Programming and Applications of the Lattice GAL16V8D-15LJ Generic Array Logic (GAL) Device

The Lattice GAL16V8D-15LJ stands as a seminal device in the history of programmable logic. As a member of the Generic Array Logic (GAL) family, it revolutionized digital circuit design by offering a reprogrammable and cost-effective alternative to fixed-function TTL logic and one-time programmable PAL devices. Its architecture, centered around an output logic macrocell (OLMC), provides a flexible framework for implementing a wide range of combinatorial and sequential logic functions.

Architecture and Programming

The core of the GAL16V8 comprises a programmable AND array feeding into fixed OR arrays. This structure allows designers to create custom sum-of-products logic expressions. The key to its versatility lies in its eight Output Logic Macrocells (OLMCs). Each macrocell can be individually configured by the designer to operate in several modes:

Combinatorial Mode: The output is solely a function of the current input states.

Registered Mode: The output is stored in a D-type flip-flop, synchronously clocked by a dedicated pin, enabling the design of state machines and counters.

Complex Mode: Allows for additional configurations, such as using a pin as either an input or an output.

Programming the GAL16V8D-15LJ is achieved using a JEDEC file, a standard format that contains the fuse map information. This file is generated by Hardware Description Languages (HDLs) like CUPL or Abel, or by schematic capture tools. A dedicated GAL programmer, often connected to a PC, then electrically configures the internal fuses based on this JEDEC file. The -15LJ suffix specifically denotes a 15ns maximum propagation delay and a plastic leaded chip carrier (PLCC) package.

Key Applications

The reprogrammability of the GAL device made it immensely popular for numerous applications:

Address Decoding: In microprocessor systems, it was ideal for generating chip select (CS) and read/write signals by decoding address bus patterns.

State Machine Design: Its registered outputs allowed for the efficient implementation of finite state machines (FSMs) for control logic.

Glue Logic Integration: It served as a "glue logic" consolidator, replacing multiple small- and medium-scale integration (SSI/MSI) ICs (e.g., gates, decoders, flip-flops) into a single, compact chip. This significantly reduced board space, component count, and power consumption.

Interface Logic: It was commonly used to adapt and translate signal levels and protocols between different subsystems, such as between a CPU and its peripheral devices.

Prototyping and Education: Its ease of use and reprogrammability made it a perfect tool for prototyping digital designs and for teaching fundamental logic concepts.

Legacy and Modern Context

While largely superseded by more advanced Complex Programmable Logic Devices (CPLDs) and Field-Programmable Gate Arrays (FPGAs) offering vastly greater capacity and performance, the GAL16V8 remains in use today for simple logic replacement, legacy system maintenance, and educational purposes. Its design principles directly informed the development of modern programmable logic.

ICGOODFIND: The Lattice GAL16V8D-15LJ is a foundational programmable logic device whose innovative output logic macrocell architecture provided unprecedented design flexibility. It was instrumental in consolidating glue logic, enabling more compact and reliable digital systems, and served as a critical bridge between discrete logic and high-density CPLDs/FPGAs.

Keywords:

1. Programmable Logic Device (PLD)

2. Output Logic Macrocell (OLMC)

3. JEDEC File

4. Glue Logic

5. Hardware Description Language (HDL)