PCF2127 sometimes doesn't switch over to battery supply

[t_ruiter]

Hi,

I am facing some problems with the PCF2127 RTC. After a powerdown, it sometimes has the OSF flag set.

According to the datasheet, the OSF flag will only be set after a powerup or a powerfailure(when the internal voltage has been below 1.2V).

I have measured the vcc, bbs and battery voltage and made 2 images. In one image we can see that the RTC changes supply when the vcc drops below 1.83V but this should be 2.5V according to the datasheet.

And in the other picture we see that the RTC changes supply at 0.8V and this can cause the OSF flag because it is below 1.2V.

But none of the measurements make sense according to the datasheet because it says it should always switch over at 2.5V.

What can cause this problem?

Kind regards,

Thijs Ruiter

[guoweisun]

Hi

Could you please share with us your schematic?

[t_ruiter]

Luckily I just solved the problem 5 minutes ago.

It is written in 5.3 of this document: https://www.nxp.com/docs/en/application-note/AN11186.pdf 

And also in this document: https://www.nxp.com/docs/en/application-note/AN11266.pdf 

To me it is weird that it is not written anywhere in the datasheet.