Digilent 410-182P-KIT - Manual (Page 14)

Nexys3 Reference Manual
Doc: 502-182 page 14 of 22
FE Resend. FE directs keyboard to re-send most recent scan code.
FF Reset. Resets the keyboard.
The keyboard can send 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 must check to see whether the host is sending data
before driving the bus. To facilitate this, the clock line is used as a “clear to send” signal. If the host
pulls the clock line low, the keyboard must 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 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
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
PS/2 Keyboard Scan Codes