Lattice GAL20V8B-25LPN: Architecture, Key Features, and Applications in Digital Logic Design

The Lattice GAL20V8B-25LPN stands as a quintessential example of a high-performance, second-generation Generic Array Logic (GAL) device. As a cornerstone of programmable logic design in its era, it provided a flexible and reliable alternative to fixed-function TTL logic and more complex PLDs. Its architecture, speed, and reprogrammability made it a popular choice for a vast array of digital logic applications, from simple glue logic to complex state machines.

Architecture: A Look Inside

The architecture of the GAL20V8B-25LPN is built around a programmable AND array feeding into a fixed OR array, a structure known as a Programmable Logic Device (PLD). The "20V8" designation is key to understanding its structure:

20 Inputs: The device features up to 20 dedicated inputs.

8 Outputs: It provides 8 output logic macro cells (OLMCs), which are the heart of its flexibility.

Programmable AND Array: This is the fusible link matrix that allows designers to create custom product terms by defining the connections between input variables and their complements.

Each of the eight OLMCs can be individually configured by the user to operate in various modes (registered or combinatorial) with programmable polarity. This output logic macro cell (OLMC) architecture allows each pin to be defined as an input, an output, or a bidirectional pin, providing tremendous design flexibility from a single, standard chip.

Key Features and Specifications

The GAL20V8B-25LPN is defined by a set of robust features that cemented its utility:

High Performance: The -25 suffix denotes a maximum pin-to-pin propagation delay of 25 ns, enabling operation at high clock frequencies for its time.

Low Power Consumption: Fabricated in low-power CMOS technology, it offers a significant power reduction compared to bipolar PLDs like PAL devices.

Electrically Erasable: Utilizing E²CMOS (Electrically Erasable CMOS) technology, the device is reprogrammable and reusable. Designs can be easily modified and tested, drastically reducing development time and cost.

100% Testability: The built-in logic functionality ensures 100% programming and functional testability, guaranteeing high production yields and design reliability.

28-Pin PDIP (Plastic Dual In-line Package): The LPN package type indicates a plastic DIP, which is easy to prototype with on breadboards and in test systems.

Applications in Digital Logic Design

The flexibility of the GAL20V8B-25LPN made it suitable for a wide spectrum of digital logic tasks, effectively consolidating multiple simple ICs into a single, programmable chip. Its primary applications include:

Address Decoding: Generating chip select signals for microprocessors and memory systems.

State Machine Control: Implementing finite state machines (FSMs) for controlling complex sequential logic operations.

Glue Logic Integration: Replacing numerous small-scale integration (SSI) and medium-scale integration (MSI) components like decoders, multiplexers, and latches, thereby reducing board space and improving system reliability.

Bus Interface and Control: functioning as bus drivers, translators, and controllers in data path management.

Code Converters and Pattern Generators: Implementing custom logic functions for data encoding, decoding, and signal generation.

ICGOOODFIND: The Lattice GAL20V8B-25LPN is a classic and highly influential PLD that demonstrated the power of reprogrammable logic. Its blend of high-speed performance, CMOS low-power operation, and flexible macrocell architecture made it an indispensable tool for engineers, bridging the gap between discrete logic and more complex FPGAs and CPLDs. It remains a foundational device for understanding the principles of programmable logic.

Keywords:

Programmable Logic Device (PLD)

Output Logic Macro Cell (OLMC)

CMOS Low-Power

Glue Logic

Electrically Erasable