Lattice GAL20V8B-15LD: Architecture, Features, and Key Applications in Digital Logic Design
The Lattice GAL20V8B-15LD stands as a quintessential example of a Generic Array Logic (GAL) device, a type of Programmable Logic Device (PLD) that played a pivotal role in the evolution of digital logic design. This particular device, with its 15ns maximum propagation delay, offered a powerful blend of speed, flexibility, and cost-effectiveness for a wide range of applications, bridging the gap between simple fixed-function logic and more complex FPGAs.
Architecture: A Look Inside
The architecture of the GAL20V8B is ingeniously designed for maximum programmability. The "20V8" designation reveals its core structure: it has up to 20 inputs and 8 outputs. Its internal logic is based on a Programmable AND array feeding into a fixed OR array. The key to its versatility lies in its Output Logic Macro Cells (OLMCs). Each of the eight outputs is controlled by a dedicated OLMC, which can be configured by the user to operate in various modes—combinatorial, registered, or as a dedicated input. This programmability is achieved through a sophisticated switch matrix and architecture control word, which are defined by the JEDEC fuse map file generated during compilation.
Salient Features and Specifications
The GAL20V8B-15LD is defined by a set of robust features that made it a workhorse for engineers:
High Performance: The -15 suffix denotes a maximum propagation delay of 15 nanoseconds, making it suitable for high-speed logic applications.
Re-programmability: Unlike its predecessor, the PAL, which was one-time programmable (OTP), the GAL20V8B uses an EEPROM (Electrically Erasable Programmable Read-Only Memory) technology. This allows the device to be erased and reprogrammed hundreds of times, significantly accelerating design iteration and prototyping.
Low Power Consumption: Fabricated in CMOS technology, it consumes significantly less power than bipolar PLDs, enhancing its suitability for portable and power-sensitive applications.
Pin-compatibility: It was designed to be a drop-in replacement for a wide range of older PAL devices, allowing for easy upgrades and design migration without board re-layout.
Key Applications in Digital Logic Design
The flexibility of the GAL20V8B-15LD made it indispensable for numerous functions:
Address Decoding: It was extensively used in microprocessor and microcontroller-based systems to generate chip-select signals for memory (RAM, ROM) and peripheral ICs, simplifying complex decoding logic into a single, programmable chip.
State Machine Design: The device's ability to implement registered outputs made it ideal for designing finite state machines (FSMs) for control logic, sequence detection, and system management.
Bus Interface and Control Logic: It served as a glue logic component, interfacing between different digital subsystems with varying protocols by implementing custom data routing, latching, and buffering logic.
Function Emulation: Engineers could use it to emulate the functionality of multiple standard 74-series logic gates, consolidating board space and reducing component count, thus improving system reliability.
The Lattice GAL20V8B-15LD was a cornerstone of digital design, offering a perfect balance of speed, density, and re-programmability. It empowered a generation of engineers to implement complex, custom logic functions efficiently, paving the way for the sophisticated programmable devices we use today.
Keywords:
1. Programmable Logic Device (PLD)
2. Output Logic Macro Cell (OLMC)
3. Re-programmable
4. Address Decoding
5. Finite State Machine (FSM)
