Lattice GAL20V8B-25LJ: Architecture, Key Features, and Application Design Considerations
The Lattice GAL20V8B-25LJ stands as a quintessential example of a high-performance Generic Array Logic (GAL) device, a technology that played a pivotal role in the evolution of programmable logic. As a 24-pin, electrically erasable CMOS device, it offers designers a flexible and reliable alternative to fixed-function logic ICs and one-time programmable (OTP) PALs. Its architecture, performance characteristics, and design versatility make it suitable for a wide range of logic integration applications.
Architecture Overview
The architecture of the GAL20V8B-25LJ is centered around a programmable AND array feeding into a fixed OR array, connected to output logic macro cells (OLMCs). The "20" in its name denotes the number of inputs, while the "8" refers to the maximum number of outputs. Its core components include:
Programmable AND Array: This forms the core logic gate array, allowing users to define custom sum-of-products (SOP) logic functions by programming the connections between inputs.
Output Logic Macro Cells (OLMCs): This is the defining feature of the GAL family. Each of the eight outputs can be individually configured by the designer. Key configuration options include programmable output polarity (active-high or active-low) and the output structure, which can be set as registered (with a D-type flip-flop) or combinatorial (directly from the OR array). This flexibility allows the same device to implement a vast array of state machine and combinatorial logic functions.
Erasable Technology: Unlike OTP devices, the GAL20V8B uses an EECMOS process. This allows the device to be erased by ultraviolet light or electrically, enabling countless design iterations and prototyping cycles.
Key Features and Performance
The GAL20V8B-25LJ is defined by a set of robust features tailored for complex logic design:
High Speed: The "-25" suffix indicates a maximum propagation delay (tPD) of 25 nanoseconds, ensuring operation at clock frequencies suitable for many microprocessor interfacing and control applications.
Low Power Consumption: Built on a CMOS process, it consumes significantly less power than its bipolar counterparts, making it ideal for power-sensitive designs.
100% Testability: The architecture supports functional testing, guaranteeing that the programmed device is 100% functional before it leaves the factory.
High Reliability: The CMOS technology contributes to a high noise margin and robust operation across industrial temperature ranges.
Registered or Combinatorial Outputs: The OLMCs provide the critical choice between implementing state-based sequential logic or pure combinatorial logic on a per-pin basis.
Application Design Considerations
Successfully integrating the GAL20V8B-25LJ into a design requires careful consideration of several factors:
1. Logic Consolidation: Its primary role is to replace multiple small-scale integration (SSI) and medium-scale integration (MSI) devices like 74-series logic gates, decoders, and multiplexers. This consolidation reduces board space, component count, and improves system reliability.
2. State Machine Design: The registered OLMCs are perfectly suited for designing finite state machines (FSMs), counters, and other sequential logic circuits that require clocked storage elements.
3. Clock and Pin Management: Designers must carefully plan the use of dedicated input pins, clock pins (pin 1), and output enable terms. Understanding the limitations of the number of product terms available per output is crucial for complex functions.
4. Programming and Security: Utilizing a certified programmer with the correct JEDEC file is essential. The device also features a security fuse that, once programmed, prevents the internal logic pattern from being read back, protecting intellectual property.
5. Signal Integrity: While robust, proper decoupling and attention to ground planes are necessary to maintain signal integrity, especially when operating at high speeds.
The Lattice GAL20V8B-25LJ represents a foundational technology in programmable logic. Its enduring value lies in its architectural flexibility, provided by the configurable OLMCs, its reprogrammability for design prototyping, and its role in significantly reducing system complexity. For engineers seeking to integrate discrete logic, implement control logic, or design simple state machines, it remains a highly effective and reliable solution.
Keywords:
Programmable Logic Device
Output Logic Macro Cell (OLMC)
Sum-of-Products (SOP)
Propagation Delay (tPD)
Electrically Erasable CMOS
