The PC Mouse is a pointing device shipped with almost every computer nowadays as because is an important element of the graphical user interfaces available. It was invented by Doug Engelbart and presented at the Fall Joint Computer Conference held in San Francisco in 1968.

The idea behind the mouse is that you move the device on a mouse pad and the computer responds by moving a cursor accordingly around the screen. Using buttons placed on the top of the mouse enables you to select things or move things around. Complex mouse gestures have been developped by software designers and the mouse has been enhanced with additionnal buttons and wheels to suit user's needs.

Other pointing devices that mimic what you can do with a mouse have been developped : the Touch Pad, IBM ThinkPad's TrackPoint etc.

Mice anatomy

A mouse consists of five functional units : sensors, a mouse controller, a communication link, a data interface and a driver.

Sensors detect movement and trigger the mouse controller which sends data through the communication link to the data interface. Then the driver reads the data and updates the mouse information (position, buttons pressed, etc.) available to the kernel and the applications.

Sensors and Movement

Traditional mice use a rubber ball that rolls on the mouse pad when you move the mouse. The ball is placed between three wheels : two of them form an angle of 90° and are used to detect the movement, the third one is connected to a spring that holds the rubber ball in place.

The movement wheels are connected to a disk pierced of little equaly spaced holes placed between two infrared light emitting diodes and the corresponding photo transistors.

   diodes => | >= photo transistors
             |
           --+------- shaft connected to the movement wheel
             |
             |

When the mouse moves, so does the wheel, cycling through blocking and allowing the light to reach the detector. There is a 90° phase difference between the signals collected by the two detectors.

Let 0 code for no light being detected by the sensor and 1 for light being detected. Upon mouse movement, the signal typically looks like :

   sensor #0 - 1111000011110000111100001111
   sensor #1 - 0011110000111100001111000011

The controller's job is to determine in which direction the mouse is moving. This can easily be done by constructing an integer from the two signals and seeing if it is incremented (the mouse moves one way) or decremented (the mouse moves the other way) modulus 4 :

   sensor #0 | 0 | 0 | 1 | 1
   sensor #1 | 0 | 1 | 1 | 0
  -----------+---+---+---+---
    integer  | 0 | 1 | 2 | 3

Say if the integer increases the moving direction is left to right, when the sequence 12 is encountered, the controller should send a 'mouse moved right' packet to the computer. If the sequence 03 is encountered, then it means that the mouse is moved to the left.

Mice quality

Baud rate

When the mouse's status changes (button press, mouse movement, etc.), the controller is expected to send a packet containing the new information to the computer. But due protocol limitations, there is a limit on the transmission rate. For example, the Microsoft serial mouse uses a 1200 baud protocol and each packet consists of 3 bytes. Therefore the mouse can't send more than 50 packets per second. This isn't really a problem since screen refrash rates range from 60 to 120 Hz.
The baud rate is related to the maximum display rate. It can be changed by sending the appropriate signals to the hardware (see mouse protocol). For example PS/2 mice can have refresh rates as high as 200 Hz.

Tracking speed

If mice were updated only 50 times per second, you couldn't move them fast without the controller losing track of the movement.

Say you have a classic mouse with a 1 cm diameter ball and 40 slots and 40 spokes on the optomechanical sensors. Then a complete revolution of the ball involves 40 * 2 * 2 = 160 counts of movement, which can't be done in less than 160 / 50 = 3.2 s and represents a travel distance of 2 * π * 1 = 6.28 cm.
The maximum mouse speed would be about 2 cm/s which is very low.

In fact, the controller has a higher refresh rate. Typical cheap mice are able to achieve more than 5000 counts per second. The tracking speed is the maximum mouse speed without the controller losing track of movement (ie. not skipping sensor changes).
If samples are lost, the controller can think that the mouse is going in the opposite direction and cause the mouse to jitter on the screen. To visualize this, think of a spinning disk with a black spot and lit by a stroboscope. If the disk is spinning too fast, you might think that it is turning in the opposite direction.
Tracking speed defines how fast the mouse can move without jittering.
(Of course if the baud rate is too low, the buffers might overflow)

Resolution

A complete revolution of the previous mouse involves 160 counts and represents a distance of 6.28 cm which means that it measures movement with a granularity of 0.098mm. This reprenents about 259 dpi.
The resolution is expressed in Counts Per Inch (CPI) and defines how precise the mouse can measure movement.

Conclusion

The mouse is a basic device, it is rather easy to build on your own and even easier to program. See Mouse Protocol for further information on the data sent to the computer.

PC MOUSE Implementation Using COP800 AN-681, National Semiconductor, Application Note 681
http://www.hut.fi/~then/mytexts/mouse.html