A wearable device may use a dedicated real-time clock, a clock within its microcontroller, or another timekeeping arrangement. The right choice depends on sleep behavior, timestamp requirements, available board space and the complete power budget. An external RTC is useful when its capabilities solve a defined system requirement.
1. Maintaining time while the processor sleeps
A supported low-power clock domain can keep time while other parts of the device sleep. This can let the application avoid keeping its main processor active merely to maintain a calendar. Compare the dedicated RTC option with the microcontroller's available low-power modes before assuming that another IC will reduce total consumption.
Use data-sheet current figures for the intended supply, temperature and operating mode. Include interface pull-ups, backup-source leakage and the energy used each time the processor wakes. There is no universal current limit of a few nanoamps for all RTC ICs, and an RTC alone cannot establish days or weeks of battery life.
2. Giving stored measurements a time reference
A timestamp helps users and applications place a stored measurement in context. The firmware must associate the reading with the correct time, preserve the record and indicate when the clock was not valid. The RTC does not measure activity, heart rate or sleep stages; those functions depend on sensors and application processing.
Distinguish calendar accuracy from measurement timing. A readable clock value does not prove that a sensor was sampled at a precise instant. Where sampling intervals matter, define the timing source, capture method and allowable error separately from the calendar displayed to the user.
3. Supporting offline use and later synchronization
A local clock can provide a time reference while the wearable is disconnected from a phone. It will not automatically remain synchronized with the phone or a remote service. Clock drift, initial time setting and later corrections must be handled by the application.
Keep enough information to interpret records after reconnection. A device identifier, sequence number, synchronization status and documented time convention can help distinguish delayed uploads from newly measured data. Test a backward clock correction and a change of time zone rather than assuming that timestamps alone prevent duplicates or missing records.
4. Providing a wake-up signal where supported
Some RTCs offer alarm or timer outputs that can signal a processor. Confirm that the selected part provides the required function, that the output is wired correctly and that it can wake the processor from the intended sleep mode.
Producing a vibration, sound or display notification still requires energy and supporting hardware. The RTC does not independently render a user notification, and the notification path must be tested with the actual firmware. Check alarm rearming and startup behavior as well as the first successful wake-up.
5. Preserving time across an interruption where supported
Backup timekeeping requires a suitable energy source and a device designed to use it. A depleted main battery does not mean that an RTC can continue indefinitely without power. Review backup duration, source compatibility, switchover thresholds and fault handling for the complete circuit.
Texas Instruments' BQ32000 product information illustrates a serial RTC with battery backup, trickle charging and oscillator-fail detection. It is an example of features to check, not a recommendation for every wearable. Size, current consumption and the rest of the product requirements still need evaluation.
Validation checklist for a wearable design
- Measure the complete device's sleep and active consumption in representative use.
- Test initialization with an unset clock and recovery after a timekeeping fault.
- Check main-power interruption, backup depletion and restoration.
- Compare timestamp error over the required temperature range and offline period.
- Verify alarm behavior in every supported processor sleep mode.
- Test offline recording, reconnection, time correction and data export together.
A basic RTC does not guarantee complete records, protect stored personal data or make a device suitable for a medical application. Those requirements must be addressed and validated at the system level. Event-capture and security capabilities should be confirmed for the exact part rather than assumed for every RTC.
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