Electrical Interface / Protocol:
The PS/2 mouse uses the same protocol as the PS/2
(AT) keyboard. This standard originally appeared in
the IBM technical reference manual, but I am not
aware of any current official publication of this
standard. However, you may click
here for the (detailed) information I have
gathered about that protocol.
Inputs, Resolution, and Scaling:
The standard PS/2 mouse 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.
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.
When the mouse reads its inputs, it records the
current state of its buttons, then 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.
The parameter that determines the amount by which
the movement counters are incremented/decremented is
the resolution. The default resolution is 4
counts/mm and the host may change that value using
the "Set Resolution" (0xE8) command.
There is a parameter that does not effect the
movement counters, but does effect the reported(1)
value of these counters. This parameter is
scaling. By default, the mouse uses 1:1
scaling, which has no effect on the reported mouse
movement. However, the host may select 1:2 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:
| Movement Counter |
Reported Movement |
| 0 |
0 |
| 1 |
1 |
| 2 |
1 |
| 3 |
3 |
| 4 |
6 |
| 5 |
9 |
| N > 5 |
2 * N |
Movement Data Packet:
The standard PS/2 mouse sends movement (and
button) information to the host using the following
3-byte packet (4):
Byte 1 |
| Bit 7 |
Bit 6 |
Bit 5 |
Bit 4 |
Bit 3 |
Bit 2 |
Bit 1 |
Bit 0 |
| Y
overflow |
X
overflow |
Y
sign bit |
X
sign bit |
Always 1 |
Middle Btn |
Right Btn |
Left
Btn |
|
| Byte 2 |
|
| Byte 3 |
|
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 and the counter is not
incremented/decremented until it is reset.
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.
Modes of Operation:
Data reporting is handled according to the mode
in which the mouse is operating. There are four
standard modes of operation:
- Reset - The mouse enters Reset mode
at power-up or after receiving the "Reset"
(0xFF) command.
- Stream - 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.
- Remote - Remote mode is useful in
some situations and may be entered by sending
the "Set Remote Mode" (0xF0) command to the
mouse.
- Wrap - 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.
(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.)
Reset Mode:
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 follwing default values:
- Sample Rate - 100 samples/sec
- Resolution - 4 counts/mm
- Scaling - 1:1
- Data Reporting Disabled
It then sends a BAT completion code of either 0xAA
(BAT successful) or 0xFC (Error). If the host
receives a response other than 0xAA, it may cycle
the mouse's power supply, causing the mouse to reset
and re-execute its BAT.
Following the BAT completion code (0xAA or 0xFC),
the mouse sends its device ID of 0x00. This
distinguishes it from a keyboard, or a mouse in an
extended mode. I have read documents saything the
host is not supposed to transmit any data
until it receives a device ID. However I've found
that some BIOS's will send the "Reset" (0xFF)
command immediately following the 0xAA received
after a power-on reset.
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
send any movement data packets to the host until the
"Enable Data Reporting" (0xF4) command is received.
Stream Mode:
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
sample rate. 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" (0xF3) command.
Stream mode is the default mode of operation.
Remote Mode:
In this mode, the mouse reads its inputs and
updates its counters/flags at the current sampling
rate, but it only notifies the host of movement (and
change in button state) when that information is
requested by the host. The host does this by issuing
the "Read Data" (0xEB) command. After receiveing
this command, the mouse will send a movement data
packet, and reset its movement counters.
Wrap Mode:
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" (0xFF) command and "Reset Wrap
Mode" (0xEC) command. The mouse treats these as
valid commands and does not echo them back to the
host.
Intellimouse Extensions:
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.
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 0x00.) To enter scrolling wheel mode, the
host sends the following command sequence:
Set sample rate 200
Set sample rate 100
Set sample rate 80
The host then issues the "Get device ID" command
(0xF2) and waits for a response. If a standard PS/2
mouse (ie, non-Intellimouse) is attached, it will
respond with a device ID of 0x00. 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 0x03. 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:
Byte 1 |
| Bit 7 |
Bit 6 |
Bit 5 |
Bit 4 |
Bit 3 |
Bit 2 |
Bit 1 |
Bit 0 |
| Y
overflow |
X
overflow |
Y
sign bit |
X
sign bit |
Always 1 |
Middle Btn |
Right Btn |
Left
Btn |
|
| Byte 2 |
|
| Byte 3 |
|
| Byte 4 |
|
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.
To enter scrolling wheel + 5 button mode, the host
sends the following command sequence:
Set sample rate 200
Set sample rate 200
Set sample rate 80
The host then issues the "Get device ID" command
(0xF2) and waits for a response. A Microsoft
Intellimouse will respond with a device ID of 0x04,
then use the following 4-byte movement data packet:
Byte 1 |
| Bit 7
|
Bit 6
|
Bit 5
|
Bit 4
|
Bit 3
|
Bit 2
|
Bit 1
|
Bit 0
|
| Y overflow
|
X overflow
|
Y sign bit
|
X sign bit
|
Always 1
|
Middle Btn
|
Right Btn
|
Left Btn
|
|
| Byte 2 |
|
| Byte 3 |
|
| Byte 4 |
| Always 0
|
Always 0
|
5th Btn
|
4th Btn
|
Z3 |
Z2 |
Z1 |
Z0 |
|
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.
4th Btn: 1 = 4th mouse button is pressed; 0
= 4th mouse button is not pressed.
5th Btn: 1 = 5th mouse button is pressed; 0
= 5th mouse button is not pressed.
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 horizontal wheel is scrolled right, the
Z-counter is incremented by two and if it is
scrolled left, the Z-counter is decremented by
two. 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.)
Command Set:
The following are the only commands that may be
sent to the mouse... If the mouse is in Stream mode,
the host should disable data reporting (command
0xF5) before sending any other commands...
-
0xFF (Reset) - The mouse responds to this
command with "acknowledge" (0xFA) then enters
Reset Mode.
-
0xFE (Resend) - The host sends this command
whenever it receives invalid data from the
mouse. The mouse responds by resending the last(2)
packet(3) 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.
-
0xF6 (Set Defaults) - The mouse responds
with "acknowledge" (0xFA) 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.
-
0xF5 (Disable Data Reporting) - The mouse
responds with "acknowledge" (0xFA) 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.
-
0xF4 (Enable Data Reporting) - The mouse
responds with "acknowledge" (0xFA) 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.
-
0xF3 (Set Sample Rate) - The mouse responds
with "acknowledge" (0xFA) 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.
-
0xF2 (Get Device ID) - The mouse responds
with "acknowledge" (0xFA) followed by its device
ID (0x00 for the standard PS/2 mouse.) The
mouse should also reset its movement counters.
-
0xF0 (Set Remote Mode) - The mouse responds
with "acknowledge" (0xFA) then resets its
movement counters and enters remote mode.
-
0xEE (Set Wrap Mode) - The mouse responds
with "acknowledge" (0xFA) then resets its
movement counters and enters wrap mode.
-
0xEC (Reset Wrap Mode) - The mouse responds
with "acknowledge" (0xFA) then resets its
movement counters and enters the mode it was in
prior to wrap mode (Stream Mode or Remote Mode.)
-
0xEB (Read Data) - The mouse responds with
acknowledge (0xFA) 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.
-
0xEA (Set Stream Mode) - The mouse responds
with "acknowledge" then resets its movement
counters and enters steram mode.
-
0xE9 (Status Request) - The mouse responds
with "acknowledge" then sends the following
3-byte status packet (then resets its movement
counters.):
-
Byte 1 |
| Bit 7
|
Bit 6
|
Bit 5
|
Bit 4
|
Bit 3
|
Bit 2
|
Bit 1
|
Bit 0
|
| Always 0
|
Mode |
Enable |
Scaling |
Always 0
|
Left Btn
|
Middle Btn
|
Right Btn
|
|
| Byte 2 |
|
| Byte 3 |
|
Right, Middle, Left Btn = 1 if
button pressed; 0 if button is not pressed.
Scaling = 1 if scaling is 2:1; 0 if
scaling is 1:1. (See commands 0xE7 and 0xE6)
Enable = 1 if data reporting is
enabled; 0 if data reporting is disabled.
(See commands 0xF5 and 0xF4)
Mode = 1 if Remote Mode is enabled; 0
if Stream mode is enabled. (See commands
0xF0 and 0xEA)
- 0xE8 (Set Resolution) - The mouse responds
with acknowledge (0xFA) then reads one byte from
the host and again responds with acknowledge
(0xFA) then resets its movement counters. The
byte read from the host determines the
resolution as follows:
| Byte Read from Host
|
Resolution |
| 0x00 |
1 count/mm |
| 0x01 |
2 count/mm |
| 0x02 |
4 count/mm |
| 0x03 |
8 count/mm |
- 0xE7 (Set Scaling 2:1) - The mouse responds
with acknowledge (0xFA) then enables 2:1 scaling
(discussed earlier in this document.)
- 0xE6 (Set Scaling 1:1) - The mouse responds
with acknowledge (0xFA) then enables 1:1 scaling
(discussed earlier in this document.)
The only commands the standard PS/2 mouse will send
to the host are the "Resend" (0xFE) and "Error"
(0xFC) commands. They both work the same as they do
as host-to-device commands.
Initialization:
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.)
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 may be able to detect the
mouse was reattached. Microsoft tried to support
this, but it only works about 50% of the time.
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...
The following is the communication between my
computer (running Win98SE) and mouse when it boots
up with an (emulated) Intellimouse...
-
Power-on Reset:
Mouse: AA Self-test passed
Mouse: 00 Mouse ID
Host: FF Reset command
Mouse: FA Acknowledge
Mouse: AA Self-test passed
Mouse: 00 Mouse ID
Host: FF Reset command
Mouse: FA Acknowledge
Mouse: AA Self-test passed
Mouse: 00 Mouse ID
Host: FF Reset command
Mouse: FA Acknowledge
Mouse: AA Self-test passed
Mouse: 00 Mouse ID
Host: F3 Set Sample Rate : Attempt to Enter
Microsoft
Mouse: FA Acknowledge : Scrolling Mouse
mode
Host: C8 decimal 200 :
Mouse: FA Acknowledge :
Host: F3 Set Sample Rate :
Mouse: FA Acknowledge :
Host: 64 decimal 100 :
Mouse: FA Acknowledge :
Host: F3 Set Sample Rate :
Mouse: FA Acknowledge :
Host: 50 decimal 80 :
Mouse: FA Acknowledge :
Host: F2 Read Device Type :
Mouse: FA Acknowledge :
Mouse: 03 Mouse ID : Response 03 if
microsoft scrolling mouse
Host: E8 Set Resolution
Mouse: FA Acknowledge
Host: 03 8 counts/mm
Mouse: FA Acknowledge
Host: E6 Set scaling 1:1
Dev: FA Acknowledge
Host: F3 Set Sample Rate
Mouse: FA Acknowledge
Host: 28 decimal 40
Mouse: FA Acknowledge
Host: F4 Enable device
Mouse: FA Acknowledge
If I
then press the left mouse button:
Mouse: 09 00001001 bit0 = Left button state;
bit3 = always 1
Mouse: 00 No X-movement
Mouse: 00 No Y-movement
Mouse: 00 No Z-movement
...and
then release the left mouse button button:
Mouse: 08 00001000 bit0 = Left button state;
bit3 = always 1
Mouse: 00 No X-movement
Mouse: 00 No Y-movement
Mouse: 00 No Z-movement
After
I downloaded/installed the Microsoft's Intellimouse
drivers with support for the 4th and 5th buttons,
the following sequence was found:
... (starts same as before) ...
Host: F3 Set Sample Rate : Attempt to Enter
Microsoft
Mouse: FA Acknowledge : Scrolling Mouse
mode.
Host: C8 decimal 200 :
Mouse: FA Acknowledge :
Host: F3 Set Sample Rate :
Mouse: FA Acknowledge :
Host: 64 decimal 100 :
Mouse: FA Acknowledge :
Host: F3 Set Sample Rate :
Mouse: FA Acknowledge :
Host: 50 decimal 80 :
Mouse: FA Acknowledge :
Host: F2 Read Device Type :
Mouse: FA Acknowledge :
Mouse: 03 Mouse ID : Response 03 if
microsoft scrolling mouse.
Host: F3 Set Sample Rate : Attempt to Enter
Microsoft 5-button
Mouse: FA Acknowledge : Scrolling Mouse
mode.
Host: C8 decimal 200 :
Mouse: FA Acknowledge :
Host: F3 Set Sample Rate :
Mouse: FA Acknowledge :
Host: C8 decimal 200 :
Mouse: FA Acknowledge :
Host: F3 Set Sample Rate :
Mouse: FA Acknowledge :
Host: 50 decimal 80 :
Mouse: FA Acknowledge :
Host: F2 Read Device Type :
Mouse: FA Acknowledge :
Mouse: 04 Mouse ID : Response 04 if
5-button scrolling mouse.
... rest of initialization same as before ...
Emulation/Interfacing:
- Click
here for routines that emulate a PS/2 mouse
or keyboard
- Click
here for routines that emulate a PS/2 host
(ie, interface a mouse/keyboard)
- Click
here for a fully-functional PS/2 mouse
written for the PIC16F84.
If you want to build a truely fully-implemented
mouse or host, you should implement all of the
features described in this document (except for, of
course, the Microsoft Intellimouse extensions, which
are optional). However, at an absolute minimum,
your device should operate as follows:
To Emulate a Mouse:
- Never send data when the "Clock" line
low. If the host pulls the "Data" line low,
prepare to read a byte from the host.
- ~500 milliseconds after powerup,
transmit "0xAA, 0x00".
- Wait for the host to send the enable
(0xF4) command before sending any
movement/button data.
- Emulate the various mouse functions as
follows:
| Emulated Action
|
Data sent to host
|
| Move up one
|
0x08,0x00,0x01
|
| Move down one
|
0x28,0x00,0xFF
|
| Move right one
|
0x08,0x01,0x00
|
| Move left one
|
0x18,0xFF,0x00
|
| Press left button
|
0x09,0x00,0x00
|
| Release left button
|
0x08,0x00,0x00
|
| Press middle button
|
0x0C,0x00,0x00
|
| Release middle button
|
0x08,0x00,0x00
|
| Press right button
|
0x0A,0x00,0x00
|
| Release right button
|
0x08,0x00,0x00
|
- Respond to the "Reset" (0xFF) command
with "0xFA" then goto the beginning of your
program. (ie, send 0xAA, 0x00, then wait for
the enable command before sending any
movement/button data.)
- Respond to the "Get Device ID" (0xF2)
command with "0xFA, 0x00".
- Respond to the "Status Request" (0xE9)
command with "0xFA, 0x00, 0x02, 0x64".
- Respond to all other commands with
acknowledge (0xFA).
To Interface a Mouse:
- Wait for the mouse to send "0xAA", then
send the "Enable" (0xF4) command.
- The mouse will then send a 3-byte
movement packets as described earlier in
this document.
Footnotes:
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" (0xEB) command.
2) The mouse and host do not buffer "Resend"
(0xFF) commands. This means "0xFE" will never be
sent in response to the "Resend" command.
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" [0xE9] command) a 2-byte
completion-code-ID packet (0xAA,0x00 or
0xFC,0x00), or a 1-byte response to a command.
4) A little advice from my own
experience... Even though bit 3 of the first
byte in a movement data packet is supposed to be
set, some drivers (such as the standard PS/2
mouse driver included with Windows 98SE) don't
care and just ignore that bit. However, other
drivers do check that bit and if it is not set,
it is considered an error. I mention this so
that, if you're designing a mouse, you
double-check that this bit is set in every
movement data packet sent by your mouse. If it
is not, your mouse may work properly when you
test it on your computer, but it may not work on
other computers that use different mouse
drivers.
For example, if using MS Intellimouse drivers
and bit 3 of the first byte in a movement data
packet is not set, the driver will discard that
packet, then send the "Disable Data Reporting"
(0xF5) command, followed by the "Set Defaults"
(0xF6) command, then it will reinitialize the
mouse using the same command sequence as it does
when Windows boots up (see the "Initialization"
section above.)
