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PCF2127 sometimes doesn't switch over to battery supply

Question asked by Thijs Ruiter on Sep 13, 2019
Latest reply on Sep 15, 2019 by G.w. Sun

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

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