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

The Lattice GAL16V8D-25LPN stands as a seminal device in the history of programmable logic. As a member of the Generic Array Logic (GAL) family, it provided a powerful, erasable, and electrically reprogrammable alternative to one-time programmable PAL devices. Its architecture, speed, and flexibility made it a cornerstone for countless glue logic applications in digital systems.

Architecture Overview

The GAL16V8D-25LPN is built around a programmable AND array driving a fixed OR array, a structure known as a PAL-type architecture. The "16V8" designation is key: it has up to 16 inputs and 8 outputs, with the outputs configured through its Output Logic Macro Cells (OLMCs).

Each of the eight OLMCs is the heart of the device's flexibility. An OLMC can be configured by the user to operate as:

A dedicated combinatorial output.

A dedicated input.

A combinatorial output with a feedback path to the array.

A registered output (using a D-type flip-flop) with feedback.

This configuration is determined by programming specific fuse bits that control multiplexers within each OLMC. The device's architecture is managed by a sophisticated Programmable Array Logic (PAL) architecture that allows for the implementation of a wide range of logic functions, from simple Boolean equations to state machines.

Key Features and Specifications

High Performance: The `-25` suffix denotes a maximum propagation delay of 25 ns, making it suitable for high-speed systems of its era.

Low Power: The `L` in the part number signifies a low-power CMOS process, significantly reducing power consumption compared to bipolar PAL devices.

Electrically Erasable: The GAL16V8D uses an EEPROM (E2CMOS) technology base. This allows the device to be reprogrammed and erased electrically, facilitating rapid design iteration and prototyping without requiring UV erasure.

8 Output Logic Macro Cells (OLMCs): Provides immense flexibility for I/O configuration.

100% Testability: The internal logic provides 100% functional testability and ac performance verification.

Package: The `PN` indicates a 20-pin Plastic Dual-In-Line Package (PDIP).

Application Circuit Design Considerations

Designing with the GAL16V8D-25LPN requires careful attention to several practical aspects:

1. Power Supply Decoupling: Like all high-speed CMOS devices, it requires robust power supply decoupling. A 0.1 μF ceramic capacitor should be placed as close as possible to the VCC and GND pins to suppress high-frequency noise and ensure stable operation.

2. Unused Inputs: All unused input pins must be tied to either VCC or GND, never left floating. A floating CMOS input can cause excessive current draw and unpredictable operation.

3. Reset and Preset Initialization: For designs utilizing the registered (flip-flop) configuration, ensure a proper power-on reset circuit is implemented if a specific initial state is required. The GAL's registers may not power up in a deterministic state.

4. Clock Routing: The clock signal for any registered functions (typically pin 1) must be treated as a critical high-speed signal. Keep clock traces short and away from other switching signals to minimize noise and skew.

5. Output Loading: While the outputs can source 24 mA and sink 16 mA, heavily loading multiple outputs simultaneously can cause ground bounce and VCC sag, leading to noise-related errors. Calculate total power budget and switching currents carefully.

6. Programming and Security: The device features a security fuse that, once programmed, prevents the programmed pattern from being read back, protecting intellectual property. However, the device can still be erased entirely for reprogramming.

ICGOOODFIND

The Lattice GAL16V8D-25LPN is more than a historical component; it is an icon that democratized programmable logic design. Its EEPROM-based reprogrammability, flexible OLMC architecture, and CMOS efficiency set a new standard, bridging the gap between fixed TTL logic and more complex FPGAs. For engineers, it represented a perfect blend of speed, low power, and design agility for implementing custom "glue logic," making it an indispensable tool in 80s and 90s digital system design.

Keywords:

Programmable Logic Device (PLD)

Output Logic Macro Cell (OLMC)

EEPROM (E2CMOS)

Propagation Delay

Glue Logic