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[F103] RTC_ALR not preserved after reset

Question asked by ziutek on Jan 21, 2016
Latest reply on Jan 21, 2016 by ziutek
I have a problem with RTC_ALR behaviour.

The RM0008 in 18.3.2 says:

All system registers are asynchronously reset by a System Reset or Power Reset, except for RTC_PRL, RTC_ALR, RTC_CNT, and RTC_DIV.
The RTC_PRL, RTC_ALR, RTC_CNT, and RTC_DIV registers are reset only by a Backup

In my application I use PRL, ALR, CNT, DIV and some backup registers. They all are preserved after reset or power-off (with battery connected to VBAT), except ALR which is always reset to 0xFFFFFFFF.

There is no problem writing to it and read it. When alarm is set it can generate both kind of interrupts (RTC and RTCAlarm) without any problem in run and sleep mode (I didn't tested stop nor standby mode).

This is very strange thing, especially that any other registers from backup domain which I use (RTC_PRL, RTC_CNT, BKP_DR1, BKP_DR2) aren't affected.

I tested this on STM32F103RCT6 and STM32F103C8T6 with the same result.

Errata sheet doesn't mention such bug. Internet doesn't help too...