Lattice GAL20V8B-15QP: Architecture, Features, and Application Design Considerations
The Lattice GAL20V8B-15QP stands as a classic and highly influential device in the history 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, offering designers a flexible and cost-effective solution for a vast array of digital logic applications. This article delves into its internal architecture, key features, and critical considerations for application design.
Architecture: A Look Inside
The GAL20V8B's architecture is a masterpiece of structured programmability. The "20V8" designation is key: it features 20 inputs and 8 outputs, with each output capable of being configured as either a dedicated input or an output. Its core consists of a programmable AND array followed by a fixed OR array, a structure known as PAL® (Programmable Array Logic). The 20V8's most significant architectural advancement over its predecessors was the Output Logic Macrocell (OLMC).
Each of the eight outputs is fed by its own OLMC. These macrocells are incredibly versatile, allowing each pin to be individually configured for combinatorial or registered (clocked) operation. The OLMC structure includes a programmable D-type flip-flop, multiplexers, and control bits that determine the output's functionality, including its polarity (active-high or active-low). This programmability is what made the GAL20V8B a universal replacement for dozens of fixed-function PAL devices.
Key Features and Specifications
The GAL20V8B-15QP, specifically, encapsulates a set of features that defined its utility:
High Performance: The `-15` suffix denotes a maximum pin-to-pin propagation delay of 15 ns, making it suitable for high-speed logic implementations in its era.
Electrically Erasable (EE) CMOS Technology: Unlike fusable-link PALs, the GAL20V8B uses an E²CMOS process. This allows the device to be reprogrammed and reused up to 100 times, drastically simplifying design iteration and prototyping.
Low Power Consumption: The advanced CMOS technology ensures lower power consumption compared to bipolar PAL alternatives, a critical factor for many applications.
100% Testability: The internal logic provides complete functionality testability, ensuring high reliability.
QP Package: The "QP" indicates a Plastic Quad Flat Pack (QFP), a surface-mount package suitable for automated assembly processes.
Application Design Considerations
Designing with the GAL20V8B-15QP requires careful consideration of its capabilities and limitations:
1. Logic Density: With a limited number of product terms per output, designers must carefully optimize their Boolean equations. Complex logic functions might require creative partitioning or the use of multiple devices.
2. Clock and Reset Management: The clock signal for all registered outputs is common (on pin 1). This synchronous design simplifies timing analysis but requires that all registered operations occur on the same clock edge. Similarly, an asynchronous reset is shared across all macrocells.
3. Timing Analysis: While the 15 ns speed was impressive, designers must account for worst-case propagation delays and setup/hold times for the flip-flops to ensure stable operation in synchronous systems.
4. Power-On Reset: The device features a built-in power-on reset circuit that initializes all registers to a known state (typically low), which is essential for ensuring predictable startup behavior.
5. Modern Toolchain: While modern EDA tools still support these devices, finding and maintaining a hardware programmer and the requisite software (like CUPL or Abel) can be a challenge for new projects today.
Despite being a legacy component, the GAL20V8B found extensive use in address decoding, state machine design, bus interfacing, and glue logic in countless computer, telecommunications, and industrial systems throughout the 1990s and early 2000s.
The Lattice GAL20V8B-15QP was a cornerstone of digital design, offering an unprecedented blend of reprogrammability, high speed, and design flexibility. Its macrocell-based architecture set a standard that influenced later, more complex PLDs and CPLDs. For engineers today, understanding its structure provides a fundamental insight into the evolution of programmable logic, even as its practical use has been largely superseded by modern CPLDs and FPGAs offering vastly greater logic capacity and integration.
Keywords:
1. Programmable Logic Device (PLD)
2. Output Logic Macrocell (OLMC)
3. Electrically Erasable (EE) CMOS
4. Propagation Delay
5. Logic Synthesis
