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

[Ole Bauck]

Something needs to supply the circuit while the battery is gone, this can be a capacitor. You should go into System ON sleep mode before removing the battery. If you only keep 16K RAM retained the current draw should be 2.6uA. Be aware that if any event or interrupt should happen, the current draw will be higher.

The equation for how long the chip will last before going into brown-out reset (power fail) if we assume constant current draw is:

t = C * (V0 - 1.7V) / I

Where C is the capacitor value, V0 is the start voltage (3.0V in your case) and I is the current draw. If the capacitor is 10uF, current draw 2.6uA and the start voltage 3.0V, the chip will last for 5 seconds before going into reset.

EDIT: If the RTC is running, additional current of 0.5 or 0.9 uA (depending on crystal or RC oscillator) needs to be added to the system ON sleep current. So system On sleep current with 16K RAM enabled and RTC running will be 3.1 or 3.5 uA.

[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.

[sunnyjet16]

Thanks, Ole.