nrf_clock_lf_cfg_t for SYNTH operation

[RK]

I don't have a 32KHz crystal on my board (using the stock Rigado module) and power isn't an issue as the board is mains powered in operation, so I was intending on using the HF SYNTH for the LF clock.

I've just moved to the latest S130/S132 V2 which uses the nrf_clock_lf_cfg_t structure for configuring ble, for SYNTH operation do I have to provide a xtal_accuracy field and if so should it be

1. the accuracy of the actual 16MHz crystal (which I'll have to ask Rigado but I assume is better than 50ppm)

or

2. The default (0) 250ppm which matches the old SDK's NRF_CLOCK_LFCLKSRC_SYNTH_250_PPM, which was the only SYNTH source available?

or is SYNTH not really recommended and I should use the RC source with frequent calibration? I don't really mind, I'll take whatever gives the most stable BLE performance.

[Petter Myhre]

In general NRF_CLOCK_LF_SRC_SYNTH is not recommended. If you don't have an external 32 kHz crystal, it is recommended to use NRF_CLOCK_LF_SRC_RC with the configuration in the migration document:

nrf_clock_lf_cfg_t rc_cfg = {
   .source = NRF_CLOCK_LF_SRC_RC,
   .rc_ctiv = 4, // check every 4 * 250ms
   .rc_temp_ctiv = 1, // only calibrate if temperature has changed.
};

[RK]

ok - my only thought was this device is going to be subject to some temperature fluctuations, so I'll at least turn the temperature calibration up to 'fairly often'. The next board rev I'll put the external crystal on as well, it's worth it I think.