Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet

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SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]

11063K–ATARM–05-Nov-13

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.

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.

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.

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.

Hour, minute and second matching alarm (SECEN, MINEN, HOUREN) can be enabled independently of SEC, MIN,
HOUR fields.

Note:

To change one of the SEC, MIN, HOUR, DATE, MONTH fields, it is recommended to disable the field before 
changing the value and then re-enable it after the change has been made. This requires up to 3 accesses to the 
RTC_TIMALR or RTC_CALALR registers. The first access clears the enable corresponding to the field to 
change (SECEN,MINEN,HOUREn,DATEEN,MTHEN). If the field is already cleared, this access is not required. 
The second access performs the change of the value (SEC, MIN, HOUR, DATE, MONTH). The third access is 
required to re-enable the field by writing 1 in SECEN,MINEN,HOUREn,DATEEN,MTHEN fields.

Verification on user interface data is performed when accessing the century, year, month, date, day, hours, minutes,
seconds and alarms. A check is performed on illegal BCD entries such as illegal date of the month with regard to the year
and century configured.

If one of the time fields is not correct, the data is not loaded into the register/counter and a flag is set in the validity
register. The user can not reset this flag. It is reset as soon as an acceptable value is programmed. This avoids any
further side effects in the hardware. The same procedure is done for the alarm.