Hi Raja,
It is true that you are within the operational voltage ranges for VDD and VBAT according to the datasheet, but I’m worried about the embedded Battery Switch Mode functionality for this device. Please check Section 8.11 of the datasheet: http://www.nxp.com/documents/data_sheet/PCF85263A.pdf
Due to this functionality, you may be working all the time in battery mode since VBAT is higher than VDD and when this happens, the VDD power domain is disabled. This means that I2C pins are ignored, CLK output is disabled and Hi-Z, TS pin output mode is disabled and Hi-Z, TS digital input is ignored and may be left floating. TS pin mechanical switch detector is active. INTA output is still active for interrupt output and battery switch indication, but disabled for clock output.
There is a way to disable this functionality (by setting BSOFF bit to “1”, however to do this, VDD and VBAT should be connected together, so, it not useful for your case.
However, there is a case where it would work with your voltage configuration, by setting BMS[1:0] to 00 (Switching at the Vth level) and setting the threshold Vth level LOW (1.5V) setting BSTH bit to “0”.
In this case, VDD would be higher than Vth, letting it work from VDD and permit I2C communication.
Have a great day,
Jose
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