Lattice GAL16V8D-25QJN: Architecture, Key Features, and Application Design Considerations

The Lattice GAL16V8D-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 revolutionary, erasable alternative to one-time programmable PAL devices, cementing its role in countless electronic systems from the late 1980s onwards. This article delves into its internal architecture, highlights its key specifications, and outlines critical considerations for modern design and emulation.

Architecture: A Look Inside

The GAL16V8D-25QJN's architecture is a masterpiece of structured programmability. Its name reveals its core structure: a maximum of 16 inputs and 8 outputs. The internal logic is centered around a programmable AND array that feeds into fixed OR terms. The outputs are configured through sophisticated Output Logic Macro Cells (OLMCs), which are the heart of its flexibility.

Each OLMC can be configured by the user into multiple operational modes, including:

Combinational Output: A simple, clock-independent output from the AND-OR array.

Registered Output: Where the output is stored in a D-type flip-flop synchronized to a clock signal.

Combinational I/O: A pin that can function as either an input or an output based on the logic state.

This programmable cell architecture allows the single GAL16V8D device to emulate a wide range of fixed PAL device configurations, reducing inventory needs and increasing design agility.

Key Features and Specifications

The "D" in its part number signifies a commercial-temperature-range device in a PLCC-20 package, while the "-25" denotes a maximum propagation delay (tPD) of 25 ns, making it a standard-speed part for its era. Its key features include:

High-Speed Electrically Erasable (E²) CMOS Technology: Allows for rapid reprogramming and design iteration, a significant advantage over fuse-based PALs.

8 Fully Configurable OLMCs: As described, enabling complex state machine and combinatorial logic design.

UltraMOS® Advanced CMOS Manufacturing: Provides high reliability and low power consumption.

100% Testability: With built-in logic for programming verification and functional testing.

25 ns Maximum Propagation Delay: Offering a clock frequency of up to 40 MHz.

Critical Application Design Considerations

While largely supplanted by more advanced CPLDs and FPGAs today, the GAL16V8D-25QJN remains relevant for legacy system support, educational purposes, and simple glue logic applications. Key design considerations include:

1. Power-On Reset and State Machine Design: The device features a power-on reset that initializes all registered outputs to a logic high. This is a critical consideration for designing safe and predictable state machines, ensuring the system starts in a known, valid state.

2. Security Fuse: The device includes a programmable security fuse. Once blown, it prevents the programmed pattern from being read back, protecting intellectual property. However, it does not disable verification, so the device can still be checked against the original JEDEC file.

3. Input/Output Loading: Designers must account for the input capacitance and output drive capabilities, especially when interfacing with other legacy components or driving buses.

4. Modern Tooling: Designing for this device requires legacy software tools like CUPL or WinCUPL to generate the JEDEC fuse map file. Access to a vintage or compatible programmer is also essential.

5. Emulation and Replacement: For new designs, it is almost always preferable to use a modern, in-production CPLD. However, the GAL16V8D is often used to emulate the function of obsolete PALs, serving as a bridge to maintain the functionality of older equipment.

ICGOOODFIND: The Lattice GAL16V8D-25QJN is far more than a historical artifact; it is a foundational pillar of programmable logic. Its elegant and flexible architecture, defined by its programmable AND array and configurable Output Logic Macro Cells (OLMCs), demonstrated the power of reusable logic. While its 25ns speed and limited density are eclipsed by modern devices, understanding its operation is crucial for maintaining legacy systems and appreciating the evolution of digital design. Its enduring legacy is a testament to a brilliantly executed concept in semiconductor technology.

Keywords: Programmable Logic Device (PLD), Output Logic Macro Cell (OLMC), Generic Array Logic (GAL), JEDEC File, Electrically Erasable (E²CMOS)