Using a Serial Mouse
to Create a Position Encoder

As we discussed in class, the rotating ball of a mouse turns a set of two position encoders, one for the x-movement and one for the y-movement. Using the electronics of the mouse, these position encoders (along with the two button inputs) can be used for feedback on the position of motors or user-controlled knobs. All that needs to be done is to decipher the serial data that is sent to the PC. (Note: The data interface for the PS/2 mouse is very similar and the procedures discussed here can be used for that port too.)

Procedure

To begin with, take a serial mouse and connect it to one of the free serial ports on the back of the computer. Make a note of which one you use because this information will be needed when we set up the interface to Linux.

Not all serial mice are created equal. For example, a mouse might have a third button and therefore have a slightly different protocol. We will be using a serial terminal program called minicom to access the data that comes from the mouse you are using at your PC to verify what its protocol is.

Open minicom by typing minicom from the command line. Once the program opens (usually after making some announcement about initializing the modem) follow the following procedures to set it up so that it can capture the mouse data.

• Pressing CTRL-A Z opens the configuration/special key menu. From that menu, press the letter O to configure minicom.
• Scroll to the menu item Serial port setup and press Enter. Verify that the Serial Device is set to the port you connected your mouse to. /dev/ttyS0 denotes the first serial port and /dev/ttyS1 denotes the second serial port.
• Before exiting this configuration screen, you should also press E to edit the communications parameters Bps/Par/Bits. The serial mouse communicates at 1200 baud, 7 data bits, no parity, and 1 stop bit. Press B, L, U, and W to set the communications parameters this way. Press enter to close this window.
• Before exiting this configuration screen, you should also verify that both Hardware Flow Control and Software Flow Control are set to No. Press F or G to make sure they are set this way. From the configure minicom screen, press enter to get to the top configuration screen. Scroll down to Exit and press enter to close minicom configuration menu.

At this point, if you move the mouse you should see a jumble of characters appear on the screen. The next step is to capture those characters to a file so we can determine the mouse's protocol.

• Press CTRL-A Z to open the configuration/special key menu again. This time, press E to to turn local echo on. This will make it so that anything typed on the keyboard will appear in the display. We will be using this only to place markers in the file so we know what the mouse was doing at each point.
• Press CTRL-A Z to open the configuration/special key menu again. This time, press L to begin capturing data to a file. A prompt should appear asking you to enter a file name to capture the data to. Enter a file name then press Enter.

Now we are ready to begin capturing data to analyze the mouse protocol from.

• Remove the cover to the mouse. This should be made possible by unscrewing the three or four screws from the button of the mouse. While you're doing this, some characters may appear on the screen indicating movement of one of the sensors.
• Type a string similar to "Pressing left button". This string should appear on the monitor. Press the left button multiple times to capture a number of characters.
• Type a string similar to "Pressing right button". Press the right button multiple times to capture a number of characters.
• Type a string similar to "Changing X direction". With your finger, turn the wheel that would spin if the mouse were moved horizontally.
• Type a string similar to "Changing Y direction". With your finger, turn the wheel that would spin if the mouse were moved vertically.
• Press CTRL-A Z to open the configuration/special key menu again. Press L to close the file that you are capturing data to and select close from the menu that appears.
• Press CTRL-A Z then press Q to exit minicom.

The data you viewed and captured from minicom was shown to you in ASCII or text format. The mouse, however, sent the data to you in binary format. Therefore, we need to use a binary editor to view the data in the captured file. In the lab, you should have access to a package I downloaded called shed. Shed, which stands for "simple hex editor", will allow us to open the captured file and view its contents in ASCII, hexadecimal, decimal, octal, and binary. (You can download your own copy from the URL http://sourceforge.net/projects/shed.) The following instructions show how to install shed from the downloaded package.

• Once you've copied the shed file to your computer, uncompress it with tar -xvzf shed-1.10.tar.gz.
• To install it, move into the directory shed-1.10 and run the commands ./configure and make. At this point, you should have an executable file shed in the directory.
• Run shed by typing ./shed filename where filename is the name of the file you captured from minicom.
• At this point, you should be looking at columns of values similar to those shown in the table below.
  
  

  offs	asc	hex	dec	oct	bin
  0000:	b	62	098	142	01100010
  0001:	h	68	104	150	01101000
  0002:	-	2D	045	055	00101101

  
  
  The "offs" column represents the offset into the file where the character occurred. The "asc" column shows how the character looks as text. The last column, "bin" gives the binary value. This is the column you will be most interested in to decipher the mouse data.
• Use the ASCII values to see where you began pressing the left button. See if the binary values match what you expected based on the information from our lecture on the serial mouse data. Repeat this for the right mouse button presses and the x and y movements.

At this point, you should know what the protocol for your mouse is. Now we can begin writing the code for the mouse. The remainder of the lab involves accessing the serial port, capturing the data from the mouse, and indicating the number of turns the encoders take. To access the serial port, we will use the same file-type I/O port interface that we covered when we discussed device drivers. The specifics for this method can be found in Michael Sweet's article, "Serial Programming Guide for POSIX Operating Systems".

When prompted either with a keypress from the user or a timed output, your program should be able to give the number of pulses in both the x-direction and the y-direction since the last output. Good luck!

This lab was created by David Tarnoff for exclusive use by his sections of CSCI 4956 at ETSU.