2
AVR313
1235B–AVR–05/02
Timing The timing for the data transferred from the keyboard to the host is shown in Figure 2.
The protocol is: one start bit (always 0), eight data bits, one odd parity bit and one stop
bit (always 1). The data is valid during the low period of the clock pulse. The keyboard is
generating the clock signal, and the clock pulses are typically 30-50 µs low and 30-50 µs
high.
The host system can send commands to the keyboard by forcing the clock line low. It
then pulls the data line low (the start bit). Now, the clock line must be released. The key-
board will count 10 clock pulses. The data line must be set up to the right level by the
host before the trailing edge of the clock pulse. After the tenth bit, the keyboard checks
for a high level on the data line (the stop bit), and if it is high, it forces it low. This tells the
host that the data is received by the keyboard. The software in this design note will not
send any commands to the keyboard.
Scan Codes TheATkeyboardhasascancodeassociatedwitheachkey.Whenakeyispressed,
this code is transmitted. If a key is held down for a while, it starts repeating. The repeat
rate is typically 10 per second. When a key is released, a “break” code ($F0) is transmit-
ted followed by the key scan code. For most of the keys, the scan code is one byte.
Some keys like the
Home
,
Insert
and
Delete
keys have an extended scan code, from
twotofivebytes.Thefirstbyteisalways$E0.Thisisalsotrueforthe“break” sequence,
e.g., E0 F0 xx…
AT keyboards are capable of handling three sets of scan codes, where set two is
default. This example will only use set two.
The Software The code supplied with this application note is a simple keyboard to RS-232 interface.
The scan codes received from the keyboard are translated into appropriate ASCII char-
acters and transmitted by the UART. The source code is written in C, and is easily
modified and adaptable to all AVR microconrollers with SRAM.
Note: The linkerfile (AVR313.xcl) included in the software archive has to be included instead of
the standard linkerfile. This is done from the include menu under XLINK – Options. The
linker file applies to AT90S8515 only.
Table 1. AT Keyboard Connector Pin Assignments
AT Computer
Signals
DIN41524, Female at
Computer, 5-pin DIN 180
o
6-pin Mini DIN PS2 Style
Female at Computer
Clock 1 5
Data 2 1
nc 3 2,6
GND 4 3
+5V 5 4
Shield Shell Shell
1
2
3
45
4
2
3
65
1
筱菂2013-06-15能用，资料不详细
