HFCLK start and usage

[JeanMBoones]

Dear Nordic Support,

Processor: nRF52840 / S140 soft device / SDK 15.3.0

I have a question about the use of the HFCLK in relation to the soft device. Our board has both a 32.768kHz and a 32MHz crystal. Currently, we configure the LFCLK to use the external crystal, but we do NOT configure the HFCLK to use the external crystal. My question is this: In our case -- where the HFCLK is using the internal oscillator (HFINT) -- will the soft device automatically enable the external crystal for BLE transactions? The S140 SDS clearly says it will if the radio time-slot API is used, AND the user requests the HFCLK set to the external clock and ready before the time slot. We are not using this API. We are using the stack to manage a single BLE peripheral connection.

thanks,

- J

[Vidar Berg]

Hi,

The Softdevice will always start the HFXO before a RADIO event, and release it afterward to reduce the base current (e.g., advertisement event). The reason for this is that the RADIO requires an accurate clock source to transmit at the correct frequency. If you want the HFXO always on you can call  sd_clock_hfclk_request() after enabling the Softdevice, but this will most likely increase the average power consumption. 

Regards,

Vidar