Maxim Integrated Secure Microcontroller - Manual (Page 162)

Secure Microcontroller User’s Guide
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17.5 DS2251T/DS2252T RTC
The DS2251T and DS2252T RTCs provide permanently powered time-of-day monitoring. The clock runs
from an internal 32kHz crystal (in the modules) and is generally independent of the microcontroller. It
provides time of day information including 0.01 second, seconds, minutes, hours, day, date, month and
year. The register format is shown below. The RTC keeps time to two minutes per month accuracy. It
offers a complete representation of time and calendar in a convenient BCD format, but is accessible via
the memory bus in a parallel fashion and is read or written like a RAM. It requires no security pattern or
shifting to access.
The RTC also offers powerful interrupt capability including a time of day/calendar alarm and a periodic
interval timeout. The alarm can be set for once per minute, when an exact minute occurs, when an exact
minute and hour occurs, or when an exact minute, hour, and day occurs. On the DS2251T, this alarm
signal is brought out to an external pin on the module. The RTC also supports a second interrupt with a
programmable interval. This interrupt will activate if the interval is allowed to timeout. It is
programmable between 0.01 second and 99.99 seconds. It is also independent of the time-of-day interrupt
described above. The time-out interrupt can be used as a third timer, a watchdog function or for a variety
of other uses.
Figure 17-6 illustrates the internal RTC connection for the DS2251T and DS2252T. The difference
between the two versions is the interrupt pinout. A DS2251T has all of the RTC interrupt outputs brought
to the connector, allowing the user to determine how these are connected. The DS2252T provides one
open-drain interrupt output that is connected to P3.2.