Lattice GAL18V10B-15LPN: Architecture, Features, and Application Design Considerations
The Lattice GAL18V10B-15LPN 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 provides a powerful, yet cost-effective, solution for integrating multiple TTL devices into a single chip, enabling significant reductions in board space, component count, and overall system power consumption. Its architecture, performance characteristics, and design flexibility have made it a enduring choice for a vast array of digital logic applications.
Architecture and Core Features
The GAL18V10B-15LPN is built around a well-optimized architecture centered on a programmable AND array feeding into fixed OR arrays and sophisticated Output Logic Macro Cells (OLMCs). This structure is the key to its versatility.
At its heart, the device features 18 inputs and 10 outputs, with the outputs being configurable. The "-15" in its part number denotes a maximum pin-to-pin propagation delay of 15 nanoseconds, ensuring solid performance for a wide range of medium-speed logic functions. The programmable AND array allows users to define custom sum-of-products logic terms.
The most critical architectural element is the Output Logic Macro Cell (OLMC). Each of the ten outputs is controlled by its own OLMC, which can be configured through a structural control word to operate in several modes:
Combinatorial Mode: The output is solely a function of the input product terms.
Registered Mode: The output is stored in a D-type flip-flop on the rising edge of the clock signal, enabling the implementation of sequential logic like counters and state machines.
Complex Mode: Allows for a mix of registered and combinatorial outputs on the same device.
This programmability is non-volatile, achieved through Electrically Erasable (E2) CMOS technology. This allows the device to be reprogrammed multiple times, facilitating rapid design iteration and prototyping without being subject to UV erasure like earlier EPROM-based parts.
Key Design Considerations
Successfully implementing the GAL18V10B-15LPN requires careful attention to several design factors:
1. Power-On Reset and State Machine Design: The registered outputs have a guaranteed power-on reset state, which is crucial for ensuring deterministic behavior upon startup. Designers must verify that this default state aligns with their system's requirements, especially for critical control logic.
2. Clock and Signal Integrity: The global clock signal must be clean and free of noise. For high-speed designs, proper signal termination and board layout are essential to prevent glitches that could be clocked into the registers. The 15ns timing rating sets a clear upper limit on operating frequency.
3. Terminology and Fitting: Designers work with Boolean equations, state diagrams, or schematics. These are compiled into a JEDEC file using a PLD compiler. A primary consideration is logic fitting—ensuring the required logic functions do not exceed the available product terms per output. Efficient logic minimization is often necessary.
4. I/O and Fan-Out Capabilities: The outputs can typically source 4 mA and sink 24 mA, which is sufficient to drive multiple TTL inputs. Designers must calculate the total load on each output to ensure they are not overdrawn, which would lead to voltage level degradation and potential malfunction.
5. Testability and Security: The device includes a security fuse that, once programmed, prevents the programmed pattern from being read back, protecting intellectual property. However, this also means the device cannot be easily tested in-circuit after being programmed, making pre-programming verification vital.
Application Space
The GAL18V10B-15LPN excels in numerous glue logic and interface applications, including:
Address Decoding: in microprocessor and microcontroller-based systems.
Bus Interface and Control: Managing read/write operations, chip selects, and wait-state generation.
State Machine Implementation: for simple control sequences.
Code Converters and Parallel-to-Serial Conversion.
Replacing multiple small-scale integration (SSI) and medium-scale integration (MSI) TTL chips like 74-series logic.
The Lattice GAL18V10B-15LPN remains a quintessential and highly effective PLD. Its elegant macrocell-based architecture provides a perfect balance of flexibility and simplicity. Key advantages like non-volatile E2CMOS technology, a 15ns propagation delay, and fully configurable I/Os have cemented its role as a go-to solution for system integration and glue logic. For engineers designing legacy systems or modern boards requiring simple, reliable, and compact logic consolidation, the GAL18V10B-15LPN continues to be an excellent and dependable choice.
Keywords:
Programmable Logic Device (PLD)
Output Logic Macro Cell (OLMC)
E2CMOS Technology
Glue Logic
Propagation Delay
