Programmable Logic Devices (PLDs)
Programmable Logic Devices (PLDs) are digital electronic components used to build
reconfigurable logic circuits. Unlike fixed-function logic devices (such as ASICs), PLDs allow
users to program their logic functions after manufacturing, making them highly flexible for
various applications.
Types of PLDs
PLDs are classified based on their complexity and programmability:
1. Simple Programmable Logic Devices (SPLDs)
These are the most basic PLDs, primarily used for small-scale logic functions.
Programmable Logic Array (PLA)
o Consists of a programmable AND array and a programmable OR array.
o Allows implementation of any combinational logic function.
o More flexible than a PROM but less commonly used due to high cost.
Programmable Array Logic (PAL)
o Has a fixed OR array but a programmable AND array.
o Faster and simpler than PLAs.
o More widely used in early digital design applications.
Generic Array Logic (GAL)
o Improved version of PALs with reprogrammability.
o Uses EEPROM-based memory, making it more versatile.
2. Complex Programmable Logic Devices (CPLDs)
CPLDs consist of multiple interconnected SPLD blocks.
Contain macrocells, which include logic gates and flip-flops for sequential and combinational
logic.
Offer moderate complexity and are used in applications like address decoding and control
circuits.
Example: Xilinx XC9500 series.
FPGA (Field-Programmable Gate Array) and Its Structures
Introduction to FPGA
An FPGA (Field-Programmable Gate Array) is a reconfigurable integrated circuit that allows
users to program and reprogram it to perform different logical operations. Unlike traditional
ASICs (Application-Specific Integrated Circuits), which have fixed functionality, FPGAs
provide flexibility by allowing the user to configure the logic after manufacturing.
FPGAs are widely used in applications such as digital signal processing, telecommunications,
embedded systems, and AI/ML acceleration.
FPGA Architecture and Structure
FPGAs consist of a combination of configurable logic blocks, interconnects, and I/O blocks. The
primary structures in an FPGA include:
1. Configurable Logic Blocks (CLBs)
CLBs contain logic elements (LEs) that implement combinational and sequential logic
functions.
Each CLB typically consists of:
o Lookup Tables (LUTs): Used for implementing logic functions.
o Flip-Flops (FFs): Used for storing state information.
o Multiplexers (MUXs): Used for signal routing.
2. Interconnect Matrix (Routing Resources)
The interconnect fabric connects different CLBs to form complex circuits.
Routing resources include:
o Global Interconnects: Connect distant CLBs.
o Local Interconnects: Connect adjacent logic blocks.
o Switch Matrices: Allow dynamic routing of signals.
3. Input/Output Blocks (I/O Blocks)
These blocks provide interfaces for external communication.
They support various voltage standards and protocols (e.g., LVTTL, LVCMOS, PCIe).
Some advanced FPGAs include high-speed serial transceivers for high-bandwidth
applications.
4. Embedded Memory Blocks
FPGAs contain embedded memory such as:
o Block RAM (BRAM): Used for storing data during processing.
o Distributed RAM: Small memory within logic blocks for fast access.
o FIFO Buffers: Used for data queuing.
5. Digital Signal Processing (DSP) Blocks
Some FPGAs include dedicated DSP slices for:
o Multiplication and Accumulation (MAC) operations.
o Fast Fourier Transform (FFT) and signal processing.
These blocks improve performance for signal processing and machine learning
applications.
6. Clock Management (PLL/DLL)
Phase-Locked Loops (PLLs) and Delay-Locked Loops (DLLs) manage the clock signals.
These help in:
o Clock frequency synthesis.
o Jitter reduction.
o Phase alignment of signals.
7. Hard IP Cores and Soft IP Cores
Hard IP Cores: Predefined circuits (e.g., PCIe, DDR controllers, Ethernet controllers).
Soft IP Cores: Customizable blocks implemented using FPGA logic (e.g., custom
processors).
Types of FPGA Architectures
1. Fine-Grained FPGA
o Consists of simple logic elements (LUTs, FFs).
o Provides high flexibility but increases routing complexity.
2. Coarse-Grained FPGA
o Uses larger logic blocks (e.g., ALUs, multipliers).
o Suitable for DSP and AI applications.
3. Heterogeneous FPGA
o Combines standard FPGA logic with hard IP cores.
o Examples: Xilinx Zynq (FPGA + ARM processor), Intel Stratix (FPGA + high-
speed transceivers).
Memory Elements in Digital Systems
Memory elements are essential components in digital circuits used to store and retain data. These
elements can be classified based on their functionality, storage duration, and structure.
Types of Memory Elements
1. Sequential Memory Elements
Sequential memory elements store data based on clock signals and are commonly used in
registers, flip-flops, and latches.
a) Latches
Level-triggered storage elements that hold data as long as the enable signal is active.
Types of latches:
o SR Latch (Set-Reset Latch): Uses NOR/NAND gates to store a single bit.
o D Latch (Data Latch): Ensures no invalid state by having only one input (D).
b) Flip-Flops
Edge-triggered memory elements that store data on clock transitions.
Types of flip-flops:
o SR Flip-Flop: Controlled version of the SR latch with clocking.
o D Flip-Flop: Stores the value of input D at each clock pulse.
o JK Flip-Flop: Modified SR flip-flop that removes the invalid state.
o T Flip-Flop: Toggles its state on each clock pulse.
2. Random-Access Memory (RAM)
RAM is volatile memory that allows both read and write operations.
a) Static RAM (SRAM)
Stores data using flip-flops.
Faster and requires more power than DRAM.
Used in caches and high-speed memory applications.
b) Dynamic RAM (DRAM)
Stores data in capacitors and requires periodic refreshing.
More compact and cheaper than SRAM.
Used as main memory in computers.
3. Read-Only Memory (ROM)
ROM is non-volatile memory used for permanent data storage.
Types of ROM:
Mask ROM: Hardwired at the manufacturing stage.
PROM (Programmable ROM): Can be programmed once.
EPROM (Erasable PROM): Can be erased using UV light.
EEPROM (Electrically Erasable PROM): Can be erased and reprogrammed electronically.
Flash Memory: A type of EEPROM used in SSDs and USB drives.
4. Other Memory Elements
Register Files: Small memory units used in CPUs for temporary storage.
Cache Memory: High-speed memory located between RAM and the processor.
Shift Registers: Stores and shifts data in a sequential manner.
FIFO and LIFO Buffers: Used for data queuing and stack operations.
