Designing deterministic wake-up.

Wake-up is a hardware function.

With the main rails removed, the microcontroller cannot sample an input. An external element must remain active: the RV-3028-C7 RTC for scheduled events or a detection path for an external edge. ELPM-S3LW exposes gate, wake and force-on; their logic has to be designed together with the sensor and carrier board.

Boot must know why it started.

A robust sequence is: stabilize supplies, read and retain the cause, initialize only the required peripherals, perform sensing or communication, save persistent state, schedule the next event and finally release the latch. This order prevents losing the wake cause or switching off before a write completes.

Firmware should be idempotent: after an unexpected reset it must be able to repeat an operation without duplicating counters, packets or commands. A small record containing a version, sequence number and checksum is often safer than saving the whole application state.

Design for simultaneous events too.

An RTC alarm can coincide with an external edge; an input can remain asserted while power is removed; a transmission can fail while the next alarm is already close. Define priority, bound retries and ensure at least one path always reaches controlled shutdown.

Test waveforms and persistent state together.

• Measure delay from wake source to stable rails.
• Generate short pulses, bounce and simultaneous events.
• Interrupt power during each persistent write.
• Verify minimum battery voltage, temperature and worst-case boot times.
• Record current and duration for the complete cycle, not only Deep Stop.