Atmel SAM9X25, Manual

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15.4 Product Dependencies
15.4.1 Power Management
The Real-time Clock is continuously clocked at 32768 Hz. The Power Management Controller
has no effect on RTC behavior.
15.4.2 Interrupt
Within the System Controller, the RTC interrupt is OR-wired with all the other module interrupts.
Only one System Controller interrupt line is connected on one of the internal sources of the inter-
rupt controller.
RTC interrupt requires the interrupt controller to be programmed first.
When a System Controller interrupt occurs, the service routine must first determine the cause of
the interrupt. This is done by reading each status register of the System Controller peripherals
successively.
15.5 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.
15.5.1 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.
15.5.2 Timing
The RTC is updated in real time at one-second intervals in normal mode for the counters of sec-
onds, 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, sec-
onds) 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.
15.5.3 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.