Lattice GAL16V8D-15LJN: Architecture, Programming, and Application in Digital Logic Design
The Lattice GAL16V8D-15LJN stands as a seminal device in the history of programmable logic. As a member of the Generic Array Logic (GAL) family, it provided a powerful, erasable, and reconfigurable alternative to fixed-function TTL logic and one-time programmable PAL devices. Its impact on digital logic design, particularly in prototyping and medium-complexity state machines, remains a key reference point for understanding the evolution of modern programmable logic.
Architecture of the GAL16V8D
The architecture of the GAL16V8 is ingeniously structured around a programmable AND array feeding into a fixed OR array, with its key feature being the Output Logic Macrocell (OLMC). The "16V8" designation indicates 16 inputs and 8 outputs, though the pins are shared. The device features a programmable AND array with 64 product terms. The true flexibility arises from the eight OLMCs, each of which can be configured independently to operate in various modes:
Registered Mode: The output is routed through a D-type flip-flop (register), clocked by a dedicated pin, making it ideal for implementing state machines and synchronous counters.
Combinatorial Mode: The output is a direct combinatorial function of the input and product terms, suitable for decoders, multiplexers, and other logic gates.
Complex Mode: The output pin can also function as an additional input, providing greater flexibility in routing signals.
The "-15LJ" suffix specifies the operating speed (15ns maximum propagation delay) and the package type (PLCC-20). The JEDEC standard fuse map defines its programming, representing the state of every connection in the AND array.
Programming the Device
Programming the GAL16V8D is a hardware-software process. A designer first writes a logic description using a Hardware Description Language (HDL) like ABEL, CUPL, or VHDL, or creates a schematic capture. Software tools then compile this description into a standard JEDEC file containing the fuse map. This file is transferred to a dedicated GAL programmer (or universal programmer), which applies the necessary voltages to physically configure the floating-gate transistors within the silicon, "burning" the desired logic function into the device. Its electrically erasable (EE) CMOS technology is its greatest advantage over PALs, allowing it to be reprogrammed thousands of times, which drastically accelerates design iteration and debugging.
Application in Digital Logic Design
The GAL16V8D found widespread use as a "universal glue logic" component. Its ability to replace multiple standard TTL chips (like the 7400-series) with a single, customized device was revolutionary. Key applications included:
Address Decoding: Generating chip select signals for microprocessors and memory systems.
State Machine Implementation: Designing finite state machines (FSMs) for control units.
Bus Interface Logic: Creating custom interfaces between components with different signaling protocols.
Code Converters and Multiplexers: Implementing complex combinatorial logic functions.
Its 15ns speed rating made it suitable for many microprocessor systems of its era. While its capacity is limited by today's standards, it perfectly served the role of integrating the "random logic" that glued larger system components together.
In summary, the Lattice GAL16V8D-15LJN was a cornerstone of digital design. Its reprogrammable macrocell architecture broke away from the inflexibility of fixed logic and one-time programmable devices. By offering a single-chip solution for replacing multiple TTL ICs, it accelerated prototyping, reduced board space, and lowered system costs. It served as a critical bridge between simple PLDs and the more complex CPLDs and FPGAs that followed, teaching a generation of engineers the fundamentals of programmable logic.
Keywords:
1. Programmable Logic Device (PLD)
2. Output Logic Macrocell (OLMC)
3. JEDEC File
4. Generic Array Logic (GAL)
5. Digital Logic Design
