Digital Systems

Chapter 21

CPLDs and FPGAs

OBJECTIVE

We will learn in this chapter about CPLDs and FPGAs. These are advanced high-density field-programmable devices.

21.1 CPLDS

The PROM, PLA and PAL (PLDs) are programmable logic devices for combinational circuits. Following are the needs observed when designing the various application circuits:

1. The output from the OR gate(s) is sometimes needed in complement form.
2. Sometimes an output is needed through a tristate or tristate NOT form also. [Figure 21.1(a)]. Tristate output (s) enables the output availability on a common bus from several sources.
3. Sometimes an output with or without complementing (or both) is needed to be feedback to an input of the same stage to a previous stage or next stage. [Refer lower ...