RE: Initialising the SoftDevice at boot vs. when required

SoWizard — Fri, 01 Apr 2022 09:16:47 GMT

This is exactly the response I was looking for. Brilliant. Thanks Einar.

RE: Initialising the SoftDevice at boot vs. when required

Einar Thorsrud — Thu, 31 Mar 2022 11:23:00 GMT

Hi,

I do not see any good reason for not starting the SoftDevice immediately after reset. First of all, the current consumption consequence is negligible (a single RTC instance contributing 0.1 uA). Secondly, your primary reason for not wanting the SoftDevice is that it does not allow you to access the clocks directly, like triggering LFRC calibration using nrf_drv_clock_calibration_start(). However, the SoftDevice will calibrate the LFRC for you, based on the parameters it has been configured with.

Assuming you use the SoftDevice handler module, which is used in the Bluetooth examples in recent SDK versions, the LF clock configuration is typically done in sdk_config.h. When using LFRC the following configuration is normally most sensible:

//==========================================================
// <o> NRF_SDH_CLOCK_LF_SRC  - SoftDevice clock source.
 
// <0=> NRF_CLOCK_LF_SRC_RC 
// <1=> NRF_CLOCK_LF_SRC_XTAL 
// <2=> NRF_CLOCK_LF_SRC_SYNTH 

#ifndef NRF_SDH_CLOCK_LF_SRC
#define NRF_SDH_CLOCK_LF_SRC 0
#endif

// <o> NRF_SDH_CLOCK_LF_RC_CTIV - SoftDevice calibration timer interval. 
#ifndef NRF_SDH_CLOCK_LF_RC_CTIV
#define NRF_SDH_CLOCK_LF_RC_CTIV 16
#endif

// <o> NRF_SDH_CLOCK_LF_RC_TEMP_CTIV - SoftDevice calibration timer interval under constant temperature. 
// <i> How often (in number of calibration intervals) the RC oscillator shall be calibrated
// <i>  if the temperature has not changed.

#ifndef NRF_SDH_CLOCK_LF_RC_TEMP_CTIV
#define NRF_SDH_CLOCK_LF_RC_TEMP_CTIV 2
#endif

// <o> NRF_SDH_CLOCK_LF_ACCURACY  - External clock accuracy used in the LL to compute timing.
 
// <0=> NRF_CLOCK_LF_ACCURACY_250_PPM 
// <1=> NRF_CLOCK_LF_ACCURACY_500_PPM 
// <2=> NRF_CLOCK_LF_ACCURACY_150_PPM 
// <3=> NRF_CLOCK_LF_ACCURACY_100_PPM 
// <4=> NRF_CLOCK_LF_ACCURACY_75_PPM 
// <5=> NRF_CLOCK_LF_ACCURACY_50_PPM 
// <6=> NRF_CLOCK_LF_ACCURACY_30_PPM 
// <7=> NRF_CLOCK_LF_ACCURACY_20_PPM 
// <8=> NRF_CLOCK_LF_ACCURACY_10_PPM 
// <9=> NRF_CLOCK_LF_ACCURACY_5_PPM 
// <10=> NRF_CLOCK_LF_ACCURACY_2_PPM 
// <11=> NRF_CLOCK_LF_ACCURACY_1_PPM 

#ifndef NRF_SDH_CLOCK_LF_ACCURACY
#define NRF_SDH_CLOCK_LF_ACCURACY 1
#endif

With this configuration LFRC calibration is automatically performed when the SoftDevice is initialized, and regularly, but only if the temperature has changed (as the LFRC is temperature dependent).

A last reason for not waiting to start the SoftDevice is that many resources are blocked or protected by the SoftDevice, and requires you to use SoftDevice APIs to access them. If you are not consistent, you need to handle both these cases (some SDK libraries and drivers do this for you though).