66 MicroBlaze Development Kit Spartan-3E 1600 Edition User Guide
www.xilinx.com UG257 (v1.1) December 5, 2007
Chapter 8: PS/2 Mouse/Keyboard Port R
The keyboard sends data to the host in 11-bit words that contain a ‘0’ start bit, followed by
eight bits of scan code (LSB first), followed by an odd parity bit and terminated with a ‘1’
stop bit. When the keyboard sends data, it generates 11 clock transitions at around 20 to
30 kHz, and data is valid on the falling edge of the clock as shown in Figure 8-2.
Mouse
A mouse generates a clock and data signal when moved; otherwise, these signals remain
High, indicating the Idle state. Each time the mouse is moved, the mouse sends three 11-bit
words to the host. Each of the 11-bit words contains a ‘0’ start bit, followed by 8 data bits
(LSB first), followed by an odd parity bit, and terminated with a ‘1’ stop bit. Each data
transmission contains 33 total bits, where bits 0, 11, and 22 are ‘0’ start bits, and bits 10, 21,
and 32 are ‘1’ stop bits. The three 8-bit data fields contain movement data as shown in
Figure 8-4. Data is valid at the falling edge of the clock, and the clock period is 20 to 30kHz.
A PS/2-style mouse employs a relative coordinate system (see Figure8-5), wherein
moving the mouse to the right generates a positive value in the X field, and moving to the
left generates a negative value. Likewise, moving the mouse up generates a positive value
in the Y field, and moving it down represents a negative value. The XS and YS bits in the
status byte define the sign of each value, where a ‘1’ indicates a negative value.
The magnitude of the X and Y values represent the rate of mouse movement. The larger the
value, the faster the mouse is moving. The XV and YV bits in the status byte indicate when
Figure 8-4: PS/2 Mouse Transaction
Figure 8-5: The Mouse Uses a Relative Coordinate System to Track Movement
LR0 1XSYSXVYV P X0X1 X2X3X4 X5 X6X7 P Y0 Y1Y2 Y3Y4 Y5 Y6 Y7 P
10 10
0 11
Idle state Idle state
Start bit
Start bit
Stop bit
Mouse status byte X direction byte Y direction byte
U257_08_04_060506
Start bit
Stop bit Stop bit
(YS=0)
-Y values
+Y values
(YS=1)
+X values-X values
(XS=1) (XS=0)
UG257_08_05_060506

Mouse Arrow