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Low Power VLSI Design


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Low Power VLSI design architecture for EDA (Electronic Design Automation) and Modern Power Estimation, Reduction and Fixing technologies including clock gating and power gating

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Low Power VLSI Design

  1. 1. Low Power VLSI Design VLSI POWER ARCHITECTURE Mahesh Dananjaya
  2. 2. Electronic Design Automation (EDA)  Integrated Circuit design has evolved from basic logic design to very large scale integrated circuits (VLSI)  FPGA, ASIC, SOC, SOPC, MPSOC, NOC and BOC (Brain-on-Chip) will be the pathway to next generation  Technology Scaling and high speed clocking  Complex Digital designs with millions of transistors will not be easy to design manually  Need a Computer aided intelligent design solutions
  3. 3. VLSI DESIGN FLOW Design Specification Architectural Design RTL Modeling Synthesis Physical Design Layout sign off Fabrication Package and Test Partitioning and Clustering Floor Planning Placement Clock Tree Synthesis Signal Routing Timing Closure
  4. 4. NETLIST VS RTL SIGNOFF  Netlist Signoff  RTL Signoff RTL Synthesis Layout Layout Signoff Fab RTL RTL Signoff Synthesis Layout Sign Off Fab
  5. 5. RTL MODELING AND HDL  Structural and Behavioral Modeling  Hardware Descriptive Language (HDL)  Verilog  VHDL  System Verilog  Mixed Languages  HDL Abstraction Level  Behavioral Model  Data Flow Model  Gate Level Model
  6. 6. SYNTHESIS Synthesis Elaboration RTL Optimization Technology Hierarchical Design Mapping to Native GatesOptimizing the design Technology Map d o b c a b c b c d o b c d a’bc | abc | d A B D E C F
  7. 7. VLSI POWER  Power is becoming caliber behind the VLSI design  Dynamic Power is the dominant culprit of the prevailing design  Leakage power is emerging their counterpart as technology scaling makes design  Trade off between power ,performance and area should be optimized for an efficient design  Electronic Design Automation (EDA) should focus on power estimation, reduction and fixing techniques  Challenge to assure power aware VLSI architecture with technology scaling and fastening the clock
  8. 8. WHY POWER ?  Battery Life  Cost of packaging and cooling  Reliability and performance degradation  Slower, leakier circuits at high temperature, higher rate of electro migration  Technology scaling impose more features to be integrated on small area  Physical design is becoming more and more complex  performance systems has a barrier of large power consumptions
  9. 9. POWER ESTIMATION System Power Dynamic Power Switching Power Internal Power Static power Leakage Power
  10. 10. SWICTHING POWER  Power generated due to output changes, thus charging and discharging the load capacitance.  Switching power dissipates mainly depend on the,  System Clock Frequency  Activity Switching Frequency Switching Power Calculation depends on the three factors  𝑪 − 𝑳𝒐𝒂𝒅 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒂𝒏𝒄𝒆  𝒇 − 𝑺𝒘𝒊𝒕𝒄𝒉𝒊𝒏𝒈 𝑭𝒓𝒆𝒒𝒆𝒏𝒄𝒚  𝑽 − 𝑫𝒓𝒊𝒗𝒊𝒏𝒈 𝑽𝒐𝒍𝒕𝒂𝒈𝒆 𝑃𝑆 = 𝐶 ∗ 𝑉2 ∗ 𝑓
  11. 11. INTERNAL POWER  Short circuit path has been created between power and ground at the transition stage  Thus the short circuit current is generated  Both NMOS and PMOS transistors are conducting for a short period of time  Power dissipation due to this temporary short circuit path and the internal capacitance is Internal Power  Depends on some factors,  Input edge time  Slew Rate  Internal Capacitances 𝑃𝐼 = 𝑉 ∗ 𝐼𝑆𝐶
  12. 12. DYNAMIC POWER  Dynamic power is the sum of switching power and internal power 𝑷 𝑫 = 𝑷 𝑺 + 𝑷 𝑰 𝑷 𝑫 = 𝑪 ∗ 𝑽 𝟐 ∗ 𝒇 + 𝑷 𝑰 𝑷 𝑫 = 𝑪 ∗ 𝑽 𝟐 ∗ 𝒇 + 𝑽 ∗ 𝑰 𝑺𝑪 𝑷 𝑫 ⩭ 𝑪 𝒆𝒇𝒇 ∗ 𝑽 𝟐 ∗ 𝒇 𝒔𝒘𝒊𝒕𝒄𝒉
  13. 13. STATIC POWER  Due to non-idle characteristic of the transistor the leakages can be taken place  Static power is nothing, but leakage power  There are two main types of leakages and their subsidiaries  𝐼 𝑂𝐹𝐹 − Sub-threshold leakage (Drain Leakage Current)  𝐼 𝐷,𝑤𝑒𝑎𝑘 − 𝑆𝑢𝑏 − 𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑 𝐷𝑟𝑎𝑖𝑛 𝐶𝑢𝑟𝑟𝑒𝑛𝑡  𝐼𝑖𝑛𝑣 − 𝑅𝑒𝑣𝑒𝑟𝑠𝑒 𝐵𝑖𝑎𝑠𝑒𝑑 𝐶𝑢𝑟𝑟𝑒𝑛𝑡  𝐼 𝐺𝐼𝐷𝐿 − 𝐺𝑎𝑡𝑒 𝐼𝑛𝑑𝑢𝑐𝑒𝑑 𝐷𝑟𝑎𝑖𝑛 𝐿𝑒𝑎𝑘𝑎𝑔𝑒  𝐼 𝐺𝐴𝑇𝐸 − Gate Leakage Current  𝐼 𝑇𝑈𝑁𝑁𝐸𝐿 − 𝐺𝑎𝑡𝑒 𝑇𝑢𝑛𝑛𝑒𝑙𝑖𝑛𝑔  𝐼 𝐻𝐶 − 𝐻𝑜𝑡 𝐶𝑎𝑟𝑟𝑖𝑒𝑟 𝐼𝑛𝑗𝑒𝑐𝑡𝑖𝑜𝑛
  15. 15. LEAKAGE POWER 𝑰 𝑳𝑬𝑨𝑲𝑨𝑮𝑬 = 𝐼 𝑂𝐹𝐹 + 𝐼 𝐺𝐴𝑇𝐸 𝑷 𝑺 = 𝑽 ∗ 𝑰𝒍𝒆𝒂𝒌 𝑰 𝑮𝑨𝑻𝑬 = 𝐼 𝐻𝐶 + 𝐼 𝑇𝑈𝑁𝑁𝐸𝐿 𝑰 𝑶𝑭𝑭 = 𝐼𝑖𝑛𝑣 + 𝐼 𝐷,𝑤𝑒𝑎𝑘 + 𝐼 𝐺𝐼𝐷𝐿
  16. 16. VARIOUS OTHER POWER  Metastability  Output of the flops are remains on the undefined states which s caused by the violation of setup time and hold time.  Set Up Time  Amount of time that the input signal needs to be stable before clocking the flop  Hold Time  Amount of time that input signal wants to be stable after clocking the flop  Glitches  Glitches are unwanted or undesired changes in signals which are resilient (self correcting).  caused by delays in lines and propagation delays of cells.  Latchups  LatchUps is a short circuit path between supply and the ground
  17. 17. EDA POWER ESTIMATION  Mostly based on the tech libraries  Based on two major calculations  Activity  The number of toggles per clock cycle on the signal, averaged over many cycles  Probability  Percentage of the time that the signal will be high
  18. 18. POWER REDUCTION  Power reduction is very important  Can be classified into three main categories based on their implementation and occurrence  Device Engineering  This refers to techniques that are implemented on the underlying transistor that form digital circuitry. This is mostly involved with the transistor level components.  Circuit Engineering  These refer to techniques that are applied to gate/logic level, which are clusters of transistors that perform a small computation like NAND, NOR etc.  System Engineering  These are referring to techniques that can be applied to macro-blocks that are part of a big data path or micro-chip.
  19. 19. LOW POWER LEVERAGES  Parallelism and Pipelined micro-architecture  Clock Gating  Power Gating  Voltage Islands  Gate Sizing  Multi VDD  DVFS – Dynamic Voltage Frequency Scaling  Device Level  Multi Threshold Devices  Low Capacitance in device  High k Hf based MOS
  20. 20. DYNAMIC POWER REDUCTION  Dynamic power reduction is very important because,  Clock tree consume more than 50% of dynamic power consumption.  Power consumed by combinational logic whose values are changing on each clock edge  Power consumed by flops  Power consumed by the clock buffer tree  Asynchronous Logic Circuits which is not driven by the global clock, is also changing de to state changes in the flops.
  21. 21. CLOCK GATING  Major dynamic power reduction technique  Gate the clock as much as the flop is not necessary to be toggled  Otherwise in every clock cycle flop will toggle and dissipate more power  Local clock gating has a new enable to every flop where clock gating is necessary  But with complex VLSI design it is not sustainable to use local clock gating  We need to derive a logic for new enable with the current logic
  22. 22. LOCAL CLOCK GATING  Local enable is used to gate the flop  Enable and clock are and gated and the gated clock is provided to the flop  Local enable, do not have a global perception AND FLOP D Q Enable Clock Data IN Data Out
  23. 23. CLOCK GATING METHODS  Latch Free Clock Gating  Latch Based Clock Gating AND FLOP D Q Enable Clock Data IN Data Out Gated CLK AND FLOP D Q Enable Clock CLK Data Out Data CLK D Q FLOP Gated CLK
  24. 24. MULTI LEVEL BOOLEAN LOGIC  Satisfiability Don’t Care (SDC)  Design spots where certain input/ input combination to a circuit can never occur. There may be possible causes for the SDC conditions.  𝒚 = 𝒂 + 𝒃 , 𝒕𝒉𝒆𝒏 𝒚 = 𝟎, 𝒂 = 𝟏, 𝒃 = ~ 𝒘𝒊𝒍𝒍 𝒏𝒆𝒗𝒆𝒓 𝒐𝒄𝒄𝒖𝒓 (𝑺𝑫𝑪)  Observability Don’t Care (ODC)  Design spots where local changes cannot be observed at the primary outputs.  𝒚 = 𝒂 + 𝒃, 𝒘𝒉𝒆𝒏 𝒂 = 𝟏, 𝒄𝒉𝒂𝒏𝒈𝒆 𝒐𝒏 𝒃 𝒊𝒔 𝒏𝒐𝒕 𝒐𝒃𝒔𝒆𝒓𝒗𝒂𝒃𝒍𝒆
  25. 25. NEW TRENDS OF CLOCK GATING  Based on the multi level Boolean logic derivations  There are two ways of clock gating to derivate new enable based on the input and output logics.  Stability Condition (STC)  Stability condition is defined with the stability of the input to the flop when upstream flop is stable, no new data or changes come to the downstream flop  Observability Don’t Care (ODC)  There are Situations where the output of the flop is changing or staying constant, but that output is not used in the downstream and read only for a certain time period of time
  26. 26. STABILITY CONDITION (STC)  Stability condition is defined as stability of the input to the flop when upstream flop is stable, no new data or changes come to the downstream flop.  If the input to the flop is not changing with the (Stable) for a period of time, there is no use of toggling the flop for state changes.  In such situation input to the flop is just remain constant thus output of the flop also stable without changing.  Then we can stop providing clock to the flop and save more power EN1 Upstream register Downstream register
  28. 28. OBSERVABILITY DON’T CARE (ODC)  There are Situations where the output of the flop is changing or staying constant, but that output is not used in the downstream and read only for a certain time period of time.  Then toggling and state changes of the flop for entire time period is not required.  Therefore we can shut down that flop for a relevant time period where the output of the flop will not be read and unnecessary.  And we can reactivate the flop when someone is actually reading its output. 0 1 Q
  29. 29. OBSERVABILITY DON’T CARE (ODC)  Before ODC  After ODC SEL 0 1 Q X CLK SEL 0 1 Q X
  30. 30. CLOCK GATING EFFICIENCY & ENABLE STRENGTHENING  Most of the devices have explicit or already instantiated clock enables in the digital designs according to records advanced SOC designs such as mobile application units is recommended to have around 90% of clock gating cross designs.  Although the digital designs consist of explicit or instantiated clock enables, all of them are not efficient and provided an efficient clock gating.  Therefore modern approaches are focusing on finding a new enable which strengthen the existing enable.  This process and new enable are often known as Enable Strengthening and the Strengthened Enable respectively.  Basis behind this approach is to strengthen the existing one with new one, if the percentage of power reduction through the new enable surpasses the existing enable.
  31. 31. ENABLE STRENGTHENING  There are two types of strengthening methodologies based on the logic they are acquired.  Strong STC  In a gated flop, if the input is not changing for a period of time and the flop is still clocking or toggling then we can find out a condition for causing input to be stable. We can use this new logic to strengthen the existing enable.  Strong ODC  In a gated flop, if the output is not read for a period of time but the flop is still clocking, we can find out the conditions for output not t be observed. Then we can enable the existing enable with this new logic. This is known as strong ODC.
  32. 32. MEMORY POWER REDUCTION Most off the digital systems are associated with memory systems. There are different techniques for memory power reduction.  Remove redundant read  Remove redundant write  Memory as steering point for register power reduction  Light sleep power reduction
  33. 33. REDUNDANT READ REMOVAL  Any read access occurring when the memory output is not observable is a redundant read and can be removed based on the ODC technique.  And also if the read address is stable then every read after the first one is redundant, if no new address write is taken. This is based on the STC techniques.
  34. 34. REDUNDANT WRITE REMOVAL  If the data and write addresses are stable, then ever write access after the first one is redundant and can be removed
  35. 35. STATIC POWER REDUCTION  In the past few decades dynamic power is the major concern of design engineers due to fastening the system clock and frequency.  But prevailing technology revolution with advanced fabrication techniques with technologies such as photolithography, the device or technology scaling is happening with an exponential growth.  Thus semiconductor devices scale down and leakages are becoming paramount important for the overall power consumption.  Therefore VLSI power architecture predicts that static power (Leakage Power) will become a dominant component of the power architecture and most researches are carrying through to support that concept.  Power gating are effectively mitigating leakage losses and becomes a major static power reduction technique.
  37. 37. POWER GATING  The basic strategy of power gating is to establish two power modes, Active Mode, Low Power Mode and switch between these power modes where necessary  Establishment of two power modes is a pragmatic remedy for accurate switch between these modes at the appropriate time and in the appropriate manner to maximize power saving while minimizing the impact on the performance  Therefore switching and controlling process is also complex  Due to power gating implementations there may be three modes of operations  Active Mode  Sleep (Low power mode)  Wake Up
  38. 38. SLEEP TRANSISTORS  Head Sleep Transistor  Foot Sleep Transistor
  39. 39. Switch Sizing  Smaller Switches: Smaller area, large resistance and good leakage reduction  Bigger Switches: Larger area, smaller resistance and relatively low leakage reduction
  40. 40. Switch Placing Architecture  Switch in Cell: Switch transistor in each standard cell. Area overhead is a disadvantage and physical design easiness of EDA is an advantage
  41. 41. Switch Placing Architecture  Grid of Switches: Switches placed in an array across the power gated block. 3 rails routed through the logic block (Power, GND and Virtual).
  42. 42. Switch Placing Architecture  Ring of Switches: Used primarily for legacy design where the physical design of the block may not be disturbed.
  43. 43. Signal Isolation  Powering Down the region will not result in crowbar current in many inputs of powered up blocks.  None of the floating outputs of the power-down block will result in spurious behavior in the power-up blocks. Clams will add some delays to the propagation paths.
  44. 44. POWER GATING MODES  Fine Grained Power Gating  Process of adding a sleep transistor to every cell is called a fine-grained power gating  Coarse Grained Power Gating  Implementation of grid style sleep transistor, to stack of logic cell, which drive cell locally through shared virtual power network, is known as coarse grain power gating  Ring Based  Power gates (Switches) are places around the perimeter of the module that is being switched off as a ring  Column Based  Power gates are inserted within the module with the cells abutted to each other in the form of columns
  45. 45. CONTROLLING MECHANISM  Non-State Preserving Power Gating  Cut-off (CO)  Multi-Threshold (MTCMOS  Boosted-Gate (BGMOS)  Super Cut-off (SCCMOS)  State Preserving Power Gating  Variable Threshold (VTMOS)  Zigzag Cut Off (ZZCO)  Zero Delay Ripple Turn On (ZZRTO)  State Preserving use some retention registers to store states.
  46. 46. State Retention Techniques  When power gating taking place we have to retain some critical state content (FSM State)  Software Based Register Read and Write  Scan Based approach based on using scan chains to store state off chip  Retention Registers
  47. 47. Retention Registers  When power gating taking place we have to retain some critical register content (FSM State).  Saving and restoring state quickly and efficiently is the faster and power efficient method to get the block fully functional after power up.  There can be various methods for state retention.  DSP Unit: data flow driven DSP unit can start from reset on new data input.  Cache Processor: This mechanism is good for large residual state retention.
  48. 48. SYNCHRONOUS & ASYNCRONOUS LOGIC POWER GATING  Clock gating for dynamic power reduction which reduce the power consumption of idle section of synchronous circuits  Asynchronous circuits has a inherent strength of data driven capability and active while performing useful tasks  Asynchronous circuits implement the equivalent of a fine grain power gating network  Power gating can be efficiently implemented in Pipelined flows
  49. 49. EDA Power Gating Designp power gating library cells Determine which blocks to power gate Determine state retention mechanism Determine Rush Current Control Scheme Design power gating controller Power gating aware synthesis Determine floor plan power gating aware placement clock tree synthesis Route Verify virtual rail electrical charateristics verify timing
  50. 50. POWER VERIFICATION  Power verification process in the EDA is consisting of the steps of analyzing, monitoring and validating power rules related to EDA tool.  Each and every power estimation and reduction rule and algorithms should be test and check against digital cores.  It is essential to have verification process in the EDA development cycle e to ensure that the software infrastructure is working properly for electronic prototyping.  Perl, C++ is used to write scripts  HDL (Verilog and Vhdl) is used to generate test cases
  51. 51. VERIFICATION OF POWER MONITORS  Power monitors where all mathematics and physics come to engineering  A and B nets have simulation data  C does not have a simulation data A B C AND A B C
  52. 52. POWER MONITORS  According to the mathematical formulas,  If A, B are two independent event,  If those events are independent  In the Above example,  But in actual scenario
  53. 53. POWER MONITOR  Some Correlative or Partitioning approach need  Power Monitor Solution  Divide the simulation time into the fastest clock slots. And find the probability for each and every portion and integrate them together. A B Slot
  54. 54. FUTURE TRENDS OF POWER & EDA  Physical Aware Power Reduction and Fix  Leakage Reduction with advanced power gating  Asynchronous Pipelined micro-architecture  Neural Network Approach for Design Automation  Neuro-Synaptic Computing
  55. 55. THANK YOU For more information visit