nRF52832 external XL LF Clock

RE: nRF52832 external XL LF Clock

MartinBL — Mon, 29 Jan 2018 15:14:38 GMT

RE: nRF52832 external XL LF Clock

Mickael — Mon, 29 Jan 2018 12:31:00 GMT

HI there, just to double check: all of this should also apply to NRF52840, right?

RE: nRF52832 external XL LF Clock

Tmac — Fri, 30 Sep 2016 09:39:50 GMT

Thanks for the quick feedback. So functionally it should work, but there is a risk of performance degradation, right? What kind of noise is the issue here? maybee its possible to filter the clock signal close to the nRF52 XC1 pin. If XC1 doesn't have to be driven rail to rail, and high frequency noise is the issue, we could add a simple R-C LPF close to the XC1 pin cutting off even below the 32MHz. This would attenuate the 32MHz to some degree and result in a sinewave input to the XC1 pin. It would also attenuate any high frequency noise coupled into the clock signal. Any idea if this would be feasible.

RE: nRF52832 external XL LF Clock

Håkon Alseth — Fri, 30 Sep 2016 08:30:33 GMT

@Tmac,

It's because it 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.

Cheers, Håkon

RE: nRF52832 external XL LF Clock

Tmac — Thu, 29 Sep 2016 09:53:55 GMT

We have a design with a 32MHz clock avaliable, its a digital clock rail to rail from 0-1.8V. We would like to use this signal to drive the HFXO and save the cost of the Xtal an synchronize the nRF52 to the external clock. Can you shed some light on why this should not be possible to do with the nRF52 as stated below. I should think that most xtal oscillators would work with an external clock directly or maybee we need to add some level shifting.

PS: For those who might wonder, external clock source for the 32 MHz clock is not supported. It used to be on the nRF51, but the clock module has been redesigned on the nRF52.

RE: nRF52832 external XL LF Clock

MartinBL — Thu, 21 Jul 2016 14:20:33 GMT

Something like this maybe:

    NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos) | 
                        (1 << 16) | // Set bit 16 in LFCLKSRC to enable (use with rail-to-rail external source)
                        (1 << 17); // Set bit 17 in LFCLKSRC to enable use of external source instead of Xtal (SRC needs to be set to Xtal)

I'll report to the SDK team that the defines are missing.

RE: nRF52832 external XL LF Clock

apuret — Tue, 19 Jul 2016 11:17:48 GMT

@martinbl: Ah ! Thanks for the LFCLKSRC link, I was using the 1.0 PDF specification and "bypass" and "external clock" wasn't described. But now, how I set these bits with the SDK 11.0.0 (neither in defines, nor in nrf_clock_lf_cfg_t struct). See this question for the details.

RE: nRF52832 external XL LF Clock

MartinBL — Tue, 19 Jul 2016 09:17:47 GMT

@apuret: Have you configured the LFCLKSRC register to enable bypass and use external clock?

@aelbancol: Difficult to say. There is no guarantee if you leave it floating, so my advice is as always to stick to the recommendations in the PS.

RE: nRF52832 external XL LF Clock

apuret — Tue, 19 Jul 2016 08:16:05 GMT

How much rail-to-rail the signal must be ? I have a (~0 V, 2.7 V) signal on XL1 and no grounding for XL2 (3 V for VDD). The LFCLK configuration is set for a crystal oscillator. This configuration is working on the PCA10040 (real crystal) but not on my BMD-300 board (external clock).

RE: nRF52832 external XL LF Clock

aelbancol — Mon, 20 Jun 2016 21:36:56 GMT

How critical is it to have XL2 connected to ground for a single ended input? In our application, this is left floating.

RE: nRF52832 external XL LF Clock

MartinBL — Mon, 21 Mar 2016 10:29:17 GMT

Yes, the nRF52832 can accept an external 32.768 kHz clock feed.

The requirements are:

Signal shall be fed into XL1 pin
If low-swing sine: XL2 pin shall be grounded. Recommend >200 mVpp to ensure higher S/N ratio.
If rail-to-rail: XL2 can be left open or grounded. Voltage requirement from GND to VDD_NRF.

Now you can start the LFCLK as if it was an external crystal source.

PS: For those who might wonder, external clock source for the 32 MHz clock is not supported. It used to be on the nRF51, but the clock module has been redesigned on the nRF52.