The table in this slide presents the key features in the PCF2127A and PCF2129A Accurate RTCs and also highlights the differences between the two devices. Two features that are present in both devices, but not common with most RTCs, are the battery switch-over function and the battery low detection function. The battery switch-over circuit monitors the main power supply and automatically switches to the backup supply when a power failure condition is detected. The battery low detection circuit monitors the status of the battery and when the battery voltage goes below a certain threshold value, a flag is set to indicate that the battery must be replaced soon.