Lattice GAL20V8B-25QJN: Architecture, Key Features, and Application Design Considerations

The Lattice GAL20V8B-25QJN stands as a classic and highly influential device in the realm of programmable logic. As a member of the Generic Array Logic (GAL) family, it provided a powerful, erasable, and CMOS-based alternative to fixed-function TTL logic and one-time-programmable PAL devices. Its architecture, a precursor to more complex CPLDs and FPGAs, remains relevant for understanding fundamental programmable logic concepts and for maintaining legacy electronic systems.

Architecture: A Look Inside

The GAL20V8B's architecture is elegantly structured around a programmable AND array feeding into fixed OR terms and configurable output logic macrocells (OLMCs). The "20" in its name denotes the number of inputs, while the "V8" indicates eight versatile output pins.

At its core, the device features a programmable AND array, which generates product terms from the input signals. This array is the heart of the device's logic functionality, allowing designers to create custom sum-of-products logic functions. The array's outputs are then routed to the Output Logic Macrocell (OLMC). Each of the eight OLMCs is a key architectural feature, providing tremendous flexibility. Each macrocell can be configured by the designer to operate as a combinatorial output, a registered (clocked) output, or even a dedicated input. The registers are positive-edge-triggered D-type flip-flops, essential for implementing synchronous state machines and counters. This configurability is what made the GAL20V8 a universal replacement for dozens of fixed-function logic parts.

Key Features and Specifications

The GAL20V8B-25QJN encapsulates several critical features that defined its success:

High-Performance CMOS Technology: This provided a significant advantage in low power consumption compared to bipolar alternatives, a crucial consideration for many applications.

Electrically Erasable (E²) Cells: The technology utilizes E²CMOS, making the device reprogrammable and reusable. This was a major improvement over fusable-link PALs, allowing for rapid design iteration and bug fixes.

25ns Maximum Propagation Delay (tPD): The "-25" suffix indicates a maximum propagation delay of 25 nanoseconds, allowing for clock speeds upwards of 40 MHz, which was sufficient for a vast array of microprocessor interfacing and glue logic applications.

100% Testability: The architecture supports full functional testability, ensuring high manufacturing yields and design reliability.

28-Pin PLCC Package (QJN): The J-lead plastic chip carrier package was a common and reliable surface-mount format for the era.

Application Design Considerations

Designing with the GAL20V8B requires careful consideration of its capabilities and limitations.

1. Logic Density: The device has a finite number of product terms per output. Complex functions requiring many AND terms might need to be simplified or distributed across multiple outputs. Careful logic minimization is essential.

2. Registered vs. Combinatorial: Plan the use of each pin meticulously. Pins configured as registered outputs utilize the internal flip-flop and require a clock signal, which is shared globally across all macrocells.

3. Timing Analysis: Designers must account for setup and hold times for registered configurations and the combinatorial propagation delays. The 25ns rating sets the ultimate speed limit for the design.

4. Power-On Reset: The internal registers feature a power-on reset, ensuring the device starts in a known state, which is critical for state machine design.

5. Legacy System Support: Today, its primary application is in retrofitting and maintaining legacy industrial and military systems where a direct, pin-compatible replacement is necessary for long-term sustainability.

In summary, the Lattice GAL20V8B-25QJN was a cornerstone of digital design, offering a unique blend of flexibility, low power, and reprogrammability. Its macrocell-based architecture became a template for future programmable logic devices.

ICGOODFIND: The Lattice GAL20V8B-25QJN is a quintessential programmable logic device that revolutionized digital design by offering an erasable, low-power, and flexible solution for implementing glue logic, state machines, and address decoding. Its macrocell-based architecture remains a fundamental concept for understanding CPLD and FPGA operation.

Keywords:

Programmable Logic Device (PLD)

Output Logic Macrocell (OLMC)

E²CMOS

Glue Logic

Sum-of-Products