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<title>PS/2 Mouse Interfacing</title>
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<small><b><font face="Arial,Helvetica"><font size="+3"><small>The PS/2
Mouse Interface</small></font></font></b></small><br>
<center></center>
<center>
<hr size="1" width="400" align="left" noshade="noshade"></center>
<br>
<font face="Arial,Helvetica">Source: <a href="http://www.computer-engineering.org/">http://www.Computer-Engineering.org</a></font><br>
<font face="Arial,Helvetica">Author: Adam Chapweske<br>
Last Updated: 04/01/03<br>
<br>
<br>
</font><b>Legal Information:</b><br>
<br>
All information within this article is provided "as is" and without any
express or implied warranties, including, without limitation, the implied
warranties of merchantibility and fitness for a particular purpose. <br>
<br>
This article is protected under copyright law. This document may
be copied only if the source, author, date, and legal information is included.<br>
<br>
<b>Abstract:</b><br>
<br>
This article attempts to explain every aspect of the PS/2 mouse interface
including the physical and electrical interface, low-level protocol, modes
of operation, commands, and extensions. All code samples involving
the mouse encoder are written in assembly for <a href="http://www.microchip.com/">Microchip's</a> PIC microcontrollers.
All code samples related to the auxiliary device controller (keyboard
controller) are written in x86 assembly<br>
<br>
<b>General Description:</b><br>
<br>
There are many types of pointing devices available for modern PCs, including
mice, trackballs, touchpads, electronic whiteboards, etc. Virtually
all of these pointing devices communicate with a computer using one of two
interfaces: Universal Serial Bus (USB) or the PS/2 Mouse Interface. See
the following table for a comparison:<br>
<blockquote>
<table width="500" border="1" cellpadding="2" cellspacing="0">
<tbody>
<tr>
<td valign="top" width="33%" align="center"><br>
</td>
<td valign="top" width="33%" align="center"><b>USB (v1.1)</b><br>
</td>
<td valign="top" align="center"><b> PS/2 Mouse Interface
</b><br>
</td>
</tr>
<tr>
<td valign="top" width="33%" align="center">Number of Devices
Supported<br>
</td>
<td valign="top" width="33%" align="center">Up to 127<br>
</td>
<td valign="top" align="center">One<br>
</td>
</tr>
<tr>
<td valign="top" width="33%" align="center">Maximum Data Rate<br>
</td>
<td valign="top" width="33%" align="center">12 Mbps<br>
</td>
<td valign="top" align="center">40 kbps<br>
</td>
</tr>
<tr>
<td valign="top" width="33%" align="center">Power (max)<br>
</td>
<td valign="top" width="33%" align="center">5V @ 500mA<br>
</td>
<td valign="top" align="center">5V @ 100mA<br>
</td>
</tr>
<tr>
<td valign="top" width="33%" align="center">Hot-Pluggable?<br>
</td>
<td valign="top" width="33%" align="center">Yes<br>
</td>
<td valign="top" align="center">No<br>
</td>
</tr>
<tr>
<td valign="top" width="33%" align="center">Documentation<br>
</td>
<td valign="top" width="33%" align="center">Well-documented: <a href="http://www.usb.org/">http://www.usb.org</a><br>
</td>
<td valign="top" align="center">Out-of-print <br>
(IBM Tech Reference)<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
Older pointing device interfaces include the Apple Desktop Bus (ADB),
RS-232 serial port, and the bus mouse interface. These are obsolete
and are not covered in this article.<br>
<br>
The PS/2 mouse interface originally appeared in IBM's "Personal System/2"
computers in the late 80's. It still remains a widely-supported interface
for the sake of constantly maintaining backward compatibility. However,
USB has quickly caught on these last few years and will eventually replace
the PS/2 mouse interface entirely.<br>
<br>
The PS/2 mouse interface uses a bidirectional serial protocol to transmit
movement and button-position data to the computer's auxiliary device controller
(keyboard controller). The computer, in turn, may send a number of
commands to the mouse to set the report rate, resolution, reset the mouse,
disable the mouse, etc. The computer also provides the mouse with an
overload-protected 5V power supply.<br>
<p><b>Electrical Interface / Protocol:</b> </p>
<p>The PS/2 mouse uses the same protocol as the PS/2 keyboard (aka AT keyboard).
Click <a href="http://www.computer-engineering.org/ps2protocol">here</a> for detailed information on this
protocol. </p>
<p><b>Inputs, Resolution, and Scaling:</b> </p>
<p>The standard PS/2 mouse interface supports the following inputs: X (right/left)
movement, Y (up/down) movement, left button, middle button, and right button.
The mouse reads these inputs at a regular freqency and updates various
counters and flags to reflect movement and button states. There are
many PS/2 pointing devices that have additional inputs and may report data
differently than described in this document. One popular extension
I cover later in this document is the Microsoft Intellimouse, which includes
support for the standard inputs as well as a scrolling wheel and two additional
buttons. </p>
<p>The standard mouse has two counters that keep track of movement: the X-movement
counter and the Y-movement counter. These are 9-bit 2's complement
values and each has an associated overflow flag. Their contents, along
with the state of the three mouse buttons, are sent to the host in the form
of a 3-byte movement data packet (as described in the next section.)
The movement counters represent the amount of movement that has occurred
since the last movment data packet was sent to the host (ie, they do not
represent absolute positions.) </p>
<p>When the mouse reads its inputs, it records the current state of its buttons
and checks for movement. If movement has occurred it increments (for +X
or +Y movement) or decrements (for -X or -Y movement) its X and/or Y movement
counters. If either of the counters has overflowed, it sets the appropriate
overflow flag. </p>
<p>The parameter that determines the amount by which the movement counters
are incremented/decremented is the <i>resolution</i>. The default resolution
is 4 counts/mm and the host may change that value using the "Set Resolution"
(0xE8) command. </p>
<p>There is a parameter that does not effect the movement counters, but does
effect the reported<a href="#Footnotes">(1)</a> value of these counters.
This parameter is <i>scaling</i>. By default, the mouse uses 1:1
scaling, which has no effect on the reported mouse movement. However,
the host may select 2:1 scaling by sending the "Set Scaling 2:1" (0xE7)
command. If 2:1 scaling is enabled, the mouse will apply the following
algorithm to the counters before sending their contents to the host: <br>
</p>
<blockquote>
<table width="300" border="1" cellpadding="2" cellspacing="0">
<tbody>
<tr>
<td valign="top" align="center">Movement Counter<br>
</td>
<td valign="top" align="center">Reported Movement<br>
</td>
</tr>
<tr>
<td valign="top" align="center">0<br>
</td>
<td valign="top" align="center">0<br>
</td>
</tr>
<tr>
<td valign="top" align="center">1<br>
</td>
<td valign="top" align="center">1<br>
</td>
</tr>
<tr>
<td valign="top" align="center">2<br>
</td>
<td valign="top" align="center">1<br>
</td>
</tr>
<tr>
<td valign="top" align="center">3<br>
</td>
<td valign="top" align="center">3<br>
</td>
</tr>
<tr>
<td valign="top" align="center">4<br>
</td>
<td valign="top" align="center">6<br>
</td>
</tr>
<tr>
<td valign="top" align="center">5<br>
</td>
<td valign="top" align="center">9<br>
</td>
</tr>
<tr>
<td valign="top" align="center">N > 5<br>
</td>
<td valign="top" align="center">2 * N<br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<center>
<div align="center"> </div>
</center>
<center></center>
<center>
<div align="center"> </div>
</center>
<p><b><br>
Movement Data Packet:</b> </p>
<p>The standard PS/2 mouse sends movement/button information to the host
using the following 3-byte packet <a href="#Footnotes">(4)</a>:
</p>
<blockquote>
<table width="650" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td> <br>
<font face="Arial,Helvetica"><font size="-1">Byte 1 </font></font></td>
<td>
<table width="100%" border="0" cellpadding="0" cols="8">
<tbody>
<tr>
<td>
<center>Bit 7</center>
</td>
<td>
<center>Bit 6</center>
</td>
<td>
<center>Bit 5</center>
</td>
<td>
<center>Bit 4</center>
</td>
<td>
<center>Bit 3</center>
</td>
<td>
<center>Bit 2</center>
</td>
<td>
<center>Bit 1</center>
</td>
<td>
<center>Bit 0</center>
</td>
</tr>
</tbody>
</table>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="8">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y overflow</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X overflow</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y sign
bit</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X sign
bit</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Always
1</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Middle
Btn</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Right
Btn</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Left
Btn</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 2</font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X Movement</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 3 </font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y Movement</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</blockquote>
The movement counters are 9-bit 2's complement integers, where the
most significant bit appears as a sign bit in Byte 1 of the movement data
packet. These counters are updated when the mouse reads its input and finds
movement has occurred. Their value is the amount of movement that has occurred
since the last movement data packet was sent to the host (ie, after a packet
is sent to the host, the movement counters are reset.) The range of values
that can be expressed by the movement counters is -255 to +255. If this
range is exceeded, the appropriate overflow bit is set.
<p>As I mentioned earlier, the movement counters are reset whenever a movement
data packet is successfully sent to the host. They are also reset after
the mouse receives any command from the host other than the "Resend" (0xFE)
command. </p>
<p><b>Modes of Operation:</b> </p>
<p>Data reporting is handled according to the mode in which the mouse is
operating. There are four standard modes of operation: </p>
<ul>
<li> <i>Reset</i> - The mouse enters Reset mode at power-up or after
receiving the "Reset" (0xFF) command. </li>
<li> <i>Stream</i> - This is the default mode (after Reset finishes
executing) and is the mode in which most software uses the mouse.
If the host has previously set the mouse to Remote mode, it may re-enter
Stream mode by sending the "Set Stream Mode" (0xEA) command to the mouse. </li>
<li> <i>Remote</i> - Remote mode is useful in some situations and may
be entered by sending the "Set Remote Mode" (0xF0) command to the mouse. </li>
<li> <i>Wrap</i> - This mode isn't particularly useful except for testing
the connection between the mouse and its host. Wrap mode may be entered
by sending the "Set Wrap Mode" (0xEE) command to the mouse. To exit
Wrap mode, the host must issue the "Reset" (0xFF) command or "Reset Wrap
Mode" (0xEC) command. If the "Reset" (0xFF) command is recieved,
the mouse will enter Reset mode. If the "Reset Wrap Mode" (0xEC)
command is received, the mouse will enter the mode it was in prior to Wrap
Mode.</li>
</ul>
(Note: The mouse may also enter "extended" modes of operation, as described
later in this document. However, this is not a feature of the standard
PS/2 mouse.)
<p><b>Reset Mode:</b> </p>
<p>The mouse enters reset mode at power-on or in response to the "Reset"
(0xFF) command. After entring this mode, the mouse performs a diagnostic
self-test referred to as BAT (Basic Assurance Test) and sets the following
default values: </p>
<ul>
<li> Sample Rate = 100 samples/sec</li>
<li> Resolution = 4 counts/mm</li>
<li> Scaling = 1:1</li>
<li> Data Reporting Disabled</li>
</ul>
It then sends a BAT completion code of either AAh (BAT successful)
or FCh (Error). The host's response to a completion code other than AAh
is undefined.
<p>Following the BAT completion code (AAh or FCh), the mouse sends its device
ID of 00h. This distinguishes it from a keyboard, or a mouse in an extended
mode. I have read documents saything the host is not <i>supposed</i> to
transmit any data until it receives a device ID. However I've found
that some BIOS's will send the "Reset" command immediately following the
0xAA received after a power-on reset. </p>
<p>After the mouse has sent its device ID to the host, it will enter Stream
Mode. Note that one of the default values set by the mouse is "Data
Reporting Disabled". This means the mouse will not issue any movement
data packets until it receives the "Enable Data Reporting" command. </p>
<p><b>Stream Mode:</b> </p>
<p>In stream mode, the mouse sends movement data when it detects movement
or a change in state of one or more mouse buttons. The maximum rate at
which this data reporting may occur is known as the <i>sample rate</i>.
This parameter ranges from 10 samples/sec to 200 samples/sec. Its default
value is 100 samples/sec and the host may change that value by using the
"Set Sample Rate" command. Stream mode is the default mode of operation
following reset. </p>
<p><b>Remote Mode:</b> </p>
<p>In this mode the mouse reads its inputs and updates its counters/flags
at the current sample rate, but it does not automatically issue data packets
when movement has occured. Instead, the host must poll the mouse using
the "Read Data" command. Upon receiving this command the mouse will
send a single movement data packet and reset its movement counters.</p>
<p> </p>
<p><b>Wrap Mode:</b> </p>
<p>This is an "echoing" mode in which every byte received by the mouse is
sent back to the host. Even if the byte represents a valid command, the
mouse will not respond to that command--it will only echo that byte back
to the host. There are two exceptions to this: the "Reset" command and "Reset
Wrap Mode" command. The mouse treats these as valid commands and does not
echo them back to the host. </p>
<p><b>Intellimouse Extensions:</b> </p>
<p>A popular extension to the standard PS/2 mouse is the Microsoft Intellimouse.
This includes support for a total of five mouse buttons and three axises
of movement (right-left, up-down, and a scrolling wheel). These additional
features require the use of a 4-byte movement data packet rather than the
standard 3-byte packet. Since standard PS/2 mouse drivers cannot
recognize this packet format, the Microsoft Intellimouse is required to
operate exactly like a standard PS/2 mouse unless it knows the drivers
support the extended packet format. This way, if a Microsoft Intellimouse
is used on a computer which only supports the standard PS/2 mouse, the
Microsoft Intellimouse will still function, except for its scrolling wheel
and 4th and 5th buttons. </p>
<p>After power-on or reset the Microsoft Intellimouse operates just like
a standard PS/2 mouse (ie, it uses a 3-byte movement data packet, responds
to all commands in the same way as a standard PS/2 mouse, and reports a
device ID of 00h.) To enter "scrolling wheel" mode, the host sends
the following command sequence: </p>
<blockquote>Set sample rate 200 <br>
Set sample rate 100 <br>
Set sample rate 80</blockquote>
The host then issues the "Get device ID" command and waits for a response.
If a standard PS/2 mouse (ie, non-Intellimouse) is attached, it will respond
with a device ID of 00h. In this case, the host will recognize the
fact that the mouse does have a scrolling wheel and will continue to treat
it as a standard PS/2 mouse. However, if a Microsoft Intellimouse
is attached, it will respond with an ID of 03h. This tells the host
that the attached pointing device has a scrolling wheel and the host will
then expect the mouse to use the following 4-byte movement data packet:
<blockquote>
<table width="650" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td> <br>
<font face="Arial,Helvetica"><font size="-1">Byte 1 </font></font></td>
<td>
<table width="100%" border="0" cellpadding="0" cols="8">
<tbody>
<tr>
<td>
<center>Bit 7</center>
</td>
<td>
<center>Bit 6</center>
</td>
<td>
<center>Bit 5</center>
</td>
<td>
<center>Bit 4</center>
</td>
<td>
<center>Bit 3</center>
</td>
<td>
<center>Bit 2</center>
</td>
<td>
<center>Bit 1</center>
</td>
<td>
<center>Bit 0</center>
</td>
</tr>
</tbody>
</table>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="8">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y overflow</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X overflow</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y sign
bit</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X sign
bit</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Always
1</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Middle
Btn</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Right
Btn</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Left
Btn</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 2</font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X Movement</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 3 </font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y Movement</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 4</font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Z Movement</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
Z Movement is a 2's complement number that represents the scrolling
wheel's movement since the last data report. Valid values are in the
range of -8 to +7. This means the number is actually represented only by
the least significant four bits; the upper four bits act only as sign extension
bits.</blockquote>
To enter "scrolling wheel + 5 button" mode, the host sends the following
command sequence:
<blockquote>Set sample rate 200 <br>
Set sample rate 200 <br>
Set sample rate 80</blockquote>
The host then issues the "Get device ID" command and waits for a response.
A Microsoft Intellimouse will respond with a device ID of 04h, then use
the following 4-byte movement data packet:
<blockquote>
<blockquote>
<table width="650" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td> <br>
<font face="Arial,Helvetica"><font size="-1">Byte 1 </font></font></td>
<td>
<table width="100%" border="0" cellpadding="0" cols="8">
<tbody>
<tr>
<td>
<center>Bit 7</center>
</td>
<td>
<center>Bit 6</center>
</td>
<td>
<center>Bit 5</center>
</td>
<td>
<center>Bit 4</center>
</td>
<td>
<center>Bit 3</center>
</td>
<td>
<center>Bit 2</center>
</td>
<td>
<center>Bit 1</center>
</td>
<td>
<center>Bit 0</center>
</td>
</tr>
</tbody>
</table>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="8">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y overflow</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X overflow</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y sign
bit</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X sign
bit</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Always
1</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Middle
Btn</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Right
Btn</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Left
Btn</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 2</font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">X Movement</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 3 </font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Y Movement</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 4</font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="8">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Always
0</font></font></center>
</td>
<td>
<center>Always 0</center>
</td>
<td>
<center>5th Btn</center>
</td>
<td>
<center>4th Btn</center>
</td>
<td>
<center>Z3</center>
</td>
<td>
<center>Z2</center>
</td>
<td>
<center>Z1</center>
</td>
<td>
<center>Z0</center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
Z0-Z3 is a 2's complement number which represents the amount of movement
that has occurred since the last data report. Valid values range
from -8 to +7. <br>
4th Btn: 1 = 4th mouse button is pressed; 0 = 4th mouse button is not
pressed. <br>
5th Btn: 1 = 5th mouse button is pressed; 0 = 5th mouse button is not
pressed.</blockquote>
</blockquote>
You may have seen mice with two scrolling wheels--one vertical and
the other horizontal. These mice use the Microsoft Intellimouse data
packet format as described above. If the vertical wheel is scrolled
upward, the Z-counter is incremented by one and if that wheel is scrolled
down, the Z-counter is decremented by one. This is normal operation
for a scrolling wheel. However, if the <i>horizontal</i> wheel is
scrolled right, the Z-counter is incremented by <i>two</i> and if it is
scrolled left, the Z-counter is decremented by <i>two</i>. This seems
like an odd way to implement the second scrolling wheel, but it works since
the placement of the two wheels make it impossible to use both of them at
the same time (and if you try to trick the software and use both at the
same time, it will ignore the horizontal wheel.)
<p><b>Command Set:</b> </p>
<p>The following is the set of command accepted by the standard PS/2 mouse.
If the mouse is in Stream mode, the host should disable data reporting
(command F5h) before sending any other commands. </p>
<ul>
<li> FFh (Reset) - The mouse responds to this command with "acknowledge"
(FAh) then enters Reset Mode.</li>
<li> FEh (Resend) - The host sends this command whenever it receives
invalid data from the mouse. The mouse responds by resending the last<a href="#Footnotes">(2)</a> packet<a href="#Footnotes">(3)</a> it sent to
the host. If the mouse responds to the "Resend" command with
another invalid packet, the host may either issue another "Resend" command,
issue an "Error" command, cycle the mouse's power supply to reset the mouse,
or it may inhibit communication (by bringing the Clock line low).
The action taken depends on the host.</li>
<li> F6h (Set Defaults) - The mouse responds with "acknowledge" (FAh)
then loads the following values: Sampling rate = 100, Resolution =
4 counts/mm, Scaling = 1:1, Disable Data Reporting. The mouse then resets
its movement counters and enters stream mode.</li>
<li> F5h (Disable Data Reporting) - The mouse responds with "acknowledge"
(FAh) then disables data reporting and resets its movement counters.
This only effects data reporting in Stream mode and does not disable sampling.
Disabled stream mode funcions the same as remote mode.</li>
<li> F4h (Enable Data Reporting) - The mouse responds with "acknowledge"
(FAh) then enables data reporting and resets its movement counters.
This command may be issued while the mouse is in Remote Mode (or Stream
mode), but it will only effect data reporting in Stream mode.</li>
<li> F3h (Set Sample Rate) - The mouse responds with "acknowledge" (FAh)
then reads one more byte from the host. The mouse saves this byte
as the new sample rate. After receiving the sample rate, the mouse again
responds with "acknowledge" (0xFA) and resets its movement counters.
Valid sample rates are 10, 20, 40, 60, 80, 100, and 200 samples/sec.</li>
<li> F2h (Get Device ID) - The mouse responds with "acknowledge" (FAh)
followed by its device ID (00h for the standard PS/2 mouse.) The mouse
should also reset its movement counters.</li>
<li> F0h (Set Remote Mode) - The mouse responds with "acknowledge" (FAh)
then resets its movement counters and enters remote mode.</li>
<li> EEh (Set Wrap Mode) - The mouse responds with "acknowledge" (FAh)
then resets its movement counters and enters wrap mode.</li>
<li> ECh (Reset Wrap Mode) - The mouse responds with "acknowledge" (FAh)
then resets its movement counters and enters the mode it was in prior to
wrap mode (Stream Mode or Remote Mode.)</li>
<li> EBh (Read Data) - The mouse responds with acknowledge (FAh) then
sends a movement data packet. This is the only way to read data in Remote
Mode. After the data packets has been successfully sent, it resets
its movement counters.</li>
<li> EAh (Set Stream Mode) - The mouse responds with "acknowledge" then
resets its movement counters and enters steram mode.</li>
<li> E9h (Status Request) - The mouse responds with "acknowledge" then
sends the following 3-byte status packet (then resets its movement counters.): </li>
<ul>
<li> </li>
<table width="650" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td> <br>
<font face="Arial,Helvetica"><font size="-1">Byte 1 </font></font></td>
<td>
<table width="100%" border="0" cellpadding="0" cols="8">
<tbody>
<tr>
<td>
<center>Bit 7</center>
</td>
<td>
<center>Bit 6</center>
</td>
<td>
<center>Bit 5</center>
</td>
<td>
<center>Bit 4</center>
</td>
<td>
<center>Bit 3</center>
</td>
<td>
<center>Bit 2</center>
</td>
<td>
<center>Bit 1</center>
</td>
<td>
<center>Bit 0</center>
</td>
</tr>
</tbody>
</table>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="8">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Always
0</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Mode</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Enable</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Scaling</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Always
0</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Left
Btn</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Middle
Btn</font></font></center>
</td>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Right
Btn</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 2</font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Resolution</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td><font face="Arial,Helvetica"><font size="-1">Byte 3 </font></font></td>
<td>
<table width="100%" border="1" cellpadding="0" cellspacing="0" cols="1">
<tbody>
<tr>
<td>
<center><font face="Arial,Helvetica"><font size="-1">Sample
Rate</font></font></center>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
<p><i>Right, Middle, Left Btn</i> = 1 if button pressed; 0 if button
is not pressed. <br>
<i>Scaling</i> = 1 if scaling is 2:1; 0 if scaling is 1:1.
(See commands E7h and E6h) <br>
<i>Enable</i> = 1 if data reporting is enabled; 0 if data
reporting is disabled. (See commands F5h and F4h) <br>
<i>Mode</i> = 1 if Remote Mode is enabled; 0 if Stream mode
is enabled. (See commands F0h and EAh) <br>
</p>
</ul>
<li> E8h (Set Resolution) - The mouse responds with acknowledge (FAh)
then reads one byte from the host and again responds with acknowledge (FAh)
then resets its movement counters. The byte read from the host determines
the resolution as follows: </li>
<br>
<center>
<table width="300" border="1">
<tbody>
<tr>
<td>
<center>Byte Read from Host</center>
</td>
<td>
<center>Resolution</center>
</td>
</tr>
<tr>
<td>
<center>0x00</center>
</td>
<td>
<center>1 count/mm</center>
</td>
</tr>
<tr>
<td>
<center>0x01</center>
</td>
<td>
<center>2 count/mm</center>
</td>
</tr>
<tr>
<td>
<center>0x02</center>
</td>
<td>
<center>4 count/mm</center>
</td>
</tr>
<tr>
<td>
<center>0x03</center>
</td>
<td>
<center>8 count/mm</center>
</td>
</tr>
</tbody>
</table>
</center>
</ul>
<ul>
<li> E7h (Set Scaling 2:1) - The mouse responds with acknowledge (FAh)
then enables 2:1 scaling (discussed earlier in this document.)</li>
<li> E6h (Set Scaling 1:1) - The mouse responds with acknowledge (FAh)
then enables 1:1 scaling (discussed earlier in this document.)</li>
</ul>
The only commands the standard PS/2 mouse will send to the host are
the "Resend" (FEh) and "Error" (FCh). They both work the same as they
do as host-to-device commands.
<p><b>Initialization:</b> </p>
<p>The PS/2 mouse is normally detected/initialized only when the computer
is booting up. That is, the mouse is not hot-pluggable and you must
restart your computer whenever you add/remove a PS/2 mouse (furthermore,
some motherboards may be damaged if you add/remove a PS/2 mouse while the
computer is running.) </p>
<p>The initial detection of the PS/2 mouse occurrs during POST. If
a mouse is detected, the BIOS will allow the operating system to configure/enable
the mouse. Otherwise, it will inhibit communication on the mouse's
bus. If you boot the computer with a mouse attached, then detach/reattach
the mouse while in Windows, the OS <i>may</i> be able to detect the mouse
was reattached. Microsoft tried to support this, but it only works
about 50% of the time. </p>
<p>The following is the communication between my computer (running Win98SE)
and mouse when it boots up with a standard PS/2 mouse attached. It
is fairly typical of how a PS/2 mouse is initialized and if you want to
emulate a PS/2 mouse it must (at minimum) be able to support the following
sequence of commands... </p>
<ul>
<li> <tt>Power-on Reset:</tt></li>
<br>
<tt>Mouse: AA Self-test passed</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: FF Reset command</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Mouse: AA Self-test passed</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: FF Reset command</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Mouse: AA Self-test passed</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: FF Reset command</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Mouse: AA Self-test passed</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: F3 Set Sample Rate : Attempt to Enter
Microsoft</tt><br>
<tt>Mouse: FA Acknowledge
: Scrolling Mouse mode</tt><br>
<tt>Host: C8 decimal 200
:</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: F3 Set Sample Rate :</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: 64 decimal 100
:</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: F3 Set Sample Rate :</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: 50 decimal 80
:</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: F2 Read Device Type :</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Mouse: 00 Mouse ID
: Response 03 if microsoft scrolling mouse</tt><br>
<tt>Host: F3 Set Sample Rate </tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: 0A decimal 10</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: F2 Read Device Type</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: E8 Set resolution</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: 03 8 Counts/mm</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: E6 Set Scaling 1:1</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: F3 Set Sample Rate</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: 28 decimal 40</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: F4 Enable</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Initialization complete...</tt>
<p><tt>If I then press the Left Button...</tt> <br>
<tt>Mouse: 09 1 1 00001001; bit0 = Left button state; bit3 =
always 1</tt> <br>
<tt>Mouse: 00 1 1 No X-movement</tt> <br>
<tt>Mouse: 00 1 1 No Y-movement</tt> <br>
<tt>... and release the Left Button:</tt> <br>
<tt>Mouse: 08 0 1 00001000 bit0 = Left button state; bit3 =
always 1</tt> <br>
<tt>Mouse: 00 1 1 No X-movement</tt> <br>
<tt>Mouse: 00 1 1 No Y-movement</tt></p>
</ul>
The following is the communication between my computer (running Win98SE)
and mouse when it boots up with an (emulated) Intellimouse...
<ul>
<li> <tt>Power-on Reset:</tt></li>
<br>
<tt>Mouse: AA Self-test passed</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: FF Reset command</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Mouse: AA Self-test passed</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: FF Reset command</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Mouse: AA Self-test passed</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: FF Reset command</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Mouse: AA Self-test passed</tt><br>
<tt>Mouse: 00 Mouse ID</tt><br>
<tt>Host: F3 Set Sample Rate : Attempt to Enter
Microsoft </tt><br>
<tt>Mouse: FA Acknowledge
: Scrolling Mouse mode</tt><br>
<tt>Host: C8 decimal 200
:</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: F3 Set Sample Rate :</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: 64 decimal 100
:</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: F3 Set Sample Rate :</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: 50 decimal 80
:</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Host: F2 Read Device Type :</tt><br>
<tt>Mouse: FA Acknowledge
:</tt><br>
<tt>Mouse: 03 Mouse ID
: Response 03 if microsoft scrolling mouse</tt><br>
<tt>Host: E8 Set Resolution </tt><br>
<tt>Mouse: FA Acknowledge </tt><br>
<tt>Host: 03 8 counts/mm</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: E6 Set scaling 1:1</tt><br>
<tt>Dev: FA Acknowledge</tt><br>
<tt>Host: F3 Set Sample Rate </tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: 28 decimal 40</tt><br>
<tt>Mouse: FA Acknowledge</tt><br>
<tt>Host: F4 Enable device </tt><br>
<tt>Mouse: FA Acknowledge</tt>
<p><tt>If I then press the left mouse button:</tt> <br>
<tt>Mouse: 09 00001001 bit0 = Left button state; bit3
= always 1</tt> <br>
<tt>Mouse: 00 No X-movement</tt> <br>
<tt>Mouse: 00 No Y-movement</tt> <br>
<tt>Mouse: 00 No Z-movement</tt> </p>
<p><tt>...and then release the left mouse button button:</tt> <br>
<tt>Mouse: 08 00001000 bit0 = Left button state; bit3
= always 1</tt> <br>
<tt>Mouse: 00 No X-movement</tt> <br>
<tt>Mouse: 00 No Y-movement</tt> <br>
<tt>Mouse: 00 No Z-movement</tt></p>
</ul>
<tt>After I downloaded/installed the Microsoft's Intellimouse drivers
with support for the 4th and 5th buttons, the following sequence was found:</tt>
<blockquote><tt>... (starts same as before) ...</tt> <br>
<tt>Host: F3 Set Sample Rate : Attempt
to Enter Microsoft </tt> <br>
<tt>Mouse: FA Acknowledge
: Scrolling Mouse mode.</tt> <br>
<tt>Host: C8 decimal 200
:</tt> <br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: F3 Set Sample Rate :</tt>
<br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: 64 decimal 100
:</tt> <br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: F3 Set Sample Rate :</tt>
<br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: 50 decimal 80
:</tt> <br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: F2 Read Device Type :</tt> <br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Mouse: 03 Mouse ID
: Response 03 if microsoft scrolling mouse.</tt> <br>
<tt>Host: F3 Set Sample Rate : Attempt
to Enter Microsoft 5-button </tt> <br>
<tt>Mouse: FA Acknowledge
: Scrolling Mouse mode.</tt> <br>
<tt>Host: C8 decimal 200
:</tt> <br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: F3 Set Sample Rate :</tt>
<br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: C8 decimal 200
:</tt> <br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: F3 Set Sample Rate :</tt>
<br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: 50 decimal 80
:</tt> <br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Host: F2 Read Device Type :</tt> <br>
<tt>Mouse: FA Acknowledge
:</tt> <br>
<tt>Mouse: 04 Mouse ID
: Response 04 if 5-button scrolling mouse.</tt> <br>
<tt>... rest of initialization same as before ...</tt></blockquote>
<b>Emulation/Interfacing:</b>
<ul>
<li> Click here for routines that emulate a PS/2 mouse or keyboard</li>
<li> Click here for routines that emulate a PS/2 host (ie, interface
a mouse/keyboard)</li>
<li> Click <a href="http://panda.cs.ndsu.nodak.edu/%7Eachapwes/PICmicro/code/Projects/mouse/ps2mouse.html">here</a>
for a fully-functional PS/2 mouse written for the PIC16F</li>
</ul>
<blockquote>
<ul>
</ul>
</blockquote>
<p><a name="Footnotes"></a><b>Footnotes:</b> </p>
<blockquote>1) 2:1 scaling only applies to the automatic data reporting in
Stream mode. It does not effect the reported data sent in response to the
"Read Data" command.
<p>2) The mouse and host do not buffer "Resend" commands. This means
"Resend" will never be sent in response to the "Resend" command.
</p>
<p>3) A "packet" may be a 3-byte movement data packet, a 4-byte
movement data packet (for the Intellimouse), a 3-byte status packet (see
"Status Request" command) a 2-byte completion-code-ID packet (AAh,00h or
FCh,00h), or a 1-byte response to a command.<br>
</p>
</blockquote>
<br>
<br>
</body></html>