Lattice GAL16V8D-15QPN: Architecture, Key Features, and Application Design Considerations
The Lattice GAL16V8D-15QPN 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. This article delves into its internal architecture, highlights its key specifications, and outlines critical considerations for modern design implementation.
Architecture: A Look Inside
The GAL16V8D-15QPN is based on a Programmable AND-Fixed OR array structure. Its architecture is fundamentally composed of two main sections:
1. The Programmable AND Array: This is the core of the device's programmability. It consists of a grid of fusible links that allow designers to create custom product terms by connecting inputs and their complements. The "16" in its name refers to the maximum number of inputs that can be fed into this array.
2. The Output Logic Macrocell (OLMC): The "8" denotes its eight output macrocells. Each macrocell is a configurable block that takes the fixed sum-of-products terms from the OR array and determines the final output configuration. Crucially, each macrocell can be individually programmed for combinatorial or registered (clocked) output operation. This flexibility allows the same device to implement simple glue logic or more complex sequential state machines. The macrocells also provide options for output polarity control (active-high or active-low).
Key Features and Specifications
The part number "GAL16V8D-15QPN" itself encodes several of its critical features:
GAL16V8: The core family and model.
D: Indicates a dedicated registered configuration for the outputs, though the OLMCs can be reconfigured.
-15: Signifies a maximum pin-to-pin propagation delay of 15 nanoseconds, making it suitable for moderate-speed applications.
QPN: Denotes the package type—a Plastic Leaded Chip Carrier (PLCC) with 20 pins.
Other essential features include its use of CMOS technology, which offers lower power consumption than its bipolar predecessors, and Electrically Erasable (EECMOS) cells. This re-programmability drastically improved design iteration time and prototyping efficiency compared to fuse-based devices.
Application Design Considerations
While considered a legacy part today, understanding its design constraints is vital for maintaining older systems or for educational purposes.
1. Logic Density Limitations: With only 8 macrocells and a limited number of product terms per output (7-8), the device is suited for small-scale logic integration. Complex functions must be partitioned wisely or spread across multiple devices.
2. Timing Analysis: The 15ns speed grade sets a ceiling on the maximum operating frequency. Designers must carefully calculate setup, hold, and propagation delay times, especially when interfacing with synchronous components like microprocessors. The registered outputs are triggered on the low-to-high transition of the clock input.
3. Power-On Reset State: The registered outputs are automatically reset to a logic low upon power-up. This predictable behavior is crucial for designing stable finite state machines (FSMs).
4. I/O Pin Management: The limited number of pins (20) means careful planning of input, output, and dedicated pin (like Clock) assignments is necessary to avoid "pin-out" problems during design.
5. Modern Tooling: Designing for the GAL16V8 requires older, but still available, software tools like CUPL or WinCUPL to compile logic equations and generate the standard JEDEC programming file.
The Lattice GAL16V8D-15QPN was a cornerstone of digital design, bridging the gap between simple logic ICs and high-density FPGAs. Its re-programmable nature, flexible macrocell architecture, and respectable speed made it an indispensable tool for a generation of engineers, primarily used for address decoding, state machine control, and bus interfacing in countless embedded systems.
Keywords:
1. Programmable Logic Device (PLD)
2. Output Logic Macrocell (OLMC)
3. Sum-of-Products
4. Electrically Erasable
5. JEDEC File
