In this lab you will create a digital stopwatch. As shown in the video below, the stopwatch will be displayed on the eight-digit seven-segment display.

• Digits 7:6 will display minutes, digits 5:4 will display seconds, and digits 3:0 will display ten-thousandths of a second.
• Switch 0 will start and stop the timer.
• Button CPU_RESET will reset the timer.

• Create a sequential circuit using multiple counters.
• Practice with SystemVerilog hierarchy and parameters.

Read about the Seven Segment Controller module that is provided to you. The module cycles through the eight seven-segment digits, lighting up one at a time.

• Why doesn't the Seven Segment Controller just display all eight digits at the same time?
• How long (in microseconds) is each digit illuminated for?

At the heart of your stopwatch will be a counter module for each of the eight digits. The counter should be a modulus counter, meaning it counts up to some predetermined value, then rolls over to 0 and continues counting.

You will use a SystemVerilog parameter, MOD_VALUE to indicate the modulus value. The counter should reach (MOD_VALUE-1) and then roll over to 0. This approach will allow us to use this module for digits that count 0-9, as well as digits that count 0-5. Consult the Parameterization in Dataflow SystemVerilog section in the textbook (Chapter 14) for an example on adding parameters to your SystemVerilog modules.

What you need to do:

• Create a Vivado project.
• Write the SystemVerilog for the mod_counter module.
  • NOTE: the vast majority (>90%?) of students write the logic for their rolling_over wrong the first time. Why? Go re-read the description for this signal above a third time. Exactly what is the logic condition for this signal? Does it involve the 'clk' signal and a register or is it purely combinational logic? If you put the code to generate this signal this inside an always_ff block will it generate a register or will it generate combinational logic? What is it that you really want?
• Create a tcl simulation script, and verify that your counter is working. Make sure your simulation is thorough; for example, check that the counter only counts when increment is high, and that the rolling_over output is high only in the appropriate condition. Also, don't forget to do the Tcl file in this general order: a) set up the clocking, b) reset the design and simulate a few cycles, and then c) exercise the rest of your counter functionality.

Include your Tcl simulation script in your lab report.

Pass-Off: Show the TA your simulation and explain how you tested the correctness of your module.

In this exercise you will create the stopwatch module which will consist of eight instances of your mod_counter module . Each of these instances will be responsible for generating the value for one digit of the display. In addition, you will create one counter which will serve as a timer module to time the refreshing of your 7-segment display. The figure below shows the overall outline of your stopwatch module.

Your stopwatch will contain eight copies of your mod_counter. Each counter's rolling_over output is fed into the increment signal for the next most significant digit, as shown below. You will need to declare those intermediate signals as local signals in your SystemVerilog code.

Make sure you set the roll-over parameter for each of your mod_counter instances. The most significant two digits represent minutes, and the next two digits represent seconds; both should roll over at 59. The lower four digits represent fractions of a second, and should behave accordingly.

Note the module in the upper left - this is a counter that rolls over every 0.0001s. When this counter rolls over, it should generate a single cycle pulse on its output. That pulse then is the input to the increment signal for the least significant digit of the 8 digits. This 0.0001s counter should increment every cycle that the run input is high, and reset to 0 if the reset input is high (where reset takes precedence).

Given that the system clock is 100MHz, what value does your counter need to count to in order to roll over every 0.0001s?

To answer this question you should a) compute how long (in seconds) one clock period is for a 100MHz clock. Then calculate how many of those will fit into a 0.0001s interval. That is the maximum count value for this counter. Also, once you compute this you should then be able to calculate how many bits wide the counter should be. Remember: you can only count from 0-15 using 4-bits, to count from 0-1023 takes 10 bits, to count from 0-2047 takes 11 bits, and so on. Once you know the maximum count value and the number of bits for the counter, you can design it.

You have two ways to design this timer, you can choose which to use.

1. The first way is to simply design this counter very similarly to how you designed the mod_counter module. In fact, you could largely just copy the code and change the number of bits in the counter.
2. The second way is to modify your mod_counter module to be parameterized for width and then just instance another copy of it. Note that it already is parameterized with a MOD_VALUE parameter for its maximum count. If you simply add a second parameter to parameterize number of bits for the counter signal you can then just instance another copy of your mod_counter design for this timer module.
   • The textbook example on parameterization shows how to parameterize signal widths.
   • To add a second parameter in the module definition, you just separate it from the first using a comma like this: #(parameter PARAM1 = val1, PARAM2 = val2).
   • Then, when you instance it, you add a second value inside the parens like this: mod_counter #(10, 495) TIMER (clk, run, …).

Now that you have all the blocks designed, create a stopwatch module and instance all of the needed modules inside it.

Some things to remember:

1. You will need to declare local signals for those wires in the diagram below that are not input or output signals.
2. Note that the your timer module output ('count') is not tied to anything. You will still need to declare a local signal for it to wire up to the timer module, but that signal will not connect to anything else in your stopwatch module.

Create a tcl simulation script to simulate the behavior of your stopwatch module. You will likely need to simulate for several milliseconds to check that the lower digits are functioning correctly. It will take too long to simulate the upper digits, so you will have to wait until next exercise to test it on the board. In your simulation, make sure to check that:

• The digitX outputs increment appropriately.
• Your least significant digit increments every 0.0001s.
• The stopwatch only runs when the run input is high.
• The digits roll over correctly.
• The rest works after it has counted for some time.

Include your Tcl simulation script in your lab report.

Pass-Off: Show the TA your simulation and explain how you tested the correctness of your module.

In this exercise you will create the top-level module and test your stopwatch on the board.

Create the top-level module as follows:

Your top module should:

• Instantiate both your stopwatch module, as well as a SevenSegmentControl module.
• Connect your digit values output from the stopwatch module to the dataIn input of the SevenSegmentControl.
• Turn on the appropriate decimal point.
• Note: The CPU_RESETN button is active-low. This means it behaves differently than the other buttons you have used: it is a 0 when pressed and a 1 otherwise. You will need to invert the signal when connecting it to your stopwatch and SevenSegmentControl modules.

Be sure to include an appropriate constraints file:

• Note: For this lab, and all subsequent labs that use the clk pin, you should also uncomment two lines near the top that refer to the clock. One connects the clock, the line after it tells informs Vivado that the clock runs at 100MHz.

Show the TAs your stopwatch working on the board.

Submit your SystemVerilog modules as part of the lab report on Learning Suite. (Make sure your SystemVerilog conforms to the lab SystemVerilog coding standards).

How many hours did you spend on this lab?

Describe any problems or challenges you had with the lab.