Keep RTC active when STM32WBA65RIV6 is powered down

[LJimé.1]

Hi,

We are currently developing a solution based on the STM32WBA65RIV6 MCU, and we would like the internal RTC to remain active while the MCU is powered down.

We assume that this is possible, and we would like to clarify the following points:

• For how long does the RTC retain its values after the MCU is powered down (by default)?
• Is it possible to connect a backup capacitor or battery to supply only the RTC or part of the MCU in order to extend the retention time?

We also want to know if there is any documentation available on this topic. We have not been able to find detailed information.

Thank you in advance.

Best regards,

Luis

[Peter BENSCH · Accepted solution]

Well, it depends on what you mean by "while the MCU is powered down": as long as VDD remains within the permitted range, the RTC continues to run. However, if you switch off VDD, you also kill the RTC (see the data sheet, section 3.29 or the reference manual RM0515, section 37.1):

As long as the VDD supply voltage remains in the operating range, the RTC never stops, regardless of the device status (Run mode, low-power mode or under reset).

Reason: unlike other STM32 families, the STM32WBA6 does not have a VBAT pin that would allow the RTC to be powered independently of VDD. A VDD within the permitted range is therefore an essential requirement for the RTC to function.

Does it answer your question?

Regards
/Peter

[Laurent LOUAZON]

Hello,

May I suggest having a look at AN4718?

I can confirm some customers did implement successfully such implementation (case 1b) with STM32WBA.

Regards
Laurent

[LJimé.1]

Hi Peter,

Thank you for your response, it is very useful.

However, we have found something strange while testing the MB2130 evaluation board for this case.

We powered the MB2130 board exclusively through pins 30 (VDD) and 49 (GND) of connector CN1. The NRST signal was held low by shorting pins 10 (NRST) and 9 (GND) of the unequipped CN4 connector. You can see the connection below:

With a supply voltage of +2.8 V and RESET asserted, a constant current consumption of approximately 1 mA was measured.

It is worth noting that the different power supply pins of the MCU are NOT short-circuited. The solder bridges SB12, SB3, SB10, SB14, SB15, SB18, and SB19 are open. Additionally, it has been verified with a multimeter that there is no continuity between these pins.

We have checked the datasheet and there is no information about the current consumption in reset state:

Is it 1 mA a reasonable value? It seems too high for us.

Thank you in advance.

Best regards,

Luis

[LJimé.1]

Hi Laurent,

Thank you for your response. The solutions proposed in AN4718 are helpful in identifying ways to keep the RTC running.

As mentioned in my reply to Peter, we are currently experiencing issues with the current consumption measurement in RESET state, as one of the possible approaches is to keep the MCU in reset for a certain period of time.

Best regards,

Luis

[Laurent LOUAZON]

Hello,

1mA is expected if setting reset pin to  low.

Entering in low power mode should (such as 1uA in standby mode) be done through dedicated API.

Laurent

[Laurent LOUAZON]

We do have many wiki pages that might be useful such as below one
https://wiki.st.com/stm32mcu/wiki/Connectivity:STM32CubeWB0_Low_Power_Management

[Laurent LOUAZON]

Sorry, just realized I mixed chipset with another topic.
I provided data for STM32WB09, but I would guess this is similar for WBA (agree data is not provided in datasheet)
I would suggest using dedicated APIs for entering low power mode.
https://wiki.st.com/stm32mcu/wiki/Connectivity:STM32CubeWBA_Low_Power_Management

[Imen.D]

Hi @LJimé.1 ,

We would appreciate it if you could provide an update on this thread and confirm whether the issue has been resolved.

This information helps clarify the current situation for other Community users who have the same question.

[LJimé.1]

Hi,

Thank you very much for your responses, and please accept the apologies for the delay.

We were still evaluating our options. As the RTC is a crucial element in our design, we have decided to include an external RTC powered by backup capacitors, in order to avoid depending on the VDD supply, which is affected by the MCU operating mode and the state of the interfaces.

Best regards.

Luis