Commodore Amiga A500 Technical Reference Manual page 225

Force
Load
Input Mode
TOD
A strobe bit allows the timer latch to be loaded into the timer
counter at any time, whether the timer is running or not.
Control bits allow selection of the clock used to decrement the timer.
TIMER A can count 0 2 pulses or external pulses applied to the CNT
pin. TIMER B can count 0 2 pulses, external CNT pulses, TIMER A un-
derflow pulses or TIMER
A
underflow pulses while the CNT pin is
held high.
The timer latch is loaded into the timer on any timer underflow, on a
force load or following a write to the high byte of the prescaler while
the timer is stopped. If the timer is running, a write to the high byte
will load the timer latch, but not reload the counter.
READ (TIMER)
REG Name
4
TA L 0
TA17
5
TAHI TAH7
6 TB L 0
TBL7
7 TB HI
TBH7
WRITE (PRESCALER)
REG Name
4 TA L 0 PAL7
5 TA HI PAH7
6 TB L 0
PBL7
7 TB HI
PBH7
TA16 TA15 TA14
TAL3 TAL2 TAL1
TALO
TAH6 TAH5 TAH4
TAH3 TAH2 TAHl
TAHO
TBL6 TB15
TB14 TBL3 TBL2
TBLl TBLO
TBH6 TBH5
TBH4 TBH3 TBH2
TBHl TBHO
PAL6
PAL5 PAL4 PAL3 PAL2
PAL1 PAL0
PAH6 PAH5 PAH4 PAH3 PAH2
PAHl PAHO
PBL6
PBL5 PBL4
PBL3 PBL2 PBLl
PBLO
PBH6 PBH5
PBH4 PBH3 PBH2 PBHl
PBHO
TOD consists of a 2 4 bit binary counter. Positive edge transitions on
this pin cause the binary to increment. The TOD pin has a passive
pull-up on it. A programmable ALARM is provided for generating an
interrupt at a desired time. The ALARM registers are located a t the
same addresses as the corresponding TOD register. Access to the
ALARM is governed by a Control Register bit. The ALARM is write-
only; any read of a TOD address will read time regardless of the state
of the ALARM access bit.
A specific sequence of events must be followed for proper setting
and reading of TOD. TOD is automatically stopped whenever a write
to the register occurs. The clock will not start again until after a
write to the LSB Event Register. This assures TOD will always start
at the desired time. Since a carry from one stage to the next can oc-
cur at any time with respect to a read operation, a latching function
is included to keep all Time of Day information constant during a