Digilent Basys Board Rev.C - Manual (Page 7)

Digilent
Basys Reference Manual
www.digilentinc.com
Copyright Digilent, Inc. Page 7/12 Doc: 502-107
by the scan code of the released key. If a key can be “shifted” to produce a new character (like a
capital letter), then a shift character is sent in addition to the original scan code, 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, an “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 Set Num Lock, Caps Lock, and Scroll Lock LEDs. After receiving an “ED”, the keyboard returns an “FA”;
then the host sends a byte to set LED status: Bit 0 sets Scroll Lock; bit 1 sets 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 “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 FE, the keyboard re-sends the last scan code sent.
FF Reset. Resets the keyboard.
The keyboard should 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 should check 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 must not send any data until the clock is released (host-to-
keyboard data transmission will not be dealt with further here).
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
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