26.5.3 Alarm

In Gregorian mode, 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 fields that are enabled in the RTC Calendar Alarm register (RTC_CALALR) and the RTC Time Alarm register(RTC_TIMALR), a large number of possibilities are available to the user ranging from minutes to 365/366 days.

Note: To change one of the RTC_TIMALR.SEC, MIN, HOUR and/or RTC_CALALR.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 three accesses to the RTC_TIMALR or RTC_CALALR. The first access clears the enable corresponding to the field to change (RTC_TIMALR.SECEN, MINEN, HOUREN and/or RTC_CALALR.DATEEN, MTHEN). If the field is already cleared, this access is not required. The second access performs the change of the value (RTC_TIMALR.SEC, MIN, HOUR and/or RTC_CALALR.DATE, MONTH). The third access is required to re-enable the field by writing 1 in RTC_TIMALR.SECEN, MINEN, HOUREN and/or RTC_CALALR.DATEEN, MTHEN.

In UTC mode, RTC_TIMALR must be configured to set the UTC alarm value and bit 0 in RTC_CALALR must be used to enable or disable the UTC alarm. If the UTC alarm is enabled, the alarm is generated once the UTC time matches the programmed UTC_TIME alarm field.

To change the UTC_TIME alarm field, proceed as follows:

  1. Disable the UTC alarm by clearing RTC_CALALR.UTCEN if it is not already cleared.
  2. Change the RTC_TIMALR.UTC_TIME alarm value.
  3. Re-enable the UTC alarm by setting RTC_CALALR.UTCEN.