ST M41T01 RTC

[AEPL_ST]

Hello Forum members,

I am using ST M41T01 RTC in one of my products. I observed a strange behavior when I remove the coin cell and other power to the RTC. I configured the RTC for today's time, say 10 Jun 2025 and kept it running for an hour. It was working fine. Later I removed all types of power sources to the RTC - coin cell, DC power supply and there is literally no power to the RTC chip. I kept it for few hours @ 12 hours overnight and later next day (11 Jun 2025) applied power to the RTC chip. What I observed is the RTC started with same date as 10 Jun 2025 which should start with default values. I checked RTC datasheet but could not locate the default values for date/time registers. Can anyone provide the information on default values for different registers of M41T01 RTC?

Thanks in advance for your any help.

Best Regards,

Suhas.

[Peter BENSCH]

Welcome @AEPL_ST, to the community!

As you have unfortunately not inserted a programme excerpt that shows the parts relevant for the RTC, we can only speculate:

• Possibility 1: you accidentally initialised the VCC with this date again when you created it, so that the RTC receives this date again and reports it back
• Possibility 2: the initialisation with your date was actually only carried out once and the RTC still shows the same date. In this case, the remaining charge of the circuit was sufficient to retain the date information despite the long off time, but was not large enough to allow the RTC to continue counting, otherwise the RTC would have reached the following day.

In any case, there is no non-volatile memory in the RTC that can permanently retain programmed values without a supply voltage.

Does it answer your question?

Regards
/Peter

[AEPL_ST]

Hi Peter,

Thanks for your reply and confirming that there is no NVM in the RTC which can retain the programmed values.

Your speculation is correct for the 2nd possibility where the device initializes the RTC with given date time, in my case, it is 10-06-2025/11:10:00 and it continues even after I remove all types of power backups for the RTC. As you mentioned, if the remaining charge of the circuit can be sufficient to retain the date/time information as I only checked for data retention next day. For VBAT signal, I am directly connecting a coin cell battery and for VDD, it is a 3.3V supply in line with 0.1uF capacitor. Do you think this circuit is sufficient for retaining the data over longer off time? How can I calculate this retention time period?

Another point is not clear which is related to the default values used for the date/time registers.

Thanks in advance.

Regards,

Suhas

[Peter BENSCH]

If both VDD and VBAT are disconnected, it depends on many parameters how long a data-preserving voltage is maintained:

• possible capacitors present at VDD/VBAT
• ambient temperature
• air humidity
• leakage currents on the board
• etc

It is therefore very difficult to calculate a possible time for the data retention.

And once again: there is no default value for the registers, and certainly none for the date you have observed.

Regards
/Peter