Commodore Amiga A500 - Page 228

Control Registers
There are two control registers in the 8520: CRA and CRB. CRA is
associated with TlMER
A and CRB is associated with TlMER B.
The register format is as follows:
CRA:
BIT
0
CRB:
BIT
NAME
START
PBON
OUTMODE
RUNMODE
LOAD
SPMODE
NAME
ALARM
FUNCTION
1 =START TlMER A. 0 =STOP TIMER A. This bit is automatically reset
when underflow occurs during one-shot mode.
1 =TIMER A output appears on PB6,O = 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 CRBO-CRB4 are identical to CRAO-CRA4 for TlMER B with the excep-
tion that bit
1 controls the output of TlMER B on PB7).
Bits CRBS and CRB6 select one of four input modes for TlMER B as:
CRB6 CRBS
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.