Digilent 410-094P - Manual (Page 3)

PS/2 Reference Manual Digilent, Inc.
www.digilentinc.com page 3 of 4
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
The keyboard sends data to the host only
when both the data and clock lines are high (or
idle). Since the host is the “bus master”, the
keyboard checks to see whether the host is
sending data before driving the bus. To
facilitate this, the clock line can be used as a
“clear to send” signal. If the host pulls the clock
line low, the keyboard will not send any data
until the clock is released.
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.
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
below. 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