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

The Keyboard
The keyboard is interfaced to the system through one pair of lines connected to the
odd-addressed 8520 CIA chip. These lines are CNT, for the keyboard clock (input from
keyboard), and SP, for keyboard data (input or output).
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 that is to be sent is stable on the SP line. The clock is an active low pulse.
The rising edge of this pulse clocks in the data.
The 8520 is set up to use the CNT line as a clock and the SP line as a data input to an
internal serial shift register. Appendix F contains most of the data sheet for the 8520
and provides more information for interested parties.
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. (The handshake is issued by the processor
pulsing the SP line low for a minimum of 75 microseconds.)
If another keystroke is received before the previous one has been accepted by the proces-
sor, the keyboard-processor (internal to keyboard) holds a type-ahead buffer apprOXI-
mately 10 "keycodes" long. (Keycodes are explained in the next section).
TYPE OF DATA RECEIVED
The keyboard data is not received in the form of ASCII characters. Instead, for max-
imum versatility, it is received in the form of keycodes. These codes include not only
the down-transition of the key, but also the up-transition. 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.
236 Interface Hardware