External 32.768KHz oscillator, not crystal

[mzarling]

In our nRF52832 application, we have an external 32.768KHz crystal as the nRF52832 low frequency clock source. We also have a 32.768KHz oscillator on the PCB that is used as the clock source for another IC. This oscillator has a pin that allows it to be disabled. Is there any reason we can't use this oscillator as the low frequency clock source for the nRF52832 and then use an nRF52832 GPIO pin to disable the oscillator when going into SYSTEM OFF mode? We only wake up from SYSTEM OFF mode by detecting a level change on a GPIO pin.

[Jørgen Holmefjord]

Hi,

The nRF52832 can support external clock input for the low frequency clock, as described in 32.768 kHz crystal oscillator (LFXO):

The following external clock sources are supported:

• Low swing clock signal applied to the XL1 pin. The XL2 pin shall then be grounded.
• Rail-to-rail clock signal applied to the XL1 pin. The XL2 pin shall then be grounded or left unconnected.

Note that you should check that the accuracy of the oscillator is good enough if using the RADIO for BLE operations, etc. Alternatively, you can also use the internal RC oscillator, however this will give you a bit higher current consumption.

The Softdevice also does not provide any APIs to use external clock source, but you can use the following workaround.

Best regards,
Jørgen

[mzarling]

So there should not be a problem if I use a GPIO pin to disable the external 32.768KHz oscillator before going into SYSTEM OFF mode and then re-enable the oscillator upon waking up from SYSTEM OFF mode? 

[Jørgen Holmefjord]

Yes, I can't see any problems with that. The 32.768 kHz clock is only required to run by a few specific peripherals (RTC, WDT), and none of these are available in System OFF mode. The LFCLK is controlled by the application, I would recommend you to stop it by triggering TASKS_LFCLKSTOP, before disabling the external oscillator.

[mzarling]

Ok, thank you for your recommendation.