You will create a debounce state machine and use this state machine to debounce the button on the NEXYS4 board to reliably count the number of times the button is pressed.

• Understand how to debounce noisy input signals
• Demonstrate the ability to synchronize an asynchronous input
• Demonstrate the benefits of a debounce circuit used to count button presses.
• Implement a state machine using behavioral SystemVerilog

Carefully read and review Chapter 26 of the textbook. This lab will closely follow that design.

Figure 26.1 in the textbook demonstrates the “noise” that occurs with mechanical buttons or switches. In this figure, the button signal starts at zero, transitions between zero and one, and then settles down to one.

Assuming a 100 MHz clock (10 ns period), how many clock cycles are needed to implement a delay of at least 5 ms?

How many bits are needed for a counter to implement a 5 ms delay with a 100 MHz clock?

The first exercise is to create the debounce state machine. Begin this exercise by creating the module name and ports for your module as described below:

As shown in Figure 26.3, the debounce circuit described in Chapter 26 is composed of two parts: a state machine and a timer. You will need to create these two parts which will be described in more detail below.

Debounce State Machine

The first part of this debounce circuit is the debounce finite state machine (Figure 26.4). You will need to create this state machine using behavioral SystemVerilog. The inputs to the state machine are “noisy”, which is a module input, and “timerDone”, which is an internal signal. The outputs of the state machine are the signals “debounced”, which is a module output, and an internal signal named “clrTimer”.

Create your debounce FSM using behavior SystemVerilog. Note that a large portion of code is given to you in Program 26.1.1. While this is a simple and straightforward approach, you will need to make some modifications to meet the given requirements. It is recommended that you create your module step-by-step without relying too heavily on what is already given.

Debounce Timer

The second part of the debounce circuit is the debounce timer. The purpose of this timer is to wait a predetermined amount of time for the noise input signal to settle down. The description of the debounce timer is found at the end of section 26.1. For this design, however, you will be required to wait precisely 5ms instead of whatever the maximum value for your timer is. At 5ms your timer should assert “timerDone” and then reset to 0. Your counter must also be cleared when clrTimer is asserted.

Exercise 1 Pass-off: Show a TA your debounce module. Explain how you implemented the state machine and how the state machine should behave when it receives a noisy input.

After completing your SystemVerilog module, you will need to simulate your module to make sure it operates as described in the textbook. Create a TCL simulation script that simulates your debounce module with the following conditions:

• A “short” noisy pulse that is not long enough to generate a valid debounce output
• A “long” noisy pulse that is long enough to generate a valid debounce output
• Enough time to see that debounced goes low after noisy goes low

Include a copy of your TCL simulation script

After completing your TCL simulation, download the following testbench file and simulate your debounce circuit with this testbench. Make sure the simulation runs as long as necessary until the “simulation done” message is received (this will take about 100ms of simulation time).

tb_debounce.v

Include a copy of your debounce state machine code

Exercise 2 Pass-off: Show a TA your tcl script and the output of the testbench.

For this exercise, you will create a top-level circuit that instances the debounce circuit for debouncing one of the push buttons. This debounce circuit will be used to increment a counter so you can verify that the counter increments only once for each button press. In addition to the debounce circuit, you will include a second counter that counts the button presses for a different button that is not debounced. You will compare the values of the two different counters to see the impact of debouncing. Begin this exercise by creating a SystemVerilog module with the following name and top-level ports:

Debounce Counter

The first section of this module will include the debounce state machine and a counter. The purpose of this circuit is to accurately count the number of times a button has been pressed. Although this function sounds simple, it takes some effort to create a circuit to reliably count button presses. As shown in the figure below, there are four components associated with this section of the circuit:

• Synchronizer
• Debounce State Machine
• One-Shot pulse detector
• Counter

The first first part of this circuit is the “synchronizer” whose purpose is to synchronize the asynchronous button signal to the global system clock. Because this signal is not synchronized to the clock, it is possible that this signal could cause metastability on later circuitry including the debounce state machine. You should use two flip-flops to synchronize this signal as described in section 24.3 (the textbook uses only one flip-flop - you will need to use two because of the 100 MHz clock rate).

The second part of this circuit is the debounce state machine. Instance the your state machine and attach the output of the synchronizer to the input of your state machine. The output of your state machine is now a synchronized, debounced version of the button input.

The third part of this circuit is an “edge detector” circuit. The purpose of this circuit is to create a single clock cycle pulse when the debounced button signal transitions from a zero to a one. This single clock cycle pulse will be used to increment the button counter. If you attach the output of the debounce state machine directly into the input of your counter, the counter will increment continuously for as long as the button is pressed. The edge detector is added to make sure that the counter is only incremented once each time the button goes through a 0→1 transition.

An edge detector circuit can be implemented as a simple Mealy state machine as shown below. This state machine has two states: S0 and S1. In state S0, the state machine is waiting for a '1' on the input and will generate a '1' on the edge output on the transition from S0 to S1. In the S1 state, the state machine is waiting for a '0' on the input.

Note that this is the same thing as Figure 26.5, just represented in a different way.

Implement this edge detection circuit within your top level module.

The final part of this circuit is a counter that counts the button transitions. Create a 16-bit counter with an enable input. Attach the enable signal to the output of the edge detection circuit. The output from the counter will be displayed on the seven segment display. Use the down button to clear this counter.

Undebounced Counter

To demonstrate the behavior of the debounce circuit, a second counter is added to the top-level design which is incremented using a signal that is not debounced. The behavior of this counter will be contrasted with the debounced counter. You will use btnc as the input, btnd to clear, an edge detector, and a 16-bit counter, but you will not use a synchronizer or a debouncer.

Seven Segment Display

After creating your two button counters, instance your seven segment display and connect the seven segment display as described below:

• You will use all 8 digits of the display
• Do not turn on any digit points
• Attach the 16-bit value of the debounced counter to the lower 16 bits of the display
• Attach the 16-bit value of the undebounced counter to the upper 16 bits of the display

Simulation

After completing your top-level module, create a TCL script to simulate at least three “noisy” button presses on your top-level design. Verify that your design counts properly in these noisy button press situations.

Include a copy of your top-level SystemVerilog module

Exercise 3 Pass-off: Show a TA your top level code and your tcl script. Your simulation must show the expected behavior of your debounce module.

After successfully verifying your top-level module, create and add a .xdc file to the project and proceed with the implementation of your design.

Provide a summary of your synthesis warnings.

After successfully synthesizing your design, proceed with the implementation and bitstream generation of your design.

Indicate the number of Look-up Tables (LUT) and Input/Output (I/O) pins for your design.

Once the bitstream has been generated, download your bitstream to verify that it works correctly. After downloading your circuit, press the button several times to see if you can see a difference between the counter using the debounce state machine and the counter that does not use the debug state machine.

Summarize the results from your experiments comparing the debounced counter value to the non-debounced counter value.

• Pass offs for Exercises 1, 2, and 3
• Demonstrate your circuit operating on the FPGA board

How many hours did you work on the lab?

Please provide any suggestions for improving this lab in the future.

Here are some ideas for personal exploration in this laboratory:

• Experiment with different buttons to see if different buttons have different bounce characteristics
• Experiment with the switches and see if the switch has different bounce characteristics than the buttons
• Discuss the affect of not using the one shot circuit on your counter values
• Review the timing reports of your laboratory to see how fast it can run.

Describe your personal exploration activities

TA Notes and Feedback