Is it possible to put an external oscillator (e.g. TCXO) instead of a crystal for generating the 32 MHz of the nRF52840 ?
Is it possible to put an external oscillator (e.g. TCXO) instead of a crystal for generating the 32 MHz of the nRF52840 ?
Hi Bjorn,
Ths topic is totally relevant to me too. Because of our application, I also need tighther timings than the BLE requirements at the HFCLK level.
So what is not clear to me is: what are the reasons why it might not work?
Surely feeding a more accurate clock will not be a problem - given that it is well selected, I do also believe TCXO can be as good or better than XO for radio purposes.
So are you worried about how feeding directly a cliiped-sine wave or HCMOS level into the XO circuit might cause issues?
Thanks for your help,
Mickael
Hi Mickael,
I am not saying that its not going to work, im just saying that this is not something that we recommend as we have not done any testing or verification with TCXOs as the nRF52 familiy is designed with XOs in mind.
The clock accuracy is not the issue, I completely agree with you that a more accurate clock is always preferred. However, the external clock source must meet the following requirement with regards to phase noise:
100k offset : -80dBc/Hz
300k offset : -100dBc/Hz
900k offset : -110dBc/Hz
Many of the TCXOs use PLLs which have phase noise above the limits stated above.
Best regards
Bjørn
Hi Bjorn,
we are going to try and this might be interesting to more people here!
Could you please specifiy if we should feed XC1 or XC2 pin (as in which is the input?) when doing this?
Thank you,
Mickael
HI Mickael,
You feed XC1 and XC2 should then, depending on what kind of signal your feeding into XC1, be one of the below:
If low-swing sine: XC2 pin shall be grounded.
If rail-to-rail: XC2 can be left open or grounded. Voltage requirement from GND to VDD_NRF.
very clear! Thank you Bjorn