Lattice GAL16V8D-7LJ: Architecture, Programming, and Application in Digital Logic Design
The Lattice GAL16V8D-7LJ 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 chapter in the evolution of modern electronics.
Architecture: A Flexible OLMC Framework
The architecture of the GAL16V8D-7LJ is ingeniously crafted around a programmable AND array feeding fixed OR arrays. The "16V8" designation is descriptive: it has up to 16 inputs and 8 outputs. However, the true genius of its architecture lies in the Output Logic Macro Cell (OLMC).
Each of the eight output pins can be individually configured by programming its associated OLMC. This allows any pin to operate as:
A dedicated combinatorial output.
A registered output (with a D-type flip-flop).
A dedicated input.
A bi-directional I/O pin.
This flexibility is the cornerstone of the device's utility. The `-7LJ` suffix specifically denotes a 7ns maximum propagation delay (tPD) for combinatorial logic and a commercial temperature range (0°C to 75°C) in a PLCC-20 package.
Programming: From Boolean Equations to JEDEC File
Programming the GAL16V8D-7LJ is a process of defining its internal logic connections. Designers typically follow this workflow:
1. Logic Definition: The desired functionality is captured using Boolean equations, a state diagram, or a truth table.
2. Design Entry: These equations are entered into a software tool known as a PLD/FPGA compiler. Hardware Description Languages (HDLs) like Abel-HDL or VHDL were commonly used, though schematic entry was also popular.
3. Compilation: The compiler translates the design into a fuse map, representing the connections to be made or broken within the programmable AND array.
4. JEDEC File Generation: The compiler outputs a standard JEDEC file (a .JED extension), which contains the fuse map data.
5. Device Programming: A dedicated GAL programmer (or universal programmer) erases the device (using UV light for the windowed ceramic package or electrically for the one-time plastic version) and then programs it by blowing the internal fuses according to the JEDEC file.
Application in Digital Logic Design
The GAL16V8D-7LJ found widespread use for integrating multiple simple TTL chips into a single, compact device. Its primary applications included:
Address Decoding: Generating chip select (CS) signals for microprocessors and memory systems was a quintessential use case, replacing 74LS138 decoders and surrounding glue logic.
State Machine Implementation: Its registered outputs made it ideal for implementing medium-speed finite state machines (FSMs) for control logic.
Bus Interface Logic: It was perfect for creating custom glue logic for interfacing between CPUs and peripherals with different timing requirements.
Signal Gating and Multiplexing: Complex combinatorial functions could be consolidated, reducing board space and improving reliability.
The 7ns speed grade made it suitable for many applications involving 25-50 MHz system clocks, where its fast pin-to-pin delays were critical for meeting timing constraints.
ICGOODFIND: The Lattice GAL16V8D-7LJ is a classic and highly influential Programmable Logic Device (PLD) that revolutionized digital design by offering in-system reconfigurability and logic integration. Its defining feature, the Output Logic Macro Cell (OLMC), provided unprecedented pin-level flexibility for its time. While largely superseded by more dense CPLDs and FPGAs today, understanding its architecture and programming model provides fundamental knowledge crucial for grasping the roots and evolution of programmable logic.
Keywords:
1. Programmable Logic Device (PLD)
2. Output Logic Macro Cell (OLMC)
3. JEDEC File
4. Boolean Equations
5. Glue Logic
