Mouse – Digilent 410-087P-KIT User Manual
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64
Spartan-3E Starter Kit Board User Guide
UG230 (v1.0) March 9, 2006
Chapter 8: PS/2 Mouse/Keyboard Port
R
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
. Data is valid at the falling edge of the clock, and the clock period is 20 to 30 kHz.
A PS/2-style mouse employs a relative coordinate system (see
), 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
the X or Y values exceed their maximum value, an overflow condition. A ‘1’ indicates
when an overflow occurs. If the mouse moves continuously, the 33-bit transmissions repeat
every 50 ms or so.
The L and R fields in the status byte indicate Left and Right button presses. A ‘1’ indicates
that the associated mouse button is being pressed.
Figure 8-4:
PS/2 Mouse Transaction
L
R
0
1 XS YS XV YV P
X0 X1 X2 X3 X4 X5 X6 X7 P
Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7 P
1
0
1
0
0
1
1
Idle state
Idle state
Start bit
Start bit
Stop bit
Stop bit
Stop bit
Start bit
Mouse status byte
X direction byte
Y direction byte
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Figure 8-5:
The Mouse Uses a Relative Coordinate System to Track Movement
(YS=0)
-Y values
+Y values
(YS=1)
+X values
-X values
(XS=1)
(XS=0)
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