I am looking to make a digital clock, using one of Microchip's RTCC (Real-Time Clock and Calendar) chip. The chip can retain the time/data when the system is turned off via a 3.3V battery. My question is: When the system power is supplied via the USB and the RTCC chip is also connected to the 3.3V battery. What is the common ground here? The ground of the USB power supply?
Please see the diagram.