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The  CNT line  is used as a clock for the keyboard. On each transition of
this line, one bit of data is clocked in from the keyboard. The keyboard
sends this clock when each data bit is stable on the  SP line . The clock
is an active low pulse. The rising edge of this pulse clocks in the data.

After a data byte has been received from the keyboard, an  interrupt  from
the 8520 is issued to the processor.  The keyboard waits for a handshake
signal from the system before transmitting any more keystrokes. This
handshake is issued by the processor pulsing the  SP line  low then high.
While some keyboards can detect a 1 microsecond handshake pulse, the pulse
must be at least 85 microseconds for operation with all models of Amiga

If another keystroke is received before the previous one has been accepted
by the processor, the keyboard microprocessor holds keys in a 10 keycode
type-ahead buffer.

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