Hello,
Functionally speaking, yes, you can use the PCF8563 instead of the PCF8523, both are RTCs with very similar characteristics, however, changes would need to be done in HW and SW to be used as a "replacement" is the additional features of the PCF8523 are not used, it sill not be a simple swap replacement.
PCF8523 and PCF8563 are not pin-to-pin compatible, there are differences on the arrange of the pins, to modifications on the HW needs to be done. You can compare this on Figures 2 and 3 of PCF8563 datasheet vs figures 2, 3 and 4 of the PCF8523 datasheet.
PCF8563 datasheet: https://www.nxp.com/docs/en/data-sheet/PCF8563.pdf
PCF8523 datasheet: https://www.nxp.com/docs/en/data-sheet/PCF8523.pdf
Data rate is different, PCF8523 can work up to 1000kbit/s, while PCF8563 max speed is 400kbit/s.
Register map is different, check table 4 of the PCF8563 vs the table 6 of the PCF8523. PCF8563 contains 16 8-bit registers, while PCF8523 contains 20 8-bit registers.
Offset correction, PCF8523 offers an offset register allows fine-tuning of the clock; PCF8563 does not include this feature.
And more important for your application, the battery backup feature, as you mentioned, PCF8563 does not include this feature, while PCF8523 has a battery backup input pin and battery switch-over circuit (check section 8.5.2 of the PCF8523 datasheet for details).
So I would say that this is the main reason why you should use PCF8523 instead of PCF8563 for your application.
Have a great day,
Jose
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