How To Build a Digital Clock Using the EPM7064SLC44-10N FPGA

FPGAs are powerful tools that allow you to implement custom logic circuits, offering flexibility that microcontrollers cannot match. The EPM7064SLC44-10N, produced by Altera (now part of Intel), is a popular FPGA model featuring 64 logic elements and is capable of implementing a variety of digital circuits. 

In this project, we will use the EPM7064SLC44-10N FPGA (Field-Programmable Gate Array) to create a simple yet functional digital clock that displays hours, minutes, and seconds on a 7-segment display.

This project focuses on creating the digital clock circuit, using the FPGA to control a 7-segment display, with basic components like resistors and capacitors for support.

In this step-by-step guide, we will design the digital clock, implement the logic on the FPGA, and assemble the hardware components to create a fully functional clock.

Required Components

1. EPM7064SLC44-10N FPGA (1 piece)

2. 7-Segment Displays (2 or 3 depending on how you want to show the time)

3. Resistors: 330Ω (for current limiting on the 7-segment displays)

4. Capacitors: 100nF (for decoupling)

5. Push Button Switches (for setting time)

6. Crystal Oscillator: 32.768 kHz (for accurate timekeeping)

7. Power Supply: 5V DC

8. Breadboard and jumper wires for prototyping

9. FPGA Programmer (for loading the design into the FPGA)

Project Overview

The goal of this project is to create a digital clock that shows the current time using a 7-segment display. The FPGA will manage the timekeeping function by counting seconds and converting that count into hours and minutes. A crystal oscillator will be used as the clock source for accuracy. The FPGA will also control the 7-segment display to show the current time in a readable format. Additionally, push buttons will be used to adjust the time.

The EPM7064SLC44-10N FPGA provides a sufficient number of logic elements to implement the necessary components, including a counter for the seconds, minutes, and hours, and a decoder for the 7-segment displays. We’ll break the design into smaller steps, focusing on clock generation, timekeeping, and display control.

Step 1: Setting Up the Clock Source

A crystal oscillator of 32.768 kHz is often used in digital clocks for accurate timekeeping. This particular frequency is ideal because it divides neatly into 1 Hz (one tick per second) when divided by a simple binary counter. The 32.768 kHz crystal will provide the necessary clock pulse for the FPGA.

1. Connect the 32.768 kHz crystal oscillator to the FPGA.

2. Attach a load capacitor to the crystal oscillator to ensure stable oscillation.

3. The FPGA will use this oscillation as its clock signal to drive the rest of the logic in the circuit.

In an FPGA design, the crystal oscillator will serve as the system clock. It will feed into a counter that increments with each clock pulse, effectively counting seconds. The key part of the FPGA design here is ensuring that the FPGA properly divides down the crystal oscillator's frequency to get a 1 Hz signal that represents one second.

Step 2: Designing the Timekeeping Circuit

The timekeeping mechanism will be built using a binary counter implemented in the FPGA. The FPGA will increment this counter on each clock pulse (every second) and convert this value into hours, minutes, and seconds.

1. Seconds Counter: The FPGA will use the 1 Hz signal from the divided clock to count seconds. Every time the counter reaches 60, it will reset to zero and increment the minute counter.

2. Minutes Counter: After the seconds counter rolls over, the minute counter will increment by one. Every time it reaches 60, it will reset to zero and increment the hour counter.

3. Hours Counter: After the minute counter rolls over, the hour counter will increment by one. The hour will reset to zero after it reaches 24, completing a 24-hour cycle.

The FPGA will store the current time in registers and increment the values in real-time. The FPGA’s internal logic will manage the transitions between seconds, minutes, and hours without requiring any external components.

Step 3: Controlling the 7-Segment Displays

The time will be displayed on a 7-segment display. A 7-segment display consists of seven LEDs arranged in the shape of the number 8. By selectively turning on and off specific segments, we can display any digit from 0 to 9. In this project, we’ll use two or three 7-segment displays to show the hours, minutes, and seconds.

To control the 7-segment displays with the FPGA:

1. Use a decoder to convert the binary values of hours, minutes, and seconds into signals that control the individual segments of the display. The FPGA will output a 4-bit value for each digit, corresponding to one of the 16 possible patterns for the 7-segment display.

2. The FPGA will drive each 7-segment display individually by cycling through them rapidly, creating the illusion of a continuously updating clock display. The displays will show the current time in real-time, refreshing at a rate fast enough to be perceived as stable.

Step 4: Adding Time Adjustment with Push Buttons

To allow the user to set the time on the clock, we will use push-button switches. These switches will allow the user to adjust the hours, minutes, and seconds manually. The FPGA will detect the button presses and increment or decrement the appropriate time register.

1. Setting the Hours: When the user presses the “set hour” button, the FPGA will increment the hour register. When it reaches 24, it will wrap around to 0.

2. Setting the Minutes: Similarly, pressing the “set minute” button will increment the minute register. When it reaches 60, it will reset to 0.

3. Setting the Seconds: The “set second” button will increment the second register, allowing the user to adjust the seconds as needed.

To handle button presses efficiently, we’ll use debouncing techniques in the FPGA design. This ensures that each press is detected once, preventing multiple increments from a single press due to contact bounce.

Step 5: Building the Hardware

With the FPGA logic in place, it’s time to build the hardware components of the clock.

Connecting the 7-Segment Display

1. Each 7-segment display has 7 LEDs, and we’ll connect each segment to an output pin on the FPGA.

2. A common cathode 7-segment display will require a low voltage on the segment pins to turn them on. We'll use resistors (typically 330Ω) in series with each segment to limit the current and protect the LEDs.

3. Connect each 7-segment display to the FPGA in such a way that each display represents a digit (hours, minutes, or seconds).

Connecting the Push Buttons

1. The push buttons will be connected to the FPGA's input pins. Each button will be associated with one time-setting function (hours, minutes, or seconds).

2. Use pull-up or pull-down resistors on the button lines to ensure stable logic levels when the button is not pressed.

Power Supply

1. Power the FPGA and associated components with a stable 5V DC supply. Ensure proper decoupling by placing a 100nF capacitor near the power pins of the FPGA to minimize noise and voltage spikes.

Step 6: Loading the Design into the FPGA

Once the hardware is assembled, the next step is to load the design onto the FPGA.

1. Use an FPGA programmer (such as a USB-Blaster) to load the configuration bitstream into the EPM7064SLC44-10N FPGA.

2. The bitstream contains the logic for the clock, which controls the counters, 7-segment displays, and push-button inputs.

Once the bitstream is loaded into the FPGA, the clock should begin functioning, displaying the current time and allowing the user to adjust it using the push buttons.

Conclusion

By using the EPM7064SLC44-10N FPGA, this digital clock project demonstrates the power and flexibility of FPGAs in implementing custom digital logic. The FPGA not only manages the timekeeping and display but also handles user input for setting the time, all while providing the performance needed for a responsive and reliable clock.

Through this project, you gain valuable experience in FPGA design, 7-segment display control, and timekeeping circuits. The project is a great way to get started with FPGAs and develop a deeper understanding of digital electronics.

With a bit of creativity, this basic digital clock can be expanded into a more advanced project, such as adding alarms, integrating real-time clock ICs, or creating a 12-hour format display. FPGAs offer an incredible amount of flexibility, and projects like this one are just the beginning of what you can build.