Digilent DIO2 - Manual (Page 9)

Digilab DIO2 Reference Manual Digilent, Inc.
www.digilentinc.com page 9 of 19
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Bus timings are shown below. The clock and data signals are only driven when data transfers occur,
and otherwise they are held in the “idle” state at logic ‘1’. The timings define signal requirements for
mouse-to-host communications and bi-directional keyboard communications.
Keyboard
The keyboard uses open collector drivers so that either the keyboard or an attached host device can
drive the two-wire bus (if the host device will not send data to the keyboard, then the host can use
simple input-only ports). On the DIO2 board, the clock and data signals (PS2C and PS2D) bypass the
CPLD, and are connected directly to pins on the B connector.
A PS2-style keyboard uses scan codes to communicate key press data (nearly all keyboards in use
today are PS2 style). Each key has a single, unique scan code that is sent whenever the corresponding
key is pressed. If the key is pressed and held, the scan code will be sent repeatedly once every 100ms
or so. When a key is released, a “F0” key-up code is sent, followed by the scan code of the released
key. If a key has a “shift” character that is different than the non-shift character, the same scan code is
sent whether the shift key is pressed or not, and the host device must determine which character to use.
Some keys, called extended keys, send an “E0” ahead of the scan code (and they may send more than
one scan code). When an extended key is released, a “E0 F0” key-up code is sent, followed by the scan
code. Scan codes for most keys are shown in the figure below.
A host device can also send data to the keyboard. Below is a short list of some often-used commands.
ED Turn on/off Num Lock, Caps Lock, and Scroll Lock LEDs. The keyboard acknowledges receipt
of an “ED” by returning an “FA”, after which the host send another byte to set LED status: Bit 0
sets Scroll Lock; bit 1 sets the Num Lock; and Bit 2 sets Caps lock. Bits 3 to 7 are ignored.
EE Echo. Upon receiving an echo command, the keyboard replies with the same scan code (“EE”).
F3
Set scan code repeat rate. The keyboard acknowledges receipt of an “F3” by returning an “FA”,
after which the host sends a second byte to set the repeat rate.
FE Resend. Upon receiving a resend command, the keyboard will re-send the last scan code sent.
FF Reset. Resets the keyboard.
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