ARM project: Week 3
Mouse interface in Verilog

ARM project home | Tasks (weeks): 1 | 2 | 3 | 4


You may have noticed a few of the more interesting features on the Altera board. These include a VGA connector (used in the bonus track) and a PS/2 mouse or keyboard connector, which we are going to use now.

This week's task is to interface with a PS/2 serial mouse connected to the PS/2 connector on the Altera board and display the mouse status on the two 7-segment LEDs. In particular:

Test your mouse hardware

To test your mouse hardware download which contains mousetest.sof. This is the file normally produced by a successful compilation which the Programmer downloads to the board. To tell the Programmer to download mousetest.sof rather than the project you are working on, follow these steps:

  1. Open the Programmer, by choosing MAX+plus II | Programmer, or by clicking on the shortcut button, which looks like this:
  2. Switch on JTAG | Multi-Device JTAG chain. Turning it on may open a dialogue box, otherwise open it yourself using JTAG | Multi-Device JTAG chain setup.
  3. The white box in the centre of this dialogue box should be empty; if not, click on Delete All.
  4. Now click on Select Programming File and find mousetest.sof.
  5. Back in the Multi-Device JTAG chain setup dialogue box, click on Add to add the programming file to the list.
  6. Click on Detect JTAG chain info. If the Altera board is plugged in, and you have set up the dialogue box correctly, you will get a message telling you so.
  7. Close the dialogue box with OK, and click on Configure. This should download the data to the board, and the test will run. This test code indicates the direction that the mouse is traveling in on the LEDs. The decimal points on the LEDs indicate left and right mouse button presses. If your mouse doesn't appear to work then make sure that the mouse connector is firmly inserted and then download the test code again to reboot the mouse. If that doesn't fix it then please report the problem.
  8. Once you have successfully run a test don't forget to switch off JTAG | Multi-Device JTAG chain before continuing work on your project.


There are several useful modules that you can use for this practical. These are:

Please note that the inout statement above are not input statements! If you get this wrong it frequently crashes the MAX+plus II (sigh!).

Mouse data format

The full details of the PS/2 mouse and keyboard protocols and interface, as reverse-engineered and documented by Adam Chapweske, are very useful as a reference, but perhaps you'll appreciate a summary.

The mouse sends data to the "computer" (a.k.a. the Altera) in three bytes along the serial port.

The project code

The core of your program should be a state machine with three states—one state for each byte input. The state machine should change state each time a new byte is received. Each state will deal with decoding the corresponding input byte. Note that the first byte has a valid bit and you should remain in the same state until this valid bit is set.

Pin assignments

You haven't been given a .acf file for this workshop, so you'll have to make one yourself. In fact, the .acf file from week 1 will pretty much do, so download dice.acf to this week's project directory, and rename it to mouse.acf.

First open it using File | Open.... The first thing in the file (after Altera's copyright notice) is the assignments, but they are all for a chip called "dice". Our new chip is going to be called "mouse", so indicate this by changing CHIP dice to CHIP mouse.

Now close the .acf file. Make sure you have the right project name set, and open Assign | Pin/Location/Chip....

To add an input, type the new name into Node Name, and the pin number into Pin. Then click on Add, and then OK. You will need to add mclk on pin 30, and mdata on pin 31. Remember that they are actually inouts rather than just inputs.

There is no facility to rename an input. If you need to do that, you must first delete the old one (select it from the list and click Delete) and then add the new one.

If you experimented with the "testbed" module in week 1 you probably noticed how things can go subtly wrong (with little error reporting from the tools) when your pin assignments are incorrect. Therefore, before starting to program the mouse interface, it is a good idea to double-check that your pin assignments are correct. You may want to write a simple Verilog test program that exercises them: this would be laudable, but it is not a requirement. If you write such code, do not include it in your mini-report.

Tip of the day

You may find that the old compilations from previous workshops have consumed a lot of disk space with temporary files. It is safe to delete the files which you have not created (remember to keep your .v and .acf files!).

Question 1

W3-Q1: What is the measured resolution of the particular mouse you are using, in the traditional barbarian unit of "dots per inch"?

Question 2

W3-Q2: Are the overflow flags ever set? (To answer this you will probably want to edit the verilog, for example to display something related to the flags rather than to X,Y; but you don't have to submit this modified code.)

Weekly mini-report

Submit your mini-report before the last timetabled session of this week according to the usual rules (live demo, email submission, paper submission).

Be sure to include the answers to the two questions in a comment at the end of your source.

Please don't leave submission to the last moment: expect queues for the demonstrators' attention during the last hour. If you haven't submitted by the end of the Thursday session, your next chance is on Monday, but with a penalty of 3 marks per weekday (i.e. 6 marks). Worth avoiding.