nRF51822: will RTC work without a 32 KHz input?

[Gerry Ortoli]

OK, probably a dumb question: The nRF51822 reference manual indicates that the RTC is driven by an external 32 KHz input. Can anyone confirm whether it will or will not? I'm asking because I'm developing software for a custom board that does not have a 32 KHz crystal. I need a lightweight mechanism for determining the relative time between events. I thought to use the RTC counter for this, but it's always zero. I'm trying to avoid the overhead of an app timer. Confirmations? Suggestions? Thanks!

[ovrebekk]

You will need a 32 kHz input, but you can use the internal 32 kHz RC oscillator instead of the external crystal.

Just be aware that the internal RC oscillator is several orders of magnitude less accurate than the external crystal.

[Gerry Ortoli]

OK, knowing that I'm using the Soft Device, how do I specify use of the internal 32 KHz oscillator? The reference manual describes this option but I can't seem to find how to do it. The only thing close is the BLE_STACK_HANDLER_INIT() function, but the description implies this just affects the stack, not the chip configuration. Help?

[Ole Morten]

Take a look at this: https://devzone.nordicsemi.com/index.php/what-low-frequency-clock-sources-can-i-use

[Gerry Ortoli]

Thanks for the info. I'll pass this to the hardware team & maybe the next rev of the board will be different!