The document discusses digital logic design, focusing on programmable logic devices (PLDs), which are circuits that can be programmed to create custom logic functions. It details types of PLDs including simple programmable logic devices (SPLDs), complex programmable logic devices (CPLDs), and field-programmable gate arrays (FPGAs), highlighting their differing capabilities and applications. Additionally, it describes various programming methods and hardware description languages (HDLs) used for developing logic circuits within these devices.

1. DIGITAL LOGIC DESIGN
SIII EE
INTRODUCTION TO PLDs

2. INTRODUCTION TO PLDs
• In digital electronics, ICs are divided into two
categories;
• Fixed Function Logic ICs
• Programmable Logic Devices

3. INTRODUCTION TO PLDs
• Fixed function logic ICs contain fixed logic
circuits which are neither created by user nor
can be changed by them.
• For eg ICs of Gates, Flip Flops, Full Adders etc
contain fixed circuits, which are not alterable
by user

4. Programmable Logic Devices
• ‘PLDs’ stand for Programmable Logic Devices
• PLDs are logic ICs that are used to build
customized logic circuits
• Each PLD contains some amount of logic
components like
• Arrays of Gates (OR, AND & NOT Gates etc)
and arrays of Flip Flops etc

5. Programmable Logic Devices
• Or PLDs contain ‘macro cells’, which contain these
logic components and other etc
• Based on available logic components in a PLD user can
create/develop logic circuits as desired
• Creation/development of logic circuits in a PLD is done
by programming the PLD
• A PLD is a combination of a logic device and a memory
device. The memory is used to store the pattern that
was given to the chip during programming

6. Programmable Logic Devices
• Categories/Types of PLDs;
• PLDs are divided into different categories, as
mentioned below;
• SPLDs
• CPLDs
• FPGA

7. Programmable Logic Devices
• SPLDs = Simple Programmable Logic Devices
• CPLDs = Complex Programmable Logic Devices
• FPGA = Field Programmable Gate Arrays

8. Programmable Logic Devices
• SPLDs (Simple Programmable Logic Devices)
are further divided into following types
• PAL
• PLA
• GAL

9. Programmable Logic Devices
• Types of SPLDs;
• PAL = Programmable Array Logic
• PLA = Programmable Logic Array
• GAL = Generic Array Logic

10. Types of SPLDs
• PLA has a programmable AND gate array, which links to a
programmable OR gate array, which can then be
conditionally complemented to produce an output.
• PAL devices have arrays arranged in a "fixed-OR and
“programmable-AND" array used to implement logic
equations
• GAL This device has the same logical properties as the
PAL but can be erased and reprogrammed. The GAL is
very useful in the prototyping stage of a design, when
any bugs in the logic can be corrected by reprogramming.

11. SPLDs & CPLDs
• PALs and GALs are available only in small sizes,
equivalent to a few hundred logic gates.
• CPLDs or Complex PLDs can be used for bigger
logic circuits,. These contain the equivalent of
several PALs linked by programmable
interconnections, all in one I.C.
• CPLDs can replace thousands, or even
hundreds of thousands, of logic gates.

12. CPLDs & FPGAs
• FPGAs use a grid of logic gates, and once stored, the data doesn't
change, similar to that of an ordinary gate array. The term "field-
programmable" means the device is programmed by the
customer, not the manufacturer.
• The difference between FPGAs and CPLDs is that FPGAs are
internally based on ‘Look Up Tables’(LUTs) whereas CPLDs form
the logic functions with sea-of-gates (e.g. SOP).
• CPLDs are meant for simpler designs while FPGAs are meant for
more complex designs. In general, CPLDs are a good choice for
wide combinational logic applications, whereas FPGAs are more
suitable for large state machines such as microprocessors.

13. FPGAs
• What are FPGAs?
• FPGA is a single chip, comprised of thousands
of units called logic blocks, that are linked with
programmable interconnects. The FPGA’s circuit
is made by connecting several configurable
blocks, and it has a rigid internal structure.

14. Programmable Logic Devices
• By programming PLDs, desired logic circuit is created
inside the PLD
• PLDs can be programmed by using different methods
like;
1. Text Based Entry
2. Graphical Entry
Many PAL programming devices accept input in a
standard file format, commonly referred to as ‘JEDEC
files'. They are analogous to software compilers.

15. Programmable Logic Devices
• Languages used to program PLDs are called as “HDLs”
• HDL stands for ‘Hardware Description Language’
• Popular and commonly used HDLs are;
• PALASM, ABEL and CUPL are frequently used for low-
complexity devices
• VHDL, Verilog are popular higher-level description
languages for more complex devices
[Ref: wikipedia]