Question 16.13: Describe generic array logic (GAL) and erasable programmable......
Describe generic array logic (GAL) and erasable programmable logic devices (EPLDs).
Because of their fuse-based construction, PALs can only be programmed once. They are therefore described as one time programmable (OTP) parts. The generic-ar-
ray logic (GAL) devices are pin compatible with conventional PALs but use electrically erasable and programmable read only memory (EEPROM) technology in place of fuses to achieve reprogramability. GAL
devices are intended as pin-for-pin replacements for a wide variety of PAL devices. The GAL 16 \mathrm{~V} 8 replaces most 20-pin PAL devices, while the 20V8 replaces most 24-pin PAL devices. Early GALs were much slower than PALs but more recent devices have speeds comparable with fuse-based devices.
Other reprogrammable PAL-like devices include erasable PLDs (EPLDs). These are similar to PALs, but use erasable and programmable read only memory (EPROM) techniques in place of fuses. This allow devices to be erased by exposure to ultraviolet light. Once erased they can then be reprogrammed. EPLDs generally offer more facilities than PALs and are more flexible, However they are somewhat slower than PALs with typical delay times of between 10 and 20 \mathrm{~ns}.