What are the three source differences of low frequency clock? (precision comparison / power consumption comparison)

RE: What are the three source differences of low frequency clock? (precision comparison / power consumption comparison)

outspace — Wed, 13 Nov 2019 10:59:53 GMT

Thank you very much for your guidance.This is the answer I want.

RE: What are the three source differences of low frequency clock? (precision comparison / power consumption comparison)

Simonr — Wed, 13 Nov 2019 07:58:52 GMT

1. You use the HFXO (external High-frequency clock) as a reference for calibration, which causes the HF clock to wake up now and then for calibration, which is why the RC oscillator draws more power than the LFXO (external LF clock). Yes, it is better to use the LFXO, it's just that some modules do not ship with this clock, and it takes space on a circuit board, which in some cases, causes customers to remove it for a smaller form factor.

2. The LFSYNT's advantages are that it has the same precision as the HF clock, and can be used without the components necessary for the LFXO, so if the current consumption doesn't matter, but the form factor and precision of the clock does. The LFSYNT would be the best choice. If the form factor and current consumption matters, but the clock precision doesn't then the LFRC would be the clock of choice. For most other use-cases, the LFXO will be the optimal LF clock.

3. The accuracy is set in the sdk_config.h file, and can be modified to any of the specified values. Keep in mind that the LFRC clock can not be more accurate than 250 PPM, while (I believe) the other two can both be set up to 500 PPM.

Best regards,

Simon

RE: What are the three source differences of low frequency clock? (precision comparison / power consumption comparison)

outspace — Wed, 13 Nov 2019 03:13:09 GMT

Hi,Simon

Thank you very much for your guidance.

I still don't understand some places.

1. When LFCK selects LFRC, do you need to use external LFXO for auxiliary calibration ( Document 5.2.4.1:"The LFRC oscillator can be calibrated to improve accuracy by using the HFXO as a reference oscillator during calibration. The LFRC oscillator does not require additional external components." ), if so, what is the significance of the user selecting internal RC as the clock source? Isn't it better to use LFXO directly?

2. What are the advantages and disadvantages of LFSYNT and LFRC? What is the significance of adding LFSYNT to the design?

3. What is the value of the accuracy of the "nrf_clock_lf_cfg_t" field?

thanks!

RE: What are the three source differences of low frequency clock? (precision comparison / power consumption comparison)

Simonr — Tue, 12 Nov 2019 13:25:31 GMT

1. The LFXO crystal will be the most accurate one, you can see specifics on this here. If using LFSYNT the LF clock will have the same accuracy as the HF clock you're using to synthesize it.

2. LFSYNT is the most power-consuming, as it requires the HF clock to be running (you can see its run currents in the link above). The LFXO clock is the least power consuming, having a run current of ~0.23µA.

3. Please check the product specification for more information on this.

In general, the LFXO crystal will be the best option for high accuracy and low power consumption, so we generally recommend using that.

Best regards,

Simon