SERIAL SHIFT REGISTER (SDR)
The serial port is a buffered, 8-bit synchronous shift register. A
control bit selects input or output mode. In the Amiga system one shift
register is used for the keyboard, and the other is unassigned. Note that
the RS-232 compatible serial port is controlled by the Paula chip; see
chapter 8 for details.
INPUT MODE
In input mode, data on the SP pin is shifted into the shift register on
the rising edge of the signal applied to the CNT pin. After eight CNT
pulses, the data in the shift register is dumped into the serial data
register and an interrupt is generated.
OUTPUT MODE
In the output mode, Timer A is used as the baud rate generator. Data is
shifted out on the SP pin at 1/2 the underflow rate of Timer A. The
maximum baud rate possible is 02 divided by 4, but the maximum usable
baud rate will be determined by line loading and the speed at which the
receiver responds to input data.
To begin transmission, you must first set up Timer A in continuous mode,
and start the timer. Transmission will start following a write to the
serial data register. The clock signal derived from Timer A appears as an
output on the CNT pin. The data in the serial data register will be
loaded into the shift register, then shifted out to the SP pin when a CNT
pulse occurs. Data shifted out becomes valid on the next falling edge of
CNT and remains valid until the next falling edge.
After eight CNT pulses, an interrupt is generated to indicate that more
data can be sent. If the serial data register was reloaded with new
information prior to this interrupt, the new data will automatically be
loaded into the shift register and transmission will continue.
If no further data is to be transmitted after the eighth CNT pulse, CNT
will return high and SP will remain at the level of the last data bit
transmitted.
SDR data is shifted out MSB first. Serial input data should appear in
this same format.
- 324 Appendix F -
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