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sends data to the host in 11-bit words that contain a ‘0’ start bit, followed by 8-bits of scan code (LSB

first), followed by an odd parity bit and terminated with a ‘1’ stop bit. The keyboard generates 11 clock

transitions (at 20 to 30KHz) when the data is sent, and data is valid on the falling edge of the clock.

Scan codes for most PS/2 keys are shown in the figure below.

Mouse

The mouse outputs a clock and data signal when it is moved, otherwise, these signals remain at logic

‘1’. Each time the mouse is moved, three 11-bit words are sent from the mouse to the host device.

Each of the 11-bit words contains a ‘0’ start bit, followed by 8 bits of data (LSB first), followed by an

odd parity bit, and terminated with a ‘1’ stop bit. Thus, each data transmission contains 33 bits, where

bits 0, 11, and 22 are ‘0’ start bits, and bits 11, 21, and 33 are ‘1’ stop bits. The three 8-bit data fields

contain movement data as shown in the figure above. Data is valid at the falling edge of the clock, and

the clock period is 20 to 30KHz.

The mouse assumes a relative coordinate system wherein moving the mouse to the right generates a

positive number in the X field, and moving to the left generates a negative number. Likewise, moving

the mouse up generates a positive number in the Y field, and moving down represents a negative

number (the XS and YS bits in the status byte are the sign bits – a ‘1’ indicates a negative number).

The magnitude of the X and Y numbers represent the rate of mouse movement – the larger the

number, the faster the mouse is moving (the XV and YV bits in the status byte are movement overflow

indicators – a ‘1’ means overflow has occurred). If the mouse moves continuously, the 33-bit

transmissions are repeated every 50ms or so. The L and R fields in the status byte indicate Left and

Right button presses (a ‘1’ indicates the button is being pressed).

ESC

` ~

TAB

Caps Lock

Shift

Ctrl

1 !

2 @

3 #

4 $

5 %

6 ^

7 &

8 *

9 (

0 )

- _

= +

BackSpace

[ {

] }

\ |

; :

' "

Enter

, <

> .

/ ?

Shift

Alt

Space

Alt

E0 11

Ctrl

E0 14

F10

F11

F12

E0 75

E0 74

E0 6B

E0 72

PS/2 Keyboard Scan Codes

L R 0 1 XS YS XY YY P X0 X1 X2 X3 X4 X5 X6 X7 P Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7 P1 0 1 00 11

Idle state

Start bit

Mouse status byte X direction byte Y direction byte

Stop bit Start bit

Stop bit

Idle state

Stop bit Start bit

Mouse Data Format

1 2 ... 13 14 15 16 17 18 19 20 21 22

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