nRF52832 external XL LF Clock

[lesockz]

Hello,

is it possible to use a external clock e.g. a digital output of a TXCO. As source for the LF clock. So the same clock source as used for the host uC can be used for the nRF52832.

Greets, Andi

[MartinBL]

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.

[aelbancol]

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

[apuret]

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).

[MartinBL]

@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.

[apuret]

@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.

[MartinBL]

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.