[SOLVED]Unwanted RTC reset at reboot

[Laurent]

Hi

I'm using a STM32F405.

While playing with the RTC, I discovered that my date/time was cleared at every reboot of my device. I also found that it happens only when using LSE, switching to LSI makes it work (as long as the external power supply is plugged, of course).

Investigating further, I found the reason: when I select LSE, ChibiOS always executes a backup domain reset (hal_lld.c, line 61) because it thinks that the clock source has changed; and indeed, the RCC_BDCR register says that LSI was previously selected, which is of course wrong

So here is my problem: why is the RTCSEL bit of the RCC_BDCR register always reset to "LSI" (0x2) whereas I only use "LSE" (0x1)? This happens whenever I reset the execution. With the debugger, I found that the register has the correct value upon startup, but as soon as the execution reaches the ResetHandler (is there anything between startup and ResetHandler??), the value is already wrong again.

Of course, I have an internal battery which seems to be working fine.

If you need more details, or if I was not clear enough, don't hesitate to ask.

[Giovanni]

Probably Barthess can explain the problem, I'll wait his opinion first before attempting a fix.

Giovanni

[barthess]

Laurent
I will check it this evening. Also, it will be good to speak with you on at #chibios channel irc.freenode.net

[Laurent]

Thanks.

I'm hardly available for live chat, but we can discuss here or by e-mail, I'll do my best to always answer quickly.

[barthess]

I observe such RTC resets when resets MCU using openocd command "monitor reset halt". Changing it to "monitor soft_reset_halt" fix RTC resets but not allways correctly stops the core.

Write simple application sending time value to UART and try to reset board with reset pin or power cycle.

[Laurent]

I already wrote such an application, that outputs the time at startup, and I have tried all kind of resets, none of them works. I don't think I have a reset pin available (it's a custom board, not a development one), but both power cycle and soft reset (SCB_AIRCR = (AIRCR_VECTKEY | (SCB_AIRCR & (0x700)) | (1 << 2))) produce the same incorrect result.

[alex31]

I use same mcu, on olimex (rtc backup with battery) and custom design (rtc backup via super capacitor, sufficient for 3 days), and never notice
that problem using LSE.

rtc keep value over power cycling, reset, and watchdog reset.

the only problem we had on the custom board was to find compatible cms quartz and capacitor to make lse beat.

Alexandre

[barthess]

Laurent
Can you check (with oscilloscope for example) your LSE running good without main supply?

[Laurent]

I'll check that as soon as I can.

[Laurent]

I checked the LSE with an oscilloscope, it runs fine without the main supply.

Something important to remember: the RTC registers are correctly preserved when I reboot, so it's most likely not a power supply issue. It's just the RTCSEL bits of the RCC_BDCR register which always reset to "LSI", making ChibiOS trigger a backup domain reset; if I comment out lines 61-62 in hal_lld.c the clock is not reset after rebooting.