CS 232: Project 3

Colby

CS Department

Course

Labs and Projects

Title image Fall 2021

Project 3: Reaction Timer

Due Monday 4 October 2021

Purpose: to start exploring sequential digital circuits by creating a reaction timer.

Tasks

Problem Statement: The purpose of this project is to build a circuit that will test a (potentially simulated) user's reaction time. At the heart foof the program is a state machine written in VHDL. You will also need your 7-segment display driver and a top-level file (VHDL for simulation or BDF for programming the board).

The circuit should work as follows. It begins in an idle state with two digits displaying 00. When the user hits the start button, the circuit should light up a red LED and wait for a period of time. Then the circuit should display a green LED and start counting (approximate milliseconds), showing the count on the two digits. When the user hits the reaction button, the count should stop and freeze. If the user hits the reaction button before the green LED lights up, the counter should show FF and the system should go back to the idle state. When the user hits the start button, the digit display should be set back to zeros.

  1. If you are using Quartus and the board, follow these instructions

    + (more detail)

    1. Setup

      Make a new subdirectory for your project. Copy your 7-segment driver file from project 2 (bdf or vhd) to your new project directory.

    2. Make a new Quartus project

      Start Quartus and then create a new project. Call it reaction (you are welcome to call it whatever you like, but I will refer to the project and the top level file as reaction).

      Open your 7-segment driver file, then select Project->Add to project. Then select File->Create/Update->Make symbol file.

    3. Define the state machine behavior

      1. Define the entity and port for the circuit

        Create a new VHDL file with an entity called timer and name the file timer.vhd. Insert the Moore state machine template. The timer circuit should have a clock, a reset, a start input, and a react input. It should have three outputs: one 8-bit std_logic_vector that sends the time (in ~ms) and two std_logic output signals that control the red and green LEDs.

      2. Define internal variables for the circuit

        Name the states for your state machine as sIdle, sWait, and sCount. Your circuit should initially be in the sIdle state. When the user hits the start button it should move to the sWait state, stay there for a while, and then move to the sCount state. When the user hits the react button, the sCount state should move back to the sIdle state. If the user hits the react button while the system is in the wait state, then the counter should be set to all 1s and the system should go back to the sIdle state.

        In order to both wait and count, you need an internal counter. To make this counter provide useful information to the user, we would like to report the count in milliseconds on two 7-segment displays, which means we would like and 8-bit counter to increment at a rate of 1KHz. But the clock we will use runs at 24 MHertz, which is too fast. To find bits that increase at a rate of approximately 1 KHz, we will report the bits 22 down to 15 from a counter that has at least 22 bits. Since we will also be using this counter to control the wait time, it needs to be able to count up to a number that has units of approximately seconds (so the user can wait somewhere between 0.5 and 2 seconds). Counting this high requires at least 27 bits. We will use 28.

        Create an internal signal of type unsigned with 28 bits. Put its declaration between the architecture statement and the begin, along with the state signal declaration. You'll also need to add use ieee.numeric_std.all to your header section above the entity statement.

          signal count : unsigned (27 downto 0);
      3. Define the behavior on reset

        Set up the reset condition so it activates on a '0'. It should set the state to sIdle and set the count to all zeros (28 of them). You can do this either with a double-quoted string of 28 zeros, or using a handle VHDL short-cut notation:

        			count <= (others=>'0');
      4. Define the behavior on a rising edge

        Set up the rest of the state machine. Use the count variable to both have the circuit wait a while and to count how long the user's reaction is. If you set count to what is effectively a large number and add one to it, you will eventually get to 0 (which is an easy test).

    4. Define the output behavior

      For the output, get rid of the second process statement in the template. Replace it with a few simple conditional signal assignments (remember, they all run in parallel all the time).

      1. Assign '1' to the green LED when state is equal to sCount and otherwise '0'.
      2. Assign a '1' to the red LED when state is equal to sWait, otherwise '0'.
      3. Assign a string of 8 zeros to mstime when the state is sWait, otherwise assign it bits 22 downto 15 of the internal counter. You'll need to cast it to a std_logic_vector using the function std_logic_vector().
    5. Connect the pieces of the circuit

      Create a new bdf file to be the top level file for your project. Call it reaction.bdf. Within it, place two instances of your 7-segment driver and one instance of the timer. Set up appropriate inputs and outputs for the circuit. (As an extension, make your top level file a VHDL file and use component and port map statements to connect the various elements.)

    6. Test your circuit

      Compile your circuit, assign pins, and then test it out. Document your circuit's behavior.

  2. If you are not using the boards, follow these instructions.

    + (more detail)

    1. Setup

      Make a new subdirectory for your project. Copy your 7-segment driver file from project 2 to your new project directory. You will also be creating a state machine (timer.vhdl) and a testbench (timertest.vhd)

    2. Define the state machine behavior

      1. Define the entity and port for the circuit

        We will use the Moore state machine template from Quartus. Download four_state_moore_machine.vhd, and save it as timer.vhd. Change the entity name to timer and the ports so that you had a clock, reset, start, and react inputs (each is a std_logic) and three outputs: one 8-bit std_logic_vector that sends the time (which counts clock cycles) and two std_logic output signals that control the red and green LEDs.

      2. Define internal variables for the circuit

        Name the states for your state machine as sIdle, sWait, and sCount. Your circuit should initially be in the sIdle state. When the user hits the start button it should move to the sWait state, stay there for a while, and then move to the sCount state. When the user hits the react button, the sCount state should move back to the sIdle state. If the user hits the react button while the system is in the wait state, then the counter should be set to all 1s and the system should go back to the sIdle state.

        In order to both wait and count, you need an internal counter. Create an internal signal of type unsigned with 8 bits. Put its declaration between the architecture statement and the begin, along with the state signal declaration. You'll also need to add use ieee.numeric_std.all to your header section above the entity statement.

          signal count : unsigned (7 downto 0);
      3. Define the behavior on reset

        Set up the reset condition so it activates on a '0'. It should set the state to sIdle and set the count to all zeros (8 of them). You can do this either with a double-quoted string of 8 zeros, or using a handle VHDL short-cut notation:

                   count <= (others=>'0');
      4. Define the behavior on a rising edge

        Set up the rest of the state machine. Use the count variable to both have the circuit wait a while and to count how long the user's reaction is. If you set count to what is effectively a large negative number and add one to it, you will eventually get to 0 (which is an easy test).

    3. Define the output behavior

      For the output, get rid of the second process statement in the template. Replace it with a few simple conditional signal assignments (remember, they all run in parallel all the time).

      1. Assign '1' to the green LED when state is equal to sCount and otherwise '0'.
      2. Assign a '1' to the red LED when state is equal to sWait, otherwise '0'.
      3. Assign a string of 8 zeros to mstime when the state is sWait, otherwise assign it the value of the internal counter. You'll need to cast it to a std_logic_vector using the function std_logic_vector().
    4. Write the testbench file and debug the project

      Make a new VHDL testbench file named timingtest.vhd using brighttest as a model. Set the times of the reset, start, and react key-button presses ('0') and releases ('1') to debug your state machine. Compile it with ghdl and use GtkWave to see the signals.

      Note: if you click on the plus sign next to the top-level module name, then it will show the component modules, so that you can examine wires in the components. Also, note that the state will not show up in the list. If you want to debug the state, I suggest making a separate signal that is a number, and have that signal change whenever the state changes.

    5. Demonstrate your working circuit

      Simulate good and bad user behavior. In your report, describe your strategy and demonstrate that your circuit is correct. Be sure to include at least one scenario in which the user presses reset, then start, then react after the green light goes on. And one case in which the user presses reset, then start, but reacts too soon.

Extensions

Report

Create a wiki page with your report. For each task, write a short description of the task, in your own words.

Handin

Give your wiki page the label cs232f21project3.

Put your bdf and VHDL files in a folder called project3 in your CS232 private subdirectory on the Courses server.

Stephanie R. Taylor © 2021