PROGRAMMABLE LOGIC DEVICE (PLD)

PROGRAMMABLE LOGIC DEVICE (PLD)

This is an integrated circuit that is encoded or programmed to perform a set of complex actions. It allows very fast deployment of new designs since only the program has to be changed. PLDs perform device-to-device interfacing, data communication, signal processing, and data display, timing and control operations.

With flash memory being offered, PLDs are increasingly used in the semiconductor industry, complex digital signal processing algorithms and memory controllers, CNC machines, bus interfaces, software-based microprocessors, prototyping, embedded cores, hybrid chips, and reconfigurable computing.

How a PLD works:

PLDs have a set of connected macrocells. These macrocells have Boolean logic equations (AND and OR gates) and a flip-flop. This equation will combine the state of some number of binary inputs into a binary output and store that output in the flip-flop until the next clock edge. After the basic PLD is built, you need to use a separate appliance called a device programmer to embed the PLD with the program.

Types of PLDs:

The two main types are: field programmable gate arrays (FPGAs) and complex programmable logic devices (CPLDs).

CPLDs: multiple PLDs in a single chip, all interfaced with a logic. Used when high performance logic is needed for complex finite state machines. Have hundreds of macrocells.
FPGAs: has three parts: logic blocks, interconnect, and I/O blocks. Logic blocks can have logic gates or a look-up table and a flip-flop. Used in register-heavy and pipelined applications.

Types of devices used in PLDs are:

PROMs (programmable read only memory): Data can be written only once.
EPROM (erasable programmable read-only memory): exposing the chip to ultraviolet rays can erase Data and new program can be written.
PLAs (programmable logic array): This has a programmable AND plane and a fixed OR plane.
PAL/GALs (programmable array logic/generic array logic): These use a combination of different types of arrays.