I am using an external RTC ic connected to a micro. The RTC is been supplied by 2 separated power supplies via 2 schottky diodes. The 2 power supplies are a non-rechargable Lithium battery 3V and a 5V dc regulator.
The micro is been supplied by the 5V potential only.
In power fail mode there is no power supply for the micro, but the RTC ic is been supplied by the Lithium battery.
I would like to measure and monitor the Lithium battery Voltage level using an ADC input. This can be done in normal operation mode (VRef = Vcc). But, in power fail mode and while there is no power at the Vcc pin, there must be no power at any micro's pin, in order to protect the micro from an unexpected latch-up condition.
What are your ideas about solving this problem (for Lithium battery voltage measurement), following the simplest way?