Commodore Amiga A500 Technical Reference Manual page 229

Section 7.1
ClocWCalendar Information
The clocWcalendar is based on the OK1 MSM6242RS Direct Bus
Connected-Type Real Time Clock Chip.
The A2000 features a real time clock with a perpetual calendar
which is capable of reading and writing "YEAR", "MONTH", "DAY",
"WEEK, "HOUR", "MINUTE" and "SECOND". This time clock is a
peripheral IC, connected directly by means of a bus. It is standard on
the A2000, and can be added as an option to the A500 (included in
the A501 Memory Expander).
An interface between the time clock and a microcomputer uses
4
of
data bus lines,
4
address bus lines,
3
control bus lines and 2 chip se-
lect pins, and performs time setting, reading and other operations.
The clock function covers second, minute, hour, day, month, year and
day of week. In addition, other functions such as selection of a 24-
hour time and a 12-hour time system, automatic adjustment of leap
year in the Christian Era and 30-second correction by means of soft,
periodical interruption (or periodical wave-form output) and stop/
start of time counting.
The clock-calendar is a CMOS device, so there is low power con-
sumption.
A crystal used is capable of 32.768 KHz for a consideration over
time counting during battery backup.
When installed, the clock is located in memory a t $DC0000.
Clock Warning
The addresses used by the real time clock chip access the custom chip
registers without the memory expansion/real time clock module.
When probing to test for the existence of the clock, care must be
taken to avoid unintentional changes to the custom chip register. The
test used by the setclock utility references an address that maps t o
either the seconds register or a static read-only chip register, then
checks to see if the clock "ticks."
Note:
C91 2 can be used as a slow/fast control t o tune the clock t o
best effect.