Designing High-Speed Logic Circuits with the Lattice GAL20V8B-25LPNI PLD

In the realm of digital logic design, Programmable Logic Devices (PLDs) offer a flexible and efficient solution for integrating complex logic functions into a single chip. Among these, the Lattice GAL20V8B-25LPNI stands out as a robust and high-performance option for implementing high-speed logic circuits. This article explores the key considerations and design methodologies for leveraging this specific PLD.

The GAL20V8B-25LPNI is a 24-pin, high-speed Electrically Erasable PLD (EE PLD) built on a proven CMOS technology. Its nomenclature provides critical design parameters: "20V8" indicates 20 inputs and 8 outputs, "B" signifies an advanced architecture, and "-25" specifies a maximum propagation delay (tPD) of 25 ns, making it suitable for clock frequencies exceeding 40 MHz. The "LPNI" denotes the low-power, plastic DIP package with industrial temperature range support.

A primary advantage of this device is its fully programmable AND-OR logic structure. Unlike fixed-function ICs, designers can define any combination of the 20 inputs to form product terms, which are then summed and allocated to one of the eight output logic macrocells (OLMCs). Each OLMC can be individually configured for combinatorial or registered (clocked) output operations, providing immense design flexibility. This programmability allows for the consolidation of multiple standard logic ICs (like 74-series chips) into one compact, power-efficient package, simplifying board layout and reducing component count.

Designing with the GAL20V8B begins with a clear logic definition, typically captured using Boolean equations, state diagrams, or truth tables. This logic is then entered into a PLD development environment, such as Lattice's own development software or third-party tools like WinCUPL. The software compiles the design, performs functional simulation, and fits the logic into the device's resources. Critical to high-speed design is the management of propagation delays and pin-to-pin timing. The guaranteed 25 ns tPD ensures predictable performance, but designers must be cautious of asynchronous feedback paths and ensure that registered outputs meet setup and hold times relative to the global clock.

The global clock network is a key feature for synchronous designs. One dedicated pin (CLK) provides a low-skew clock signal to all registers within the device, essential for maintaining state machine stability and meeting timing constraints at high speeds. For combinatorial paths, careful analysis is required to ensure that signal paths do not become critical and limit the overall system frequency.

Once the design is compiled, a standard JEDEC file is generated. This file is used to program the device via a standard PLD programmer. A critical post-programming step is verification through in-circuit testing, ensuring the physical device behaves as simulated under real-world conditions, including at the intended high-speed operation.

ICGOODFIND: The Lattice GAL20V8B-25LPNI remains a highly effective PLD for consolidating high-speed glue logic, implementing state machines, and creating custom logic interfaces, offering an optimal balance of speed, flexibility, and power efficiency for countless embedded and digital systems.

Keywords: High-Speed Logic, Programmable Logic Device, Propagation Delay, Output Logic Macrocell, JEDEC File.