Lattice GAL20V8A-15QP: Architecture, Features, and Application Design Considerations
The Lattice GAL20V8A-15QP stands as a classic and highly influential device in the realm of programmable logic. As a 15ns, high-performance member of the Generic Array Logic (GAL) family, it provided a powerful and flexible solution for a wide array of digital logic designs, bridging the gap between rigid standard logic parts and more complex FPGAs. Its architecture, a precursor to modern CPLDs, remains a foundational study in programmable logic.
Architecture and Core Features
The GAL20V8A's architecture is elegantly structured around a programmable AND array feeding into fixed OR terms and sophisticated Output Logic Macro Cells (OLMCs). The "20" denotes the number of inputs, and the "8" the number of outputs. Its key architectural components include:
Programmable AND Array: This is the core of the device's logic capability. It allows users to create custom product terms by programming the connections between input signals and an array of AND gates. This array defines the combinatorial logic functions.
Output Logic Macro Cells (OLMCs): This is the most defining feature of the GAL20V8A. Each of the eight outputs can be individually configured via the OLMC. The macrocell provides tremendous flexibility, allowing each pin to be programmed as a dedicated input, a combinatorial output, or a registered (clocked) output. Crucially, the polarity of the output (active-high or active-low) is also programmable, which aids in logic minimization.
15ns Maximum Propagation Delay: The `-15` suffix indicates a maximum propagation delay of 15ns from input to output. This high-speed performance made it suitable for state machines and critical control paths in systems where timing was a paramount concern.
Electrically Erasable (E²) CMOS Technology: Unlike its one-time programmable (OTP) PAL predecessors, the GAL20V8A uses E²CMOS cells. This technology enables the device to be reprogrammed and tested repeatedly, significantly accelerating the development cycle and reducing costs associated with design revisions.
Key Application Design Considerations
Designing with the GAL20V8A-15QP requires careful consideration of its specific characteristics to ensure robust and reliable system performance.
1. Power-On Reset and State initialization: Upon power-up, the internal registers initialize to a known state (typically logic low). The designer must ensure this default state is safe for the overall system and that any necessary startup sequences are managed correctly.
2. Clock and Input Signal Management: The dedicated clock pin (Pin 1) must be used for all registered outputs to ensure clean and synchronized operation. Input signals must meet setup and hold time requirements relative to this clock to prevent metastability. The 15ns speed grade demands attention to signal integrity and board layout to avoid reflections and noise.
3. Utilization of Product Terms: Each OLMC has a limited number of product terms available. Complex logic functions might require "product term sharing" or clever logic design to fit within these constraints. Exceeding the available terms would require partitioning the logic across multiple devices.
4. Output Enable Control: The output enable terms are also derived from the programmable array. Designers must correctly implement the logic to tri-state outputs when necessary, especially when bidirectional I/O or bus interfacing is required.
5. Programming and Security: While reprogrammable, the device includes a security fuse that, once programmed, prevents the internal configuration from being read or copied, protecting intellectual property. This fuse can only be cleared by a full erase cycle.
In its prime, the GAL20V8A-15QP was ubiquitous, finding use in applications such as address decoding, bus interfacing, state machine implementation, and glue logic consolidation in computer peripherals, telecommunications equipment, and industrial control systems. It served as the perfect "glue" to integrate complex components.
The Lattice GAL20V8A-15QP was a cornerstone of digital design, offering a unique blend of high speed, reprogrammability, and flexible output configuration through its macrocell architecture. It empowered a generation of engineers to efficiently replace numerous standard logic ICs with a single, customizable chip, paving the way for the more complex programmable logic devices that followed.
Keywords:
Programmable Logic Device (PLD)
Output Logic Macro Cell (OLMC)
E²CMOS Technology
Propagation Delay
Product Term
