label:] if Condition then SequentialStatements... [elsif Condition then SequentialStatements...] ... {any number of elsif parts} [else SequentialStatements...] end if [Label]; Be careful about the spelling of elsif and end if
  275. 275. Synthesis • Assignments within if statements generally synthesize to multiplexers • Incomplete assignments, where outputs remain unchanged for certain input conditions, synthesize to transparent latches in unclocked processes, and to flip-flops in clocked processes • In some circumstances, nested if statements synthesize to multiple logic levels. This can be avoided by using a case statement instead
  276. 276. • A set of elsif branches can be used to impart priority to the conditions tested first • To decode a value without giving priority to certain conditions, use a case statement instead Example IF (x<y) THEN temp:="11111111"; ELSIF (x=y AND w='0') THEN temp := "11110000"; ELSE temp:=(OTHERS =>'0');
  277. 277. D Flip-Flop with asynchronous reset • A D-type flip-flop is the most basic building block in sequential logic circuits. In it, the output must copy the input at either the positive or negative transition of the clock signal (rising or falling edge) If rst = ‘1’, then the output must be q = ‘0’ ,regardless of the status of clk. Otherwise, the output must copy the input (that is, q = d) at the positive edge of clk