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TB001 Datasheet(PDF) 4 Page - Microchip Technology |
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TB001 Datasheet(HTML) 4 Page - Microchip Technology |
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4 / 17 page  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’ |
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