Lattice GAL20V8C-5LJ: Architecture, Key Features, and Application Design Considerations
The Lattice GAL20V8C-5LJ 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 pin-compatible alternative to standard PAL devices, revolutionizing digital design in the late 1980s and 1990s. Its enduring legacy makes understanding its architecture and applications valuable for both historical context and maintaining legacy systems.
Architecture: A Look Inside
The GAL20V8C-5LJ's architecture is a masterpiece of structured programmability. The "20" indicates it has up to 20 inputs, and the "8" denotes up to 8 outputs. Its core consists of a programmable AND array that feeds into a fixed OR array. The key to its flexibility lies in the Output Logic Macrocell (OLMC). Each of the eight outputs is driven by a dedicated OLMC, which can be configured by the user to operate in various modes:
Registered Mode: The output is clocked through a D-type flip-flop, enabling synchronous state machine design.
Combinatorial Mode: The output is a direct function of the input product terms, ideal for pure logic operations.
Complex Mode: Allows for configured inputs and outputs, providing maximum flexibility for pin usage.
This macrocells are the heart of the device, allowing it to emulate a wide range of PAL architectures with a single, universal part.
Key Features and Specifications
The "C" in its part number signifies CMOS technology, offering significant power savings over its bipolar predecessors. The "-5" denotes a maximum pin-to-pin propagation delay of 5ns, making it suitable for moderately high-speed applications. Key features include:
Electrically Erasable (EECMOS) Technology: Unlike one-time programmable (OTP) PALs, the GAL20V8C-5LJ can be reprogrammed and tested repeatedly, drastically accelerating development cycles.
100% Pin-out and Function Compatibility: It was designed as a drop-in replacement for many common 24-pin PAL devices, allowing for easy design upgrades.
UltraMOS® Advanced CMOS Technology: Provides high speed while maintaining low power consumption.
8 Output Logic Macrocells (OLMCs): Provide unparalleled functional flexibility for a device of its size.
Application Design Considerations
Designing with the GAL20V8C-5LJ requires a specific workflow centered on Hardware Description Languages (HDLs) or schematic entry.
1. Design Entry: Engineers would traditionally capture logic functions using Boolean equations, state diagrams, or schematics within a PLD development tool.
2. Logic Synthesis & Fitting: The software compiler (e.g., CUPL, Abel) synthesizes the design, maps it to the GAL's AND-OR structure, and configures the OLMCs appropriately.
3. JEDEC File Generation: The output is a JEDEC file (a standard format) containing the fuse map data for programming.
4. Programming: A dedicated device programmer is used to electrically configure the GAL20V8C-5LJ with the JEDEC file.
Critical considerations for a successful design include:
Power-On Reset: The device features a power-on reset circuit that ensures all registers are in a known state upon startup, a crucial feature for state machine stability.
Product Term Limitations: Each output has a limited number of product terms available. Complex logic functions might require clever minimization or splitting across multiple macrocells.
Security Fuse: A programmable security fuse protects the intellectual property within the device by preventing the reading back of the programmed pattern.
Clock and Pin Limitations: The clock signal is typically routed to a specific pin, and the three pins dedicated to programming (SDIN, SCLK, /SDONE) have fixed functions that must be considered in board layout.
The Lattice GAL20V8C-5LJ was a cornerstone of digital logic design, prized for its reprogrammability, flexibility through Output Logic Macrocells, and drop-in compatibility. It empowered a generation of engineers to implement complex glue logic and state machines efficiently. While modern CPLDs and FPGAs have largely superseded it for new designs, its architectural principles remain foundational, and understanding it is key for maintaining a vast installed base of legacy electronic equipment.
Keywords: Programmable Logic Device (PLD), Output Logic Macrocell (OLMC), Generic Array Logic (GAL), JEDEC File, Reprogrammable
