Lattice GAL20V8B-15LP: Architecture, Features, and Application Design Considerations
The Lattice GAL20V8B-15LP 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 provided a pioneering, erasable alternative to one-time programmable PAL devices, revolutionizing digital logic design in its era. This article delves into its internal architecture, key features, and critical considerations for modern application design.
Architecture: A Look Inside
The GAL20V8B-15LP is based on a well-optimized Programmable Logic Device (PLD) architecture. Its name reveals its core configuration: it supports up to 20 inputs and 8 outputs, with the outputs being configurable. The heart of the device is a programmable AND array that feeds into a fixed OR array. This structure allows designers to create a wide variety of sum-of-products logic functions.
The true genius of the GAL architecture lies in its Output Logic Macrocell (OLMC). Each of the eight outputs is driven by a dedicated OLMC, which can be individually configured for specific operational modes. These modes include:
Combinatorial Output: The output is a direct function of the input pins.
Registered Output: The output is stored in a D-type flip-flop, synchronized to a clock signal, enabling the implementation of sequential logic like counters and state machines.
Combinatorial I/O: The pin can act as an input or an output depending on the logic function.
This flexibility made the GAL20V8B-15LP a universal replacement for numerous fixed-function TTL logic chips and simpler PALs.
Key Features and Specifications
The "15LP" suffix is critical, denoting a maximum propagation delay (tPD) of 15 nanoseconds and "Low Power" CMOS technology. This combination of high-speed operation and low power consumption was a significant advantage over its contemporaries. Key specifications include:
High Speed: 15 ns maximum propagation delay.
Low Power: Typically 90 mA active current, far lower than bipolar PLD alternatives.
Re-programmability: Utilizing EE CMOS technology, the device can be erased and reprogrammed, facilitating rapid design iteration and prototyping.
100% Testability: The architecture supports full functional testability, ensuring high manufacturing yields and design reliability.
10-Year Data Retention: A testament to the robustness of the stored program.
Application Design Considerations
While considered a legacy device today, understanding its design constraints remains valuable for maintaining and reverse-engineering older electronic systems.
1. Logic Minimization: The finite resources of the AND array make efficient logic minimization paramount. Designers must use tools to minimize Boolean equations to fit within the available product terms, especially for complex functions.
2. Pinout and OLMC Configuration: Careful planning of the pinout is essential. The configuration of each OLMC (combinatorial vs. registered) must be correctly defined in the JEDEC fuse map file during compilation. Misconfiguration can lead to incorrect circuit behavior.
3. Clock and Reset Management: The registered mode utilizes a global clock (Pin 1) and a global output enable (Pin 11). These signals are common to all macrocells; designs cannot have multiple asynchronous clocks. The reset function is typically implemented using a dedicated product term, consuming logic resources.
4. Power-On Reset: The device features a power-on reset circuit that initializes all registers to a logic low state, ensuring predictable startup behavior for state machines.
5. Signal Integrity: For high-speed operation (15ns), board-level design considerations like clean power supply decoupling and controlled trace impedance become important to prevent glitches and ensure stable operation.
The Lattice GAL20V8B-15LP was a cornerstone of digital design, offering an unmatched blend of speed, flexibility, and low power for its time. It democratized logic design by allowing engineers to implement custom logic functions quickly and modify them with ease. Its architectural principles directly paved the way for the more complex CPLDs and FPGAs that followed, securing its place as a foundational technology in the history of programmable logic.
Keywords:
Programmable Logic Device (PLD)
Output Logic Macrocell (OLMC)
EE CMOS Technology
High-Speed Operation
Logic Minimization
