The PS/2 Mouse Interface
Source: http://www.Computer-Engineering.org
Author: Adam Chapweske
Last Updated: 04/01/03
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Abstract:
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 Microchip's PIC microcontrollers. All code samples related to the auxiliary device controller (keyboard
controller) are written in x86 assembly
General Description:
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:
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.
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.
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.
Electrical Interface / Protocol:
The PS/2 mouse uses the same protocol as the PS/2 keyboard (aka AT keyboard). Click here for detailed information on this
protocol.
Inputs, Resolution, and Scaling:
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.
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
USB (v1.1) PS/2 Mouse Interface
Number of Devices
Supported
Up to 127 One
Maximum Data Rate 12 Mbps 40 kbps
Power (max) 5V @ 500mA 5V @ 100mA
Hot-Pluggable? Yes No
Documentation Well-documented:
http://www.usb.org
Out-of-print
(IBM Tech Reference)
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PS/2 Mouse Interfacing
2009/4/30
http://www.computer
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engineering.org/ps2mouse/
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