Lattice GAL20V8B-25LJN: Architecture, Features, and Application in Digital Logic Design
The Lattice GAL20V8B-25LJN represents a pivotal component in the history of programmable logic devices (PLDs). As a member of the Generic Array Logic (GAL) family, it provided a powerful and flexible solution for implementing complex combinational and sequential logic circuits, bridging the gap between discrete logic gates and more advanced FPGAs. Its architecture, speed, and reprogrammability made it a cornerstone in digital systems design throughout the late 1980s and 1990s.
Architecture: A Look Inside
The "20V8" in its name is a direct reference to its internal structure. The architecture is centered around a programmable AND array feeding into a fixed OR array. This structure efficiently implements sum-of-products (SOP) logic functions.
Inputs and Outputs: The device features up to 10 dedicated input pins and 8 I/O pins that can be configured as inputs or outputs, making a maximum of 20 signals available to the AND array.
Output Logic Macrocell (OLMC): The heart of its flexibility lies in its eight Output Logic Macrocells (OLMCs). Each macrocell can be individually configured by the user to operate in various modes:
Combinational Mode: The output is a direct function of the input signals.
Registered Mode: The output is stored in a D-type flip-flop, synchronized to a clock signal, enabling the design of state machines and counters.
Complex Mode: Allows the I/O pin to be used as a dedicated input, freeing up the internal logic for other functions.
This programmability allows the same silicon die to be used for a vast array of different logic functions.
Key Features and Specifications
The GAL20V8B-25LJN offers a set of features that cemented its popularity:
High Performance: The `-25` suffix denotes a maximum propagation delay of 25 nanoseconds, making it suitable for high-speed logic applications of its era.
Electrically Erasable (E²) Technology: Unlike its one-time programmable (OTP) predecessor, the PAL, the GAL uses an E²CMOS process. This allows the device to be reprogrammed and reused hundreds of times, drastically accelerating design prototyping and debugging.
Low Power Consumption: The E²CMOS technology also ensures relatively low power operation compared to bipolar alternatives.
100% Testability: The built-in logic provides complete functional testability, ensuring high manufacturing yields and reliable operation.
Package: The `LJN` suffix identifies a 28-pin Plastic Leaded Chip Carrier (PLCC) package, a common surface-mount format.
Application in Digital Logic Design
The primary application of the GAL20V8B-25LJN was to consolidate multiple simple logic ICs (e.g., 74-series TTL chips) into a single, customized device. This consolidation offered significant advantages:
Reduced Board Space: Replacing several SSI/MSI chips with one PLD led to more compact and elegant PCB designs.
Improved Reliability: Fewer components on a board directly translated to higher system reliability and lower power consumption.
Design Flexibility and Security: Logic functions could be changed simply by reprogramming the device without altering the circuit board layout. The programmed logic pattern could also be secured against copying.
Rapid Prototyping: Designers could quickly implement and test logic equations written in Hardware Description Languages (HDLs) like Abel or CUPL, then program a GAL in minutes to verify functionality.
Designers used these devices to implement a wide range of functions, including address decoders, state machines, bus interfaces, counters, and complex glue logic in microprocessor-based systems.
ICGOODFIND: The Lattice GAL20V8B-25LJN is a classic E²CMOS PLD featuring a programmable AND/fixed OR architecture and versatile output macrocells. Its 25ns speed and reprogrammability made it an indispensable tool for consolidating logic, prototyping designs, and implementing everything from state machines to glue logic in digital systems.
Keywords:
1. Programmable Logic Device (PLD)
2. Output Logic Macrocell (OLMC)
3. Electrically Erasable (E²CMOS)
4. Sum-of-Products (SOP)
5. Glue Logic
