Lattice GAL20V8B-25LPI: Architecture, Features, and Key Applications

The Lattice GAL20V8B-25LJPI 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 pin-compatible replacement for a wide array of PAL devices, revolutionizing digital design in the late 1980s and 1990s. Its enduring legacy makes it a subject of study and, in some cases, continued use today.

Architecture: A Look Inside

The GAL20V8B's architecture is a masterpiece of structured programmability. The "20" denotes the number of inputs, and the "8" signifies the number of outputs. Its core structure consists of:

Programmable AND Array: This is the core logic engine. It accepts inputs from the 10 dedicated input pins and 10 output pins configured as inputs (via feedback), creating a product term for each possible combination of true and complemented input signals.

Fixed OR Array: Unlike its PAL predecessors, which had fixed AND arrays and programmable OR arrays, the GAL ingeniously inverts this. The OR array is fixed, and each of the eight output logic macrocells (OLMCs) is fed by a fixed set of product terms from the programmable AND array.

Output Logic Macrocell (OLMC): This is the key to the device's flexibility. Each of the eight outputs is controlled by a sophisticated OLMC that can be configured through programming to operate in several modes:

Combinational Logic: Dedicated input mode or combinational output mode.

Registered Logic: Outputs can be clocked through a D-type flip-flop, enabling the implementation of state machines and sequential logic.

Bi-directional I/O: Pins can be configured as inputs or outputs.

This OLMC configurability, controlled by a global architecture word, is what allowed a single GAL20V8B to replace dozens of fixed-function PALs.

Salient Features

The GAL20V8B-25LJPI, specifically, boasts a set of features that made it an industry workhorse:

High Performance: The `-25` suffix indicates a maximum propagation delay of 25 ns, which was exceptionally fast for its era, enabling its use in high-speed control applications.

Electrically Erasable (EE) CMOS Technology: Unlike one-time programmable (OTP) bipolar PALs, the GAL20V8B uses EECMOS technology. This allows it to be reprogrammed and reused countless times, drastically accelerating design prototyping, debugging, and revisions.

Low Power Consumption: The CMOS design consumes significantly less power than bipolar alternatives, making it suitable for a broader range of applications.

100% Testability: The architecture supports functional testing, ensuring high production yields and reliable operation.

Security Fuse: A programmable security bit prevents the copying or reverse-engineering of the programmed logic pattern, protecting intellectual property.

Key Applications

The flexibility of the GAL20V8B made it a universal solution for "glue logic" and system integration in countless digital systems. Its primary applications included:

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

State Machine Design: Its registered outputs were perfect for implementing finite state machines (FSMs) for system control, bus interfacing, and sequence generation.

Bus Interface and Control: It served as an interface between components with different timing requirements, functioning as a bus arbitrator, wait-state generator, or peripheral controller.

Code Converters and Shifters: It efficiently implemented simple combinational functions like binary-to-Gray code conversion or bit-wise operations.

Pin-to-Pin Replacement: Its primary role was to serve as a single, universal replacement for a vast portfolio of simpler, fixed-function PAL devices, simplifying inventory and design.

ICGOODFIND: The Lattice GAL20V8B-25LJPI is more than just a historical component; it is a foundational pillar of modern programmable logic. Its introduction of a reprogrammable, flexible macrocell architecture paved the way for the vast CPLD and FPGA families we use today. For engineers, understanding its operation provides fundamental insights into the core principles of digital logic design, from state machines to bus interfacing. While largely superseded by more advanced and dense devices, its design philosophy remains highly relevant.

Keywords: Programmable Logic Device, Output Logic Macrocell, Generic Array Logic, Address Decoding, Finite State Machine