Commodore Amiga A1000, Hardware Reference Manual

Page: 336 / 380

CONTROL REGISTERS
There are two control registers in the 8520, CRA and CRB. CRA is
associated with Timer A and CRB is associated with Timer B. The format of
the registers is as follows:
CONTROL REGISTER A
BIT NAME FUNCTION
0 START 1 = start Timer A, 0 - top Timer A.
This bit is automatically reset (= 0) when
underflow occurs during one-shot mode.
1 PBON 1 = Timer A output on PB6, 0 = PB6 is normal operation.
2 OUTMODE 1 = toggle, 0 = pulse.
3 RUNMODE 1 = one-shot mode, 0 = continuous mode.
4 LOAD 1 = force load (this is a strobe input, there is no
data storage; bit 4 will always read back a zero
and writing a 0 has no effect.)
5 INMODE 1 = Timer A count positive CNT transition,
0 = Timer A counts 02 pulses.
6 SPMODE 1 = Serial port=output (CNT is the source of the shift
clock)
0 = Serial port-input (external shift clock is
required)
7 UNUSED
- Appendix F 327 -