Lattice GAL20V8B-15LPN: Architecture, Features, and Application Circuit Design

The Lattice GAL20V8B-15LPN stands as a classic and highly influential device in the history of programmable logic. As a member of the Generic Array Logic (GAL) family, it provided a powerful, erasable, and one-time programmable (OTP) alternative to rigid standard logic parts and complex PAL devices. Its architecture, speed, and flexibility made it a cornerstone for countless digital designs in the late 20th century and it remains relevant for legacy system support and educational purposes.

Architecture: A Look Inside

The GAL20V8B's architecture is elegantly structured around a programmable AND array feeding into fixed OR terms and sophisticated Output Logic Macro Cells (OLMCs). The "20V8" designation is key: it has up to 20 inputs and 8 outputs, with the outputs configured through the OLMCs.

The core components are:

Programmable AND Array: This is the heart of the logic implementation. It consists of a grid of programmable links that allow designers to create any combination of product terms (AND operations) from the input signals.

Output Logic Macro Cells (OLMCs): This is the defining feature of the GAL family. Each of the 8 outputs can be individually configured by programming the OLMC. The key configurations include:

Combinatorial Output: The output is a direct function of the AND array.

Registered Output: The output is stored in a D-type flip-flop, synchronously clocked by a dedicated clock pin, enabling sequential logic designs like counters and state machines.

Complex Mode: Allows for feedback paths where an output can be fed back as an input to the AND array, essential for creating more complex state machines.

Fixed OR Array: Unlike earlier PAL devices, the GAL's OR array is fixed. Each OLMC is fed by a fixed number of product terms from the AND array (typically 8 for most outputs).

Key Features of the GAL20V8B-15LPN

The specific part number, GAL20V8B-15LPN, reveals its critical characteristics:

GAL20V8B: The base device family.

-15: Signifies a maximum propagation delay (tPD) of 15 ns, making it a relatively high-speed part for its time. This speed is crucial for meeting timing requirements in synchronous systems.

LPN: Indicates the package (PLCC) and operating temperature range (Commercial).

High Voltage Reprogrammability: Unlike OTP PALs, it uses an E²CMOS process, allowing it to be erased and reprogrammed numerous times using a standard programmer. This drastically accelerated prototyping and design iteration.

Low Power Consumption: The E²CMOS technology is known for its relatively low power draw compared to bipolar alternatives.

Application Circuit Design: A Simple Registered Example

A common use case for the GAL20V8B is to replace multiple standard logic ICs. Consider designing a simple 4-bit counter with decoded outputs.

1. Define Requirements: Create a 4-bit synchronous counter that increments on every rising edge of the clock. The four outputs (Q0, Q1, Q2, Q3) should be registered.

2. Logic Equations: The design is described using Boolean equations. For a counter, this involves defining the D-input for each flip-flop based on its current state (e.g., using excitation equations for a Johnson counter or directly for a binary counter).

```

/CLK = PIN 1;

Q0 = PIN 12 registered;

Q1 = PIN 13 registered;

Q2 = PIN 14 registered;

Q3 = PIN 15 registered;

Q0.D = !Q0; // Toggle the LSB every clock cycle

Q1.D = (!Q1 & Q0) (Q1 & !Q0); // Standard T-flip-flop excitation

... (equations for Q2 and Q3)

```

3. Schematic Capture/HDL: While modern designs use HDLs, traditional GAL design often used schematic capture or direct Boolean equation entry in a compiler like CUPL or Abel.

4. Programming: The compiled JEDEC file is sent to a universal programmer to configure the internal fuses of the GAL20V8B-15LPN.

5. Circuit Implementation: In the circuit, the dedicated clock pin (PIN 1) is connected to the system clock. The output pins are connected to LEDs or other logic. The device effectively replaces a package containing several flip-flops and gates, saving significant board space and improving reliability.

ICGOODFIND Summary

The Lattice GAL20V8B-15LPN is a quintessential programmable logic device that revolutionized digital design. Its reprogrammable E²CMOS technology, configurable Output Logic Macro Cells (OLMCs), and 15ns high-speed performance allowed it to consolidate complex TTL logic into a single, reliable chip. It served as a critical bridge between simple logic ICs and the high-density FPGAs and CPLDs of today, remaining a vital component for understanding the evolution of programmable logic.

Keywords:

1. Programmable Logic Device (PLD)

2. Output Logic Macro Cell (OLMC)

3. E²CMOS Technology

4. Registered Output

5. JEDEC File