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* This specification are subject to be changed without notice.

EM83049

KEYBOARD ENCODER

5.2.1997

FUNCTION DESCRIPTIONS

Keyboard buffer

The keyboard will buffer 16 bytes in a first-in-first-out order when the system is able to receive scan codes

from the keyboard. The response codes and repeated codes will not be buffered. If keystrokes generate a

multiple-byte sequence, the entire sequence must fit into the buffer or the keystroke is discarded and a buffer-

overrun condition occurs.

Power-on Reset and self test

The duration of the keyboard Power-on-Reset(POR) should be within 150 milliseconds and 2 seconds after

the power is applied to the keyboard.

After executing POR , the keyboard executes a self test. The LEDs are turned on at the beginning and off at

the end of the self test. The self test takes a minimum 300 milliseconds and a maximum 500 milliseconds.

If the self test is successful, a completion code AA hex is sent to the system and the keyboard starts scanning.

If the self test fails, and error code is sent, the keyboard is disabled and waits for a command from the system.

The completion codes are sent between 450 milliseconds and 2.5 seconds after POR, and between 300 and

500 milliseconds after a RESET command is acknowledged.

Keyboard data output

When the keyboard is ready to send data to the system, it first checks clock and data lines. If either one is in

the low state, data is stored in the keyboard buffer. If both are in the high state, keyboard starts clocking data

out. Data will be valid before the trailing edge and after the leading edge of the clock pulse. During the

transmission the keyboard checks the clock line at least every 60 microseconds . If the system lowers the clock

lines before the leading edge of the 10th clock, the keyboard should stop sending, then buffer the data and

return clock and data lines to high state.

Keyboard data input

When the system is ready to send data to the keyboard, it first checks clock line to see if keyboard is sending

data. If keyboard is not sending data or it is sending data but has not reached the 10th clock, the system can

inhibit the interface by forcing the clock line low for more than 60 microseconds and prepares to send data.

The keyboard checks clock line status at least every 5 milliseconds. If a system Request to Send(RTS) is

detected, the keyboard clocks 11 bits in. After the 10th bit, the keyboard checks for a high state in data line

then pulls it low and clocks one more bit to signal the system that data has been received. If data is low after

the 10th bit, it indicates a frame error. The keyboard should continue to count until data line goes high, then

pulls it low and issues a RESEND to the system.

Command from the system

The system can send commands to the keyboard at any time. The keyboard needs to respond within 20

milliseconds, unless the system prevents keyboard output, when doing self test or executing a RESET.

The following are the keyboard input commands and the actions that keyboard needs to take.

Set /Reset Status indicators ( Hex ED )

- Responds ACK.

- Receives option byte.