Commodore Amiga A500 Technical Reference Manual page 225

Control Registers
CRA:
BIT NAME
0 START
1 PBON
2 OUTMODE
3 RUNMODE
4 LOAD
5 INMODE
6 SPMODE
7 TODIN
CRB:
BIT NAME
5,6 INMODE
7 ALARM
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 register format is as follows:
FUNCTION
1 = START TIMER A. 0 = STOP TIMER A. This bit is automatically reset
when underflow occurs during one-shot mode.
1 = TIMER A output appears on PB6, 0 = PB6 normal operation
1 = TOGGLE, 0 = PULSE
1 = ONE-SHOT, 0 = CONTINUOUS
1 = FORCE LOAD (this is a STROBE input, there is no data storage, bit 4 will
always read back a zero and writing a zero has no effect.
1 = TIMER A counts positive CNT transitions, 0 = TIMER A counts 02
pulses.
1 = SERIAL PORT output (CNT sources shift clock). 0 = SERIAL PORT input
(external shift clock required).
1 = 50 Hz clock required on TOD pin for accurate time.
0 = 60 Hz clock required on TOD pin for accurate time.
FUNCTION
(Bits CRB0-CRB4 are identical to CRA0-CRA4 for TIMER B with the excep-
tion that bit 1 controls the output of TIMER B on PB7).
Bits CRB5 and CRB6 select one of four input modes for TIMER B as:
CRB6 CRB5
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 is high
1 = writing to TOD registers set ALARM. 0 = writing to TOD registers sets
TOD clock.
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