Basys2™ FPGA Board Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
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The keyboard 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 around 20 - 30KHz) when the data is sent, and data is valid on the falling edge of the clock.
5.2 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
76
` ~
0E
TAB
0D
Caps Lock
58
Shift
12
Ctrl
14
1 !
16
2 @
1E
3 #
26
4 $
25
5 %
2E
Q
15
W
1D
E
24
R
2D
T
2C
A
1C
S
1B
D
23
F
2B
G
34
Z
1Z
X
22
C
21
V
2A
B
32
6 ^
36
7 &
3D
8 *
3E
9 (
46
0 )
45
- _
4E
= +
55
BackSpace
66
Y
35
U
3C
I
43
O
44
P
4D
[ {
54
] }
5B
\ |
5D
H
33
J
3B
K
42
L
4B
; :
4C
' "
52
Enter
5A
N
31
M
3A
, <
41
> .
49
/ ?
4A
Shift
59
Alt
11
Space
29
Alt
E0 11
Ctrl
E0 14
F1
05
F2
06
F3
04
F4
0C
F5
03
F6
0B
F7
83
F8
0A
F9
01
F10
09
F11
78
F12
07
E0 75
E0 74
E0 6B
E0 72
Figure 11. 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 P
1
0
1
0
0
1
1
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
Figure 12. Mouse data format.
