Using an external oscillator (TCXO) with a nrf52 as input to the HFXO

[khunckl]

I am currently involved in developing a product that is using the nrf52840.  The product needs to keep precise time across a wide range of temperatures, which is why a 32Mhz TCXO is needed to be used as input to the HFXO.  We will also be using the TXCO as an oscillator for a few other devices.  The TCXO selected has a very low phase noise of -150 dBc/Hz @ 100kHz offset (TCXO component name: NT2016SA-32M-END4263A). 

I see that the topic of using a TCXO as an external oscillator with the nrf52 HFXO has been discussed in past “devzone” posts.  For example: https://devzone.nordicsemi.com/f/nordic-q-a/35620/nrf52840-external-oscillator. In this discussion Bjørn mentions that Nordic does not recommend using a TCXO as little verification or testing of them has been completed.  However, he does mention that the external clock phase noise needs to meet the following criteria if one were to be used:

100k offset : -80dBc/Hz

300k offset : -100dBc/Hz

900k offset : -110dBc/Hz

In another discussion (https://devzone.nordicsemi.com/f/nordic-q-a/29771/nrf52832-external-xl-lf-clock ), Håkon mentions that the main concern with using an external clock source for the 32Mhz clock is because the nrf52 is “sensitive to noise on the radio end, where the 32M signal is used to generate the 2.4G signal. A line that is routed between 2 ICs or more will pick up noise and then provide a clock that will degrade the RF part.” 

It seems that the main concern with using an external clock source for the 32Mhz clock is that it could degrade the radio functionality of the nrf52.

As many of the posts that I read were a year old or older, I am interested if there is any more recent documentation or information that has been made available by Nordic that helps to explain the possibility of using an external clock source, such as a TCXO, as the 32Mhz clock.

I have the following questions that I have not been able to clearly answer from prior research:

1. If an external oscillator (TCXO) is selected that meets the external clock phase noise requirements, described by Bjørn, is it possible/recommended to use this external oscillator as the input into the HFXO? If it is not recommended, why?
2. In this particular project we are NOT planning to use the radio functionality of the nrf52. Therefore, what other complications could arise from using an external oscillator aside from degraded radio functionality? Why?
3. Would Nordic recommend the use of an external oscillator with the understanding that the radio functionality of the nrf52 would not be utilized in this product?
4. If it is not recommended, is there any documentation that describes why an external oscillator is not recommended with the HFCLK?
5. Are there any settings that could be changed on the nrf52 to facilitate the use of an external oscillator (TCXO) as an input into the HFXO?

Any help or discussion of these questions is greatly appreciated.

Thanks!

[bjorn-spockeli]

Hi khunckl, 

depending on what kind of signal the TCXO is feeding into XC1, the XC2 pin should be connected as listed 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.

If the TCXO outputs a rail-to-rail clock signal, then it should have a  'high' voltage above 0.7*VDD and 'low' voltage below 0.3*VDD. It's advisable to have full rail-to-rail swing, because it minimizes current consumption in the pad cell. 

If the TCXO outpus a low-swing signal, then it needs to have a peak-to-peak amplitude between 200mV and 1000mV. It can be either square wave of sinewave. DC level is not important, as long as the clock signal does not shoot above VDD or below VSS.

Best regards

Bjørn