STM32WB55 RTC with battery backup

[ffjunq]

Hello everyone, I hope you are well.

I'm working with the STM32WB55. I'm having difficulty getting RTC to work correctly.

The VBat is always powered by 3v3, I have an on/off switch that only removes the power from the VDD, however, when I turn it off and on the microcontroller is stuck in the routine: "while (LL_RCC_LSE_IsReady() == 0U)". I noticed that when CFG_HW_RESET_BY_FW is 1, the board continues to function, but it resets the RTC registers and the time is lost and when CFG_HW_RESET_BY_FW = 0, the firmware is stuck in the "while (LL_RCC_LSE_IsReady() == 0U)" routine. Looking through the oscilloscope, the clock is all right.

Can anyone help me? Thank you very much!

[Pierre_Paris]

Hello @ffjunq,

Thank you for your question !

Here some remarks :

• Can you explain your application ? Why are you always powered by 3.3v ? Are you trying to generate an internal timestamp when a switch to Vbat occurs ?
• When you are stuck in the while, can you share the value of PWR_CR1, PWR_CR4, RCC_BDCR (especially LSEON, LSEBYP bits) ? 
• VBAT operation is automatically activated when VDD is not present. An internal VBAT battery charging circuit is embedded and can be activated when VDD is present.
• When CFG_HW_RESET_BY_FW is equal to 
  • 0 : No reset.
  • 1 : It resets the Backup domain, that's why you are loosing the time. A backup domain reset only affects the LSE oscillator, the RTC, the Backup registers and the RCC Backup domain control register.
• Are you using the CSS on LSE ? Are you monitoring Vbat with the ADC ?
• Have you read chapter 6.1.4 Battery backup domain in RM0434 ?

Best Regards,

Pierre

[ffjunq]

Hi Pierre, thank you very much for your support.

- I need to make my RTC never reset, so the VBAT is always in 3v3, remembering that I am using a V3 stlink to power the circuit. I have an on/off switch that only interferes with VDD, that is, VBat always 3v3.

- Following are the values of the registers:

- I don't need to charge the battery using VBat, as in the hardware I will use a specific chip to do this.

- In this case, I understand that the CFG_HW_RESET_BY_FW for my application always needs to be 0, so that it does not reset my RTC.

- Yes, I'm using CSS, but I've already tested with CSS off. And I'm not monitoring the VBat with the ADC, because I don't see the need for it at the moment, it may be that in the future I can use it to read the battery level, but I would have to check the possibility, since my battery is 4.2 V to 3V

- Yes, I have already read this document. However, I am performing again.

Thank you very much!

[ffjunq]

Taking advantage, I would like to know if there is a schematic of the ST, using a super capacitor so that the VBat is always at 3v3.

Thanks in advance!

[Pierre_Paris · Accepted solution]

Hello @ffjunq,

• Are you waking up from any low power modes ? Can you share your code and the LSE config ?
• I saw on your screen that LSECSSD is equal to 0x1. It means a failure is detected on LSE (32 kHz oscillator) by the CSS. Have you try with the LSI clock ? 
• Also, Have you respected the sequence ?
  • First enable LSE oscillator with LSEON 
  • Then, wait the ready flag LSERDY (set by hardware)
  • And select the clock source with RTCSEL (= 0x1 for LSE)
  • Finally, enable LSECSSON. (Once enabled LSECSSON cannot be disabled, except after a LSE failure detection (LSECSSD = 1). In that case the software MUST disable the LSECSSON bit.)
• Yes, as long as the supply voltage remains in the operating range, the RTC never stops, regardless of the device status (Run mode, low-power mode or under reset).
• For the supercap, are you aware of the AN4718 ?
• Unfortunately, I just checked ES0394 and the issue is mentionned (it's n° 2.2.2 : Unstable LSI1 when it clocks RTC or CSS on LSE) You need to apply one of the following measures :
  • Clock the RTC with LSE or HSE/32, without using the CSS on LSE.
  • If LSI1 clocks the RTC or when the LSECSSON bit is set, reset the backup domain upon each VDD power
    up (when the BORRSTF flag is set). If VBAT is separate from VDD, also restore the RTC configuration,
    backup registers and anti-tampering configuration.
• Also, are you using PC13 in your application ? (I am referring to issue 2.2.6 : PC13 signal transitions disturb LSE) 
• Can you confirm this is the problem (VBAT is separate from VDD and VDD goes off then on) ? And can you please try the workaround and let me know ?

Kind Regards,

Pierre