After finishing Watchdog Timer - Internal, we wish to achieve the same functionality using the external Real-Time Clock (https://abracon.com/Precisiontiming/AB18X5-RTC.pdf)
The AB18X5 includes on-chip oscillators to provide minimum power consumption, full RTC functions including battery backup and programmable counters and alarms for timer and watchdog functions, and either an I2C or SPI serial interface for communication with a host controller. An integrated power switch and a sophisticated system sleep manager with counter, timer, alarm, and interrupt capabilities allows the AB18X5 to be used as a supervisory component in a host microcontroller based system.
Resources:
Design and Implementation of Generic Flight Software for a CubeSat: Chapter 4.1 The I2C interface.