In this DIY electronics project, we’ll explore how to build a custom digital logic circuit using the GAL20V8B-25LP, a Programmable Logic Device (PLD). The GAL20V8B-25LP is part of the GAL (Generic Array Logic) family, which allows you to create complex combinational and sequential logic circuits by programming the device to perform specific tasks.

We will design a simple digital logic circuit that uses the GAL20V8B-25LP to implement a 4-bit binary counter with a seven-segment display as the output. The goal of the project is to create a compact, programmable solution for counting in binary and displaying the results in a human-readable format on a seven-segment display. This project will demonstrate the power and flexibility of PLDs in implementing digital logic without the need for a microcontroller or external complex logic gates.

By the end of this project, you’ll have gained hands-on experience in designing digital circuits, programming PLDs, and interfacing them with common components like LEDs and displays.

Components Required:

● GAL20V8B-25LP PLD

● Seven-Segment Display (Common Anode or Common Cathode, depending on your configuration)

● Resistors (330Ω for current limiting on each segment)

● Clock Source (e.g., a 555 timer IC or a crystal oscillator)

● Switches (for manual reset and control)

● Capacitors (for decoupling and stabilization)

● Breadboard or PCB for circuit assembly

● Power Supply (5V DC)

● Jumper wires

● ISP (In-System Programming) Cable for GAL20V8B programming

Project Overview:

In this project, we will use the GAL20V8B-25LP, which is a PLD with 20 logic gates and 8 I/O pins. These devices are versatile because they allow for custom logic configurations through programming, unlike standard ICs which are fixed in their functionality. We will program the GAL20V8B to perform the function of a 4-bit binary counter that counts from 0 to 15, with the output displayed on a seven-segment display.

The seven-segment display will show the decimal equivalent of the 4-bit binary counter output. This circuit will be a basic counting system, and the reset functionality will allow the user to restart the count from zero.

The core components of this project include:

1. A clock signal to increment the binary counter.

2. The GAL20V8B-25LP to implement the binary counting logic.

3. A seven-segment display to show the count in a human-readable format.

4. A reset button to reset the counter to zero.

Understanding the GAL20V8B-25LP:

The GAL20V8B-25LP is a programmable logic device that features a 20-macrocell array and can be configured to implement a wide variety of digital logic functions. It is part of the GAL family, which is a more modern version of programmable logic arrays like the PAL (Programmable Array Logic). The GAL20V8B provides 8 I/O pins that can be individually configured for either inputs or outputs.

The GAL20V8B has two main programmable blocks:

1. Input Macrocells: These define the logic that is applied to the incoming signals.

2. Output Macrocells: These are used to drive the outputs based on the defined logic.

With the GAL20V8B-25LP, you can program the device to perform various logic operations such as AND, OR, NOT, XOR, and more. This versatility allows it to replace many traditional discrete logic chips in a circuit. In this project, we will program the device to count in binary and display the count on the seven-segment display.

Circuit Design:

1. Clock Circuit:

To drive the binary counter, we need a clock signal that increments the counter on each pulse. For simplicity, we will use a 555 timer IC configured in astable mode to generate a clock pulse at regular intervals (e.g., 1 Hz or 2 Hz). The output of the 555 timer will be connected to the input of the GAL20V8B.

2. GAL20V8B-25LP Logic Implementation:

The GAL20V8B will be programmed to implement a 4-bit binary counter. We will map the 4-bit output to the seven-segment display, which requires some additional logic for proper segment control.

● Inputs:

—  The GAL20V8B will take the clock signal as input and increment the counter with each clock pulse.

—  A reset button will allow us to reset the counter back to 0.

● Outputs:

—  The output of the GAL20V8B will be connected to the seven-segment display. Since the display needs to light up specific segments for each digit, the logic will ensure that the correct segments are illuminated for binary values 0-15.

3. Seven-Segment Display:

A seven-segment display consists of 7 LEDs (labeled A to G), which can be lit up in different combinations to represent digits 0-9. For simplicity, we will use a common cathode display, where the segments light up when the corresponding pin is driven high.

To display the numbers 0-15 on the seven-segment display, we will need to map the 4-bit binary output to the correct segments. Each digit of the display is controlled by a set of logic states corresponding to the binary input. For example, the binary number 0000 (0 in decimal) will turn on segments A, B, C, D, E, and F to display the digit “0”.

To simplify the mapping, the GAL20V8B will drive the correct logic levels to each of the seven segments based on the binary counter’s value. The mapping will be programmed in the GAL20V8B to ensure that the correct combination of segments lights up for each possible counter value.

4. Reset Circuit:

A reset button will be used to reset the counter to 0. This will be a simple push-button switch connected to the reset pin of the GAL20V8B. When pressed, the reset button will clear the counter and set the output to zero, resetting the seven-segment display.

5. Power Supply:

The circuit will be powered by a 5V DC power supply, which is typical for TTL logic circuits. The GAL20V8B-25LP operates within this voltage range, and the 555 timer and seven-segment display are also compatible with 5V.

Step-by-Step Assembly:

Here’s how to assemble the binary counter with the GAL20V8B:

1. Prepare the Components:

● Gather the GAL20V8B-25LP, 555 timer IC, seven-segment display, resistors, capacitors, and other necessary components.

● Prepare your breadboard or PCB for assembly.

2. Clock Circuit:

● Connect the 555 timer IC in astable mode to generate a clock pulse. Connect the output of the 555 timer to the input of the GAL20V8B.

3. Programming the GAL20V8B:

● Program the GAL20V8B using the appropriate programming tool (ISP or parallel programmer). The logic should implement a 4-bit binary counter and map the output to the seven-segment display.

4. Seven-Segment Display:

● Connect the outputs of the GAL20V8B to the seven segments of the display. Use 330Ω resistors to limit the current for each segment.

● Ensure that the segments are correctly mapped to display the digits 0-15.

5. Reset Circuit:

● Connect a push-button switch to the reset pin of the GAL20V8B, ensuring that pressing the button clears the counter.

6. Testing the Circuit:

● Power on the circuit and check the functionality of the binary counter. Verify that the seven-segment display shows the correct binary count from 0 to 15.

● Test the reset button to ensure that it properly resets the counter to 0.

Programming the GAL20V8B:

Programming the GAL20V8B is done using a programmer that supports ISP (In-System Programming). You can use a simple parallel programmer or USB-based programmer to load the configuration file into the GAL20V8B. The programming process involves specifying the logic equations for the inputs and outputs of the device.

In this case, the programming file will define the counter logic, the reset functionality, and the mapping of the binary output to the seven-segment display.

Final Thoughts:

This DIY project demonstrates how to use the GAL20V8B-25LP to create a custom digital logic circuit. By programming the GAL20V8B, we were able to implement a 4-bit binary counter and map the output to a seven-segment display, resulting in a compact and efficient counting system.

The GAL20V8B’s versatility in implementing complex logic functions without the need for external components makes it a powerful tool for digital designers. Whether you are building simple counters, more complex control systems, or even custom logic circuits, the GAL family of devices offers an accessible and effective way to implement your designs.

This project serves as an excellent starting point for anyone looking to learn more about digital logic design, PLDs, and the principles of programmable logic in embedded systems.