Lattice GAL20V8B-15LJN: Architecture, Features, and Key Applications in Digital Logic Design

The Lattice GAL20V8B-15LJN stands as a classic and highly influential device in the history of programmable logic. As a member of the Generic Array Logic (GAL) family, it provided a powerful, erasable, and reconfigurable alternative to fixed-function TTL logic and one-time programmable PAL devices. Its architecture and flexibility made it a cornerstone for prototyping and implementing complex digital logic designs throughout the 1980s and 1990s.

Architecture: A Blend of Simplicity and Power

The architecture of the GAL20V8B is elegantly structured around a programmable AND array feeding into fixed OR terms and sophisticated output logic macrocells (OLMCs). The "20" in its name denotes the number of inputs, while the "8" refers to the number of outputs.

At its core, the device features a programmable AND array, which generates product terms (the fundamental building blocks of sum-of-products logic functions). This array is common to all outputs. The key to its versatility lies in its Output Logic Macrocell (OLMC). Each of the eight outputs is controlled by its own OLMC, which can be configured by the designer to operate in several modes:

Combinational Output: The output is a direct function of the AND-OR array.

Registered Output: The output is stored in a D-type flip-flop, synchronizing the output to a clock signal, which is essential for implementing state machines and counters.

Combinational I/O: The pin can act as an input or an output based on the logic function.

This reconfigurability of the OLMC allows a single GAL20V8B to replace a wide variety of standard logic parts, consolidating complex circuitry into a single, integrated chip.

Key Features and Specifications

The GAL20V8B-15LJN boasts several defining features that contributed to its widespread adoption:

High Performance: The `-15` suffix indicates a maximum propagation delay of 15 nanoseconds, making it suitable for high-speed logic applications.

Electrically Erasable (EE) CMOS Technology: Unlike its fusable-link predecessors, the GAL20V8B uses EECMOS cells for its programmable array. This allows it to be reprogrammed and erased up to 100 times, facilitating rapid design iteration and debugging.

Low Power Consumption: Built on CMOS technology, it consumes significantly less power than bipolar PLDs, making it suitable for power-sensitive applications.

100% Testability: The built-in logic and register functionality made designs easier to test and verify compared to discrete logic.

28-Pin PLCC Package (LJN): The Plastic Leaded Chip Carrier package was a common and robust surface-mount package for the era.

Key Applications in Digital Logic Design

The GAL20V8B found its way into countless digital systems, serving as a universal "glue logic" component. Its primary applications included:

Address Decoding: In microprocessor-based systems, it was extensively used to decode memory addresses for RAM, ROM, and peripheral chips, generating precise chip select (CS) signals.

State Machine Implementation: Its registered outputs were perfect for implementing finite state machines (FSMs) for control logic, sequence detection, and system management.

Bus Interface and Control: It was ideal for generating complex timing and control signals for data buses, such as read/write strobes and interrupt control logic.

Code Conversion and Data Gating: It could easily perform tasks like converting between binary and Gray code or acting as a multiplexer/demultiplexer.

Function Emulation: A single GAL could emulate the functionality of multiple small- and medium-scale integration (SSI/MSI) chips, drastically reducing board space, part count, and cost.

ICGOODFIND Summary

The Lattice GAL20V8B-15LJN was a revolutionary device that democratized programmable logic. Its blend of a reprogrammable AND array, flexible output macrocells, and high speed made it an indispensable tool for engineers. It effectively bridged the gap between simple PLDs and more complex FPGAs/CPLDs, serving as the perfect solution for integrating "glue logic," implementing state machines, and simplifying system design. While largely superseded by more advanced devices, its architectural principles remain foundational.

Keywords:

1. Programmable Logic Device (PLD)

2. Output Logic Macrocell (OLMC)

3. Generic Array Logic (GAL)

4. Glue Logic

5. Finite State Machine (FSM)