Commodore Amiga A1000, Hardware Reference Manual

Page: 337 / 380

BIT MAP OF REGISTER CRA
REG# NAME UNUSED SPMODE INMODE LOAD RUNMODE OUTMODE PBON START
E CRA unused 0=input 0=02 1=force 0=cont. 0=pulse 0=PB60FF 0=stop
unused 1=output 1=CNT load 1=one- 1=toggle 1-PB60N 1=start
(strobe) shot
|<------------Timer A Variables------------->|
All unused register bits are unaffected by a write and forced to 0 on a
read.
CONTROL REGISTER B:
BIT NAME FUNCTION
0 START 1=start Timer B, 0=stop Timer B.
This bit is automatically reset (=0) when
underflow occurs during one-shot mode.
1 PBON 1=Timer B output on PB7, 0= PB7 is normal operation.
2 OUTMODE 1=toggle, 0=pulse.
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.)
6,5 INMODE Bits CRB6 and CRB5 select one of four possible
input modes for Timer B, as follows:
CRB6 CRB5 Mode Selected
---- ---- --------------------------
0 0 Timer B counts 02 pulses
0 1 Timer B counts positive CNT transitions
1 0 Timer B counts Timer A underflow pulses
1 1 Timer B counts Timer A underflow pulses
while CNT pin is held high.
7 ALARM 1=writing to TOD registers sets Alarm
0=writing to TOD registers sets TOD clock.
Reading TOD registers always reads TOD clock,
regardless of the state of the Alarm bit.
- 328 Appendix F -