1.4 PALs

PALs are Programmable Array Logic devices. The internal architecture consists of programmable AND terms feeding fixed OR terms. All inputs to the array can be ANDed together, but specific AND terms are dedicated to specific OR terms. PALs have a very popular architecture and are possibly the most widely used type of user-programmable device. If a device contains macrocells, it will usually have a PAL architecture. Typical macrocells may be programmed as inputs, outputs, or input/output (I/O) using a tri-state enable. They normally have output registers that may or may not be used in conjunction with the associated I/O pin. Other macrocells have more than one register, various types of feedback into the arrays, and occasionally feedback between macrocells. These devices are mainly used to replace multiple discrete logic functions, commonly referred to as glue logic.