The Keyboard; How The Keyboard Data Is Received; Type Of Data Received - Commodore Amiga A1000 Hardware Reference Manual

THE KEYBOARD

The keyboard is interfaced to the system via the serial shift register on one of the 8520
CIA chips. The keyboard data line is connected to the SP pin, the keyboard clock is
connected to the CNT pin. Appendix H contains a full description of the interface.

HOW THE KEYBOARD DATA IS RECEIVED

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 keyboards.
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.

TYPE OF DATA RECEIVED

The keyboard data is not received in the form of ASCII characters. Instead, for maximum
versatility, it is received in the form of keycodes. These codes include both the down and
up transitions of the keys. This allows your software to use both sets of information to
determine exactly what is happening on the keyboard.
Here is a list of the hexadecimal values that are assigned to the keyboard. A downstroke
of the key transmits the value shown here. An upstroke of the key transmits this value
plus $80. The picture of the keyboard at the end of this section shows the positions that
correspond to the description in the paragraphs below.
Note that raw keycodes provide positional information only, the legend which is printed on
top of the keys changes from country to country.
- Interface Hardware 245 -