Datasheet

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SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
14.5 Functional Description
The RTC provides a full binary-coded decimal (BCD) clock that includes century (19/20), year (with leap years),
month, date, day, hours, minutes and seconds.
The valid year range is 1900 to 2099 in Gregorian mode, a two-hundred-year calendar.
The RTC can operate in 24-hour mode or in 12-hour mode with an AM/PM indicator.
Corrections for leap years are included (all years divisible by 4 being leap years). This is correct up to the year
2099.
14.5.1 Reference Clock
The reference clock is Slow Clock (SLCK). It can be driven internally or by an external 32.768 kHz crystal.
During low power modes of the processor, the oscillator runs and power consumption is critical. The crystal
selection has to take into account the current consumption for power saving and the frequency drift due to
temperature effect on the circuit for time accuracy.
14.5.2 Timing
The RTC is updated in real time at one-second intervals in normal mode for the counters of seconds, at one-
minute intervals for the counter of minutes and so on.
Due to the asynchronous operation of the RTC with respect to the rest of the chip, to be certain that the value read
in the RTC registers (century, year, month, date, day, hours, minutes, seconds) are valid and stable, it is
necessary to read these registers twice. If the data is the same both times, then it is valid. Therefore, a minimum of
two and a maximum of three accesses are required.
14.5.3 Alarm
The RTC has five programmable fields: month, date, hours, minutes and seconds.
Each of these fields can be enabled or disabled to match the alarm condition:
 If all the fields are enabled, an alarm flag is generated (the corresponding flag is asserted and an interrupt
generated if enabled) at a given month, date, hour/minute/second.
 If only the “seconds” field is enabled, then an alarm is generated every minute.
Depending on the combination of fields enabled, a large number of possibilities are available to the user ranging
from minutes to 365/366 days.