The 3 Button Serial Mouse mini-HOWTO

6. Switching a Mouse to 3-Button Mode

Even cheap mice can also work under the Mouse Systems protocol, with all three buttons working. The trick is to get the mouse to think it's a Mouse Systems one, something you rarely see in your instructions.

Before you power up your computer, hold down the left mouse button (and keep it held down until it has booted to be on the safe side).

When the mouse first gets power, if the left button is held down it switches into Mouse Systems mode. A simple fact, but not always publicised. Note that a soft reboot of your computer may not cut the mouse power and therefore may not work. There are a number of other ways of switching the mode, which may or may not work with your particular mouse. Some of these are less drastic than rebooting your computer, two are more so!

If your computer is get-at-able you can unplug the mouse and plug it back in with the button held down (although you shouldn't normally plug things in to a live computer, the RS232 spec says it is OK).
You may be able to reset the mouse by typing echo "*n" > /dev/mouse, which should have the same effect as unplugging it. Hold the left button down for Mouse Systems mode, not for Microsoft. You could put this in whatever script you use to start X up.
Bob Nichols (rnichols@interaccess.com) has written a small c program to do the same thing, which may work if echo "*n" does not (and vice versa). You can find a copy of his source code at http://kipper.york.ac.uk/src/fix-mouse.c
Someone has reported that the `ClearDTR' line in the Xconfig is enough to switch their mouse into Mouse Systems mode.
If you are brave enough, open the mouse up (remember that this will invalidate your warranty) and have a look inside. In some cases, the mouse may have a switch inside, for some strange reason known only to the manufacturer. More likely on the cheap mice is a jumper which you can move. The switch or jumper may have the same effect as a `MS/PC' switch described in the Switched Mice section above. You may find that the circuit board is designed for a switch between 2 & 3 buttons, but it hasn't been fitted. It will look something like:
----------- | o | o | o | SW1 ----------- 1 2 3
Try linking pins 1-2 or 2-3, and see if it changes the behaviour of the mouse. If it does, you can either fit a small switch, or solder across the contacts for a quick and permanent solution.
Another soldering solution which might be a last-resort for mice which don't understand MouseSystems at all, from Peter Benie ( pjb1008@chiark.chu.cam.ac.uk). If the middle button's switch is double-pole, connect one side of the switch to the left button's switch, and the other side to right button's switch. If it's not a double pole switch then use diodes rather than wire. Now, the middle button pushes the left and right buttons down together. Select ChordMiddle in the XF86Config and you have a working middle button.
The ultimate recourse with the soldering iron was first described to me by Brian Craft ( bcboy@pyramid.bio.brandeis.edu). Two common generic mouse chips are the 16 pin Z8350, and the 18 pin HM8350A. On each of these chips, one pin controls the mode of the chip, as follows.
Pin 3 Mode ----- ---- Open Default Microsoft. Mouse Systems if a button is held on power-up. GND Always Mouse Systems. Vdd Always Microsoft.
(Pins are numbered as follows:)
____ pin1 -| \/ |- pin2 -| |- pin3 -| |- -| |- -| |- -| |- -| |- pin8 -|____|-
(This info comes courtesy of Hans-Christoph Wirth, and Juergen Exner, who posted it to de.comp.os.linux.hardware) You can solder a link between pin 3 and gnd, which will fix the mouse into MouseSystems mode.
- Peter Fredriksson ( peterf@lysator.liu.se) has tried the SYSGRATION SYS2005 chip, and found that linking Pin 3 to Gnd forced Mouse System mode.
- Uli Drescher ( ud@digi.ruhr.de) confirms it works on an HN8348A chip; Ben Ketcham ( bketcham@anvilite.murkworks.net) confirms the HM8348A (Pin 9 is Gnd).
- Urban Widmark ( ubbe@ts.umu.se) says the same applies to the EC3567A1 chip, where Pin 8 is ground. I've tried it as well and it works fine.
- Timo T Metsala ( metsala@cc.helsinki.fi) has found that on the HT6510A chip pin 3 is mode select, pin 9 is Gnd. The same works for the HT6513A chip. Holtek also make HT6513B and HT6513F chips - on these, pin 8 is Gnd.
- Robert Romanowski ( robin@cs.tu-berlin.de) says pin 3 - pin 8 (Gnd) works on an EM83701BP chip too.
- Robert Kaiser ( rkaiser@sysgo.de) confirms that pin 3 - Gnd works on a EC3576A1 chip too.
- Sean Cross ( secross@whidbey.com) found it was pin 2 - pin 7 (Gnd) on a HM8370GP chip.
- Peter Fox ( fox@roestock.demon.co.uk) used pin 3 - pin 8 on a HM8348A chip.
- Jon Klein ( jbklein@mindspring.com) found pin 3 - pin 9 did the trick for a UA5212S chip.

As an alternative to the above soldering methods, you can get the mouse to hold it's own button down when booting: this circuit from Mathias Katzer.


         -----                       
      ---  R  ---------O------ + Supply
     |   -----   |        |                     C = 100nF capacitor
     |           | E      |                     R = 100kOhm
     |       __ /         |                     T = BC557 transistor
     |      /  \          O
     |   B | #V | T         /
     |-----|-#  |          /   Left button switch of the mouse
     |     | #\ |         O
     |      \__/          |
    ---         \  C      |
    --- C        ------O----------> (to somewhere deep inside the mouse)
     |
    ###  Ground

The test mouse was a no-name model MUS2S - whether this works in other mice depends on the circuit of the mouse; if the switch is connected to ground and not to +Supply, an npn-transistor like the BC547 should work; R and C have to be swapped then, too.

So there you have it, the choice is yours. Stick with the default Microsoft two buttons, or work out how to switch the mode and set X up to take advantage of this.

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6. Switching a Mouse to 3-Button Mode