matching network of local oscillator

[Serafino Convertini]

Hi, in the development kit of nRF52840, the oscillator for the micro is configured as follow

In the bom, it can be seen that the used oscillator is the MP06003 that correspond to GSX-223/1P3MF. This crystal needs load capacitors of 8pF even if in the DK 12pF is used.

In my custom board I follow the same indication of the DK and I used MP06003 with 12pF laod capacitance, using the same ayout proposed by Nordic, adding further two inner layer connected to ground.

It seems that the oscillator doesn't work since the execution in debug stucks at a config function that deal with clock and the entire board has a unpredictable behaviour, sometimes it works, sometimes no.

Shoud I change the capacitor value?

Does anyone have the same issue?

Thanks in advance

[mathiaso]

Hi, Serafino.

The capacitors connected to the crystal should have a value of Ccap = 2 * CL - (Cpcb + Cpin), where CL is the crystal's load capacitance. For the 32 MHz crystal, a (Cpcb + Cpin) value of 4 pF can be used. This is explained both in this guide and in the HFXO section of the PS.

Do you have a 32.768 kHz crystal connected to your SoC as well?

Best regards,
Mathias

[Serafino Convertini]

Hi Mathias,

I'm using only HFXO. Since I'm using the same firmware used on the DK (I tested the firmware on the DK before producing my custom board), if the LFCLK  is not present (in my custom board I don't need the 32.768 kHz) could the execution stuck in some initializing function?

[mathiaso]

Yes, then that could be the issue.

Try to add
CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y
in prj.conf and see if that solves the issue.

[Serafino Convertini]

Great Mathias. Now it works.

Just a curiosity: the string CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y means to activate the 32k oscillators? The value should be 'n' to disactivate the oscillator?

[mathiaso]

Wonderful, Serafino.

CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC sets the LFRC as the 32k clock source. If this isn't set, the LFXO is the chosen 32k clock source, and as you haven't mounted the crystal, you'll be stuck in a loop waiting for this to start before reaching main().