Architectural Implementation and Design Considerations for the Lattice GAL20V8B-15LJ CPLD

The Lattice GAL20V8B-15LJ represents a classic architecture in the realm of Complex Programmable Logic Devices (CPLDs). As a member of the widely adopted Generic Array Logic (GAL) family, it provides a robust and cost-effective solution for a variety of glue logic, state machine, and I/O interfacing applications. Successful implementation of this device hinges on a thorough understanding of its internal architecture and key design considerations.

Architectural Overview

At its core, the GAL20V8B-15LJ is based on a Programmable AND-Fixed OR structure. Its architecture consists of several fundamental components:

Programmable Logic Array (PLA): The device features an array of programmable AND gates that feed into fixed OR gates. This structure efficiently implements sum-of-products (SOP) logic functions.

Macrocells: The device contains 8 output logic macrocells (OLMCs), each of which can be independently configured for combinatorial or registered (clocked) operation. Each macrocell includes a programmable D-type flip-flop, multiplexers, and output control circuitry. The flexibility of the Output Logic Macrocell (OLMC) is the cornerstone of the device's versatility, allowing each pin to be defined as an input, output, or bidirectional pin.

I/O Structure: The device features 10 dedicated inputs and 8 configurable I/O pins, making for a maximum of 18 signals interacting with the external system. The -15LJ suffix denotes a 15ns maximum pin-to-pin propagation delay and a PLCC (Plastic Leaded Chip Carrier) package.

Clock and Control: A dedicated clock pin (CLK) is provided for synchronous registered functions. A dedicated output enable (OE) pin offers global control over the tri-state outputs.

Key Design Considerations

1. Logic Utilization and Fitting: The GAL20V8B has a finite number of product terms (PTs) available per macrocell. Complex logic functions can quickly consume these resources. Designers must be mindful of logic minimization techniques, such as Karnaugh maps or logic synthesis tools, to ensure the design fits within the device. Functions requiring more than the available product terms may need architectural redesign.

2. Registered vs. Combinatorial Logic: The choice between using a registered or combinatorial output for each macrocell has significant implications. Registered designs are essential for implementing synchronous state machines and data pipelines, offering synchronous operation that is immune to input glitches and race conditions. Purely combinatorial paths, while faster in theory, are more susceptible to hazards.

3. Timing Analysis: The specified -15ns speed grade is a critical parameter. Designers must perform a thorough timing analysis to ensure setup, hold, and clock-to-output times are met within the larger system context. This includes accounting for internal propagation delays and external PCB trace delays. The registered clock frequency must be derated from the inverse of the propagation delay to account for these timing parameters.

4. Pinout and Signal Assignment: Strategic assignment of signals to pins can simplify board layout and enhance signal integrity. Critical signals like clocks and global resets should be assigned to dedicated input pins. High-speed signals should be routed to minimize crosstalk. The programmable I/O architecture allows for significant flexibility in this area.

5. Power-On Reset and State Initialization: Upon power-up, the internal registers of the CPLD are set to a known state, typically logic low. The designer must ensure this default power-on state is compatible with the system's requirements to prevent uncontrolled output states at startup. This often necessitates external reset circuitry.

6. Testability and Programming: The device is programmed via a standard JTAG (Joint Test Action Group) interface. Incorporating design for test (DFT) principles, such as making key internal nodes observable on output pins during debugging, is highly recommended. The design must also be verified through simulation before programming the physical device.

ICGOOODFIND

The Lattice GAL20V8B-15LJ CPLD remains a highly effective solution for digital logic integration. Its success in any application is directly tied to a design approach that respects its architectural constraints. By focusing on efficient logic utilization, rigorous timing analysis, and thoughtful pin management, engineers can leverage this venerable device to create reliable, high-performance, and cost-efficient digital systems.

Keywords:

Programmable Logic

Macrocell

Timing Analysis

Sum-of-Products (SOP)

JTAG