Subject: Digital System Design

Topic: Programmable Logic Devices

Difficulty: High

CPLD — a more Complex PLD that consists of an arrangement of multiple SPLD-like blocks on a single chip. Alternative names (that will not be used in this paper) sometimes adopted for this style of chip are Enhanced PLD (EPLD),

Super PAL, Mega PAL, and others.

A CPLD comprises multiple circuit blocks on a single chip, with internal wiring re­ sources to connect the circuit blocks . Each circuit block is similar to a PLA or a PAL; we will refer to the circuit blocks as PAL-like blocks. An example of a CPLD is given in Figure

It includes four PAL-like blocks that are connected to a set of interconnection wires. Each PAL-like block is also connected to a subcircuit labeled //0 block, which is attached to a number of the chip's input and output pins.

Figure shows an example of the wiring structure and the connections to a PAL-like block in a CPLD. The PAL-like block includes 3 macrocells (real CPLDs typically have about 16 macrocells in a PAL-like block), each consisting of a four-input OR gate (real CPLDs usually provide between 5 and 20 inputs to each OR gate). The OR-gate output is connected to another type of logic gate that we have not yet introduced. It is called an Exclusive-OR (XOR) gate. The behavior of an XOR gate is the same as for an OR gate except that if both of the inputs are 1, the XOR gate produces a 0.

FPGA — a Field-Programmable Gate Array is an FPD featuring a general structure that allows very high logic capacity. Whereas CPLDs feature logic resources with a wide number of inputs (AND planes), FPGAs offer more narrow logic resources. FPGAs also offer a higher ratio of flip-flops to logic resources than do CPLDs.

FPGAs comprise an array of uncommitted circuit elements, called logic blocks, and interconnect resources, but FPGA configuration is performed through programming by the end user. An illustration of a typical FPGA architecture appears in Figure. As the only type of FPD that supports very high logic capacity, FPGAs have been responsible for a major shift in the way digital circuits are designed.