how can I keep RTC running when I replace battery?

[sunnyjet16]

Hello, I am using nRF51822 to build a product and employ 3.0V battery as power supply and system works on DCDC mode. I need RTC counter to do real time tracking. My question is:

1. When I replace battery, how can I keep RTC running without reset?
2. Can I do it following these steps: a, set system to slow clock working mode+. b, disable DCDC. c, when replacing battery completes, press button to go back to fast clock working mode.
3. Is there suggestion on hardware and software?

Thanks Alex

[sunnyjet16]

Ole, thanks for your answer. Usually, users will replace battery when it goes to below 2.0V which means if action time of changing battery is 20 seconds, the capacitor will be very big such as 220uF. On sleep mode, is RTC counter still working?

Regards Alex

[Ole Bauck]

There are two sleep modes on nRF51, System ON and System OFF. In system ON the RTC counter is still working. In system OFF everything is turned off and the only wakeup source is from a gpio pin, so RTC is not working. When waking up from system OFF a system reset will be applied.