Computer Laboratory > Teaching > Course material 2009–10 > ECAD Labs > ECAD Lab 1 - Pong > Up-Down counter

   8-bit binary up-down counter

Here you are going to create an 8-bit up-down counter module that will later be wired up to control the position of a bat on the screen.

Initially, create a new folder for this stage of the project. It is a good idea to do this for each stage, so you have a record of how the project progressed through each stage. Into this folder, copy, don't move, all the verilog (*.v) files from the previous projects directory, together with the DE2_Pin_Assignments.csv file. Create a new project in this folder by following the wizard again. Check this has worked by compiling the project and running it. The output should be exactly the same as the previous exercise. Do not forget to import the pin assignments again since you have created a new project.

Once you have done that, create a new Verilog file by selecting File | New and double click on Verilog HDL File in the dialog box that appears. In this file, create a module named counter that has 3 inputs - a clock, a signal for counting up, and a signal for counting down; together with an 8-bit output which is the current counter value. The signal inputs will be connected to the KEY inputs of the main module, which are active-low. This needs to be remembered when connecting the signals to the module (the module should count up when its input for counting up is held high, i.e. you should invert the KEY before sending it to the counter module, thereby removing board specific characterisitcs from the general purpose counter module).

When you have created the counter module, instantiate it in the top-level module (pong), wiring it in such a way so pressing KEY[0] decrements the counter and pressing KEY[1] increments it. The counter value should be shown on HEX1 and HEX0. The clock input should be wired up to the video_clock and the inputs enabled every time start_of_frame=1 (from the params module) so the counter only increments at each screen refresh (e.g. AND the count_up_enable with start_of_frame). You may find the following (incomplete) code useful for displaying values on the 7-segment displays:

    module hex2leds(
        input [3:0] hexval,
        output [6:0] ledcode

        reg [6:0] lc;
        assign ledcode = ~lc;

        always @(*)
            case (hexval)
		4'h0: lc <= 7'b0111111;
                4'h1: lc <= 7'b0000110;
 		4'h2: lc <= 7'b1011011;
		4'h3: lc <= 7'b1001111;
		4'h4: lc <= 7'b1100110;
		4'h5: lc <= 7'b1101101;
		4'h6: lc <= 7'b1111101;
		4'h7: lc <= 7'b0000111;

The diagram below gives the mapping of bits to the corresponding segment of the display. Note bit 0 is the LSB, bit 6 the MSB.

When you have this working, instantiate the module a second time so that pressing KEY[3] increments it, and pressing KEY[2] decrements it, with the counter value being displayed on HEX3 and HEX2. The counters (with slight modifications) will eventually be used to control the positions of the bats on the screen.


At this point it is useful to mention SignalTap. Debugging hardware is alot different to software, you can't single-step through the code as you may do when developing a Java program. SignalTap allows you to visualise the signals that are present in the hardware, and then see where the problems are occuring. If you are interested in learning more, or would like to have a go at debugging any aspect of the project, then the SignalTap tutorial provides a good place to start.