Lattice GAL20V8C-7LJ: Architecture, Features, and Key Applications in Digital Logic Design
The Lattice GAL20V8C-7LJ stands as a pivotal component in the history of programmable logic devices (PLDs). As a member of the Generic Array Logic (GAL) family, it provided designers with a flexible, erasable, and cost-effective alternative to fixed-function logic ICs and one-time programmable (OTP) PAL devices. Its architecture, performance, and re-programmability cemented its role in countless digital systems throughout the late 1980s and 1990s.
Architecture and Internal Structure
The "20V8" nomenclature is key to understanding its architecture. The device features 22 dedicated input pins and 8 output pins, each of which can be configured as either an input or an output, providing significant I/O flexibility. The core of the GAL20V8C is a programmable AND array followed by a fixed OR array. The AND array generates product terms, which are then summed by the OR array to create specific sum-of-products (SOP) logic functions.
A critical architectural advancement in the GAL family was the replacement of the fuse-based programming technology of PALs with re-programmable EECMOS (Electrically Erasable CMOS) technology. This allowed designers to erase and reprogram the device thousands of times, drastically accelerating design iteration and prototyping cycles.
Each of the eight output logic macrocells (OLMCs) is a key feature that provides the device's configurability. Each macrocell can be programmed, via a set of architecture control bits, to operate in several modes:
Combinational Mode: The output is solely a function of the input product terms.
Registered Mode: The output is stored in a D-type flip-flop, synchronizing the output to the clock signal, which is essential for implementing state machines and sequential logic.
Complex Mode: Allows for feedback of the registered output back into the AND array.
The "-7LJ" suffix indicates the device's speed and package. The 7ns maximum propagation delay (tPD) from input to output (for combinatorial logic) signifies a high-speed capability for its era, while "LJ" denotes a 28-pin PLCC (Plastic Leaded Chip Carrier) package.
Key Features and Advantages
High Flexibility: The programmable macrocell structure allows it to emulate a wide range of fixed 20-pin PAL devices, making it a universal logic solution.
Re-programmability: The EECMOS cells enable easy design changes and bug fixes without replacing the physical IC, reducing development time and cost.
High Speed: With a propagation delay of just 7ns, it was suitable for high-performance logic applications.
Low Power Consumption: Compared to its bipolar PAL counterparts, the CMOS technology offered significantly lower power dissipation.
100% Testability: The architecture supported functional testing, ensuring high reliability.
Key Applications in Digital Logic Design
The GAL20V8C-7LJ found extensive use as a glue logic component, interfacing between larger integrated circuits like CPUs, memory, and peripherals. Its primary applications included:
1. Address Decoding: In microprocessor-based systems, it was extensively used to decode address buses and generate chip select (CS) signals for memory (RAM/ROM) and peripheral ICs.
2. State Machine Implementation: The registered macrocell mode allowed designers to implement finite state machines (FSMs) for controlling complex sequential processes.
3. Bus Interface and Control Logic: It was ideal for generating timing and control signals such as read/write (R/W), interrupt request (IRQ), and wait-state generation.
4. Data Routing and Multiplexing: It could be programmed to function as multiplexers, demultiplexers, and encoders/decoders, consolidating multiple simple ICs into a single package.
5. Protocol Conversion: Simple serial communication protocols like SPI or custom parallel-to-serial conversion could be efficiently implemented.
The Lattice GAL20V8C-7LJ was a workhorse of digital design, bridging the gap between simple fixed logic and more complex FPGAs/CPLDs. Its revolutionary re-programmable EECMOS technology and highly flexible output macrocell architecture empowered a generation of engineers to create complex, reliable logic functions with unprecedented speed and efficiency. While largely superseded by modern CPLDs and FPGAs, its foundational principles remain relevant in programmable logic design today.
Keywords:
Programmable Logic Device (PLD)
Output Logic Macrocell (OLMC)
EECMOS Technology
Glue Logic
Sum-of-Products (SOP)
