topic Re: PCF85053A Bootable CPU RTC with two I2C buses in Sensors

PCF85053A Bootable CPU RTC with two I2C buses

ninja_hathori — Mon, 29 Sep 2025 12:09:50 GMT

Hi all,
I am using PCF85053a rtc in our project,
one control bit TWO, have power to control the time write access from secondary i2c,
when removing cr2032 control bit TWO reg move to zero,
do this reg move back zero when power cut happen in rtc or only after removing cr2032 battery, attached the datasheet ...
when primary system taking control over rtc , it makes control bit TWO as 1 and it should never go back until or unless cr2032 is not removed and added, but i am observing very suspicious behavior with the control bit TWO reg, why sometimes it moves back 0

Re: PCF85053A Bootable CPU RTC with two I2C buses

ErikaC — Mon, 29 Sep 2025 19:12:19 GMT

Hello,

Could you please share your schematic?

It’s not entirely clear to me. Please note that when the RTC switches to battery mode, the VDD power domain is disabled. This means the I²C interface is ignored, the CLK output is disabled, and the pin goes into high impedance (Hi-Z).
Thanks to the battery, the RTC continues keeping time, but communication is not possible until VDD rises above the threshold again.
Regarding your question: Does the TWO register reset to 0 (default) only when the CR2032 battery is removed, even if VDD is still present?

Regards.

Re: PCF85053A Bootable CPU RTC with two I2C buses

ninja_hathori — Tue, 30 Sep 2025 10:02:55 GMT

Hi Erika,
Thanks for the response, I am asking about the behaviour of the TWO reg in control bit, highlighted in the attachment,
when primary i2c set the time, it takes ownership of rtc module and make TWO bit 1,

my question is when this TWO bits come to default state zero,
case 1: when rtc coin cell cr2032 is removed or
case 2: when rtc power is turn off,

my observation, when I am removing coin cell cr2032 it comes back to zero,
but sometimes it comes back to zero, with no reason, which allow secondary i2c to set time, we not want this ...

Re: PCF85053A Bootable CPU RTC with two I2C buses

ninja_hathori — Wed, 08 Oct 2025 17:54:04 GMT