AN665 데이터 시트보기 (PDF) - Microchip Technology
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AN665 Datasheet PDF : 17 Pages
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AN665
Keyboard Transmission
The keyboard pulls the data line low (start bit) and
starts the clock. The eight data bits (least significant bit
first) are shifted out, followed by the parity (odd), and
stop bit (high). Data is valid after the falling edge of the
clock and changes after the rising edge of the clock. If
no data is transmitted, both the clock line and data line
are high. If the computer pulls the clock line low for at
least 60 ms before the tenth bit is transmitted, the key-
board stops the transmission and stores the aborted
data in a buffer for retransmission at a later time.
Keyboard Receiving
The computer pulls the data line low (start bit), after
which the keyboard starts to shift out 11 clock pulses
within 15 ms. Transmission has to be completed within
2 ms. Data from the computer changes after the falling
edge of the clock line, and is valid before the rising
edge of the clock. After the start bit, eight data bits
(least significant bit first), followed by the parity bit
(odd), and the stop bit (high) are shifted out by the com-
puter with the clock signal provided by the keyboard.
The keyboard pulls the stop bit low in order to acknowl-
edge the receipt of the data. If a transmission error
occurs (parity error or similar) the keyboard issues a
“RESEND” command to the PC.
Key Pressure Release Encoding
Key pressure is communicated to the PC by sending a
scan code. Table 1 lists the scan codes corresponding
to keys ‘0’…’F’. Release is communicated by sending
the break code (0F0), followed by the previous scan
code.
TABLE 1:
SCAN CODES
Codes
Key
45
‘0’
16
‘1’
1E
‘2’
26
‘3’
25
‘4’
2E
‘5’
36
‘6’
3D
‘7’
3E
‘8’
46
‘9’
1C
‘A’
32
‘B’
21
‘C’
23
‘D’
24
‘E’
2B
‘F’
DS00665A-page 4
ã 1997 Microchip Technology Inc.
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