AN665

DS00665A-page 4

ã 1997 Microchip Technology Inc.

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’