Clock-source selection for ANT

RE: Clock-source selection for ANT

Richard Phillips — Wed, 10 Jun 2015 14:29:37 GMT

Shall do, but can I suggest it's changed from "mark this answer as correct" (which suggests I'm confirming your answer as correct, which doesn't make sense, seeing as I asked you the question!), to "accept this answer" :)

RE: Clock-source selection for ANT

Petter Myhre — Wed, 10 Jun 2015 14:25:04 GMT

Click the grey circle with a check mark in it :)

RE: Clock-source selection for ANT

Richard Phillips — Wed, 10 Jun 2015 14:15:48 GMT

How do I do that?!

RE: Clock-source selection for ANT

Petter Myhre — Wed, 10 Jun 2015 14:09:35 GMT

Np. Please accept my answer if you find it sufficient.

RE: Clock-source selection for ANT

Richard Phillips — Wed, 10 Jun 2015 13:53:47 GMT

Great, thanks - in my application, NRF_CLOCK_LFCLKSRC_SYNTH (to give it it's new name!) should be fine, then.

RE: Clock-source selection for ANT

Petter Myhre — Wed, 10 Jun 2015 13:45:50 GMT

The LF clock source must have a frequency tolerance better than 50 ppm for ANT applications.

So you can use a 32kHz crystal oscillator, with an external 32kHz crystal with less than 50 ppm frequency tolerance. Or you can use the 32kHz synthesized oscillator, with an external 16MHz crystal with less than 42 ppm frequency tolerance.

The synthesized oscillator will draw much more current compared to the crystal oscillator, because it requires the 16MHz crystal to be running.

The run current of the 32kHz crystal oscillator is IX32k=0.4uA.

The run current of the synthesized oscillator is ISYNT32k=15uA, in addition to the run current of the 16MHz crystal oscillator, which is IX16M = 470uA.

This means that you need to replace IX32k with ISYNT32k+IX16M in(or with ISYNT32k in stages where the IX16M is missing) table 20 and 21 in the S210 v2.0 specification.

I'm not sure why NRF_CLOCK_LFCLKSRC_SYNTH_250_PPM is the only one in the list. I'm quite sure it is a bug. In my opinion it should be NRF_CLOCK_LFCLKSRC_SYNTH, since it depends on the choice of 16MHz crystal.

It should be safe to use, but please keep the current penalty in mind.