nRF52833 LFXO question

[harrykuang]

I am considering using this part from MAXIM (MAX31343).

https://datasheets.maximintegrated.com/en/ds/MAX31343.pdf

it is an RTC plus a 5ppm oscillator output that I can use for the nRF52833 LFXO. Upon checking the datasheet for the MAXIM part I see the CLKO pin is 32.875Khz and not 32.768 as needed by the nRF.  It is very accurate at 5ppm. Is the absolute frequency a deal breaker here?

Harry

[Jared]

Hi,

The nRF52833 has a +- 500 ppm requirement for the LFXO crystal for the BLE stack. The requirement for the ANT stack is even more strict at +-50 ppm.

If we calculate the allowed variation for the LFXO crystal for the BLE stack:

32.768 +- (32.768 x 500/10^6) = 32.784384 and 32.751616. 

For the oscillator:

32.875 +- ( 32.875 x 5/10^6) = 32.87516438 and 32.87483563

The oscillator is therefore not within the allowed variation for the LFXO crystal. 

Is the absolute frequency a deal breaker here?

Yes. 

best regards

Jared 

[harrykuang]

Thanks Jared

Any concerns with using a MEMS 32.768 Khz oscillator? This one is 3ppm. Looks like it can only drive 50uA of output current. I am thinking the nRF LFXO input does not consume nearly this much current?

Harry

[Jared]

I think that should be fine. We haven't specified the current consumption with an external LFXO source but for a LFXO crystal it's ca 0.23 µA. I believe it should be somewhat similar for your case.