HFXO crystal

RE: HFXO crystal

Sigurd — Wed, 07 Feb 2018 13:09:16 GMT

On the PCA10040 nRF52 DK, we use this (farnell / digikey) 32 MHz crystal. It has a 8 pF load capacitance with 10 ppm frequency tolerance and 10 ppm frequency stability. BLE have a frequency tolerance requirement at 40 ppm (fTOL_HFXO_BLE in the datasheet). It is no problem to use a 12pF crystal, it is within specification, and as long as you match C1 and C2 for this, then it's perfelty fine to use a 12pF crystal.

RE: HFXO crystal

Sergey_Kolen — Wed, 07 Feb 2018 11:09:05 GMT

hi ,

but suggestion of crystal manufacturers as : NDK and Murata , they suggest us use as a 12pF CL instead 8pF. because 12pF is more a stable.

what do you think about it?

thanks

RE: HFXO crystal

Sigurd — Wed, 07 Feb 2018 10:10:28 GMT

The crystal with 8pF is better.

As mentioned here:

It is recommended to use a crystal with lower than maximum load capacitance and/or shunt capacitance. A low load capacitance will reduce both start up time and current consumption.

RE: HFXO crystal

Sergey_Kolen — Wed, 07 Feb 2018 08:59:37 GMT

Hi,

what the answer for my a first question? which crystal 32MHz is better? with CL=8pF or CL=12pF?

thanks

RE: HFXO crystal

Sigurd — Tue, 06 Feb 2018 12:45:52 GMT

Hi,

The load capacitors are the capacitors connected to the crystal, C1 and C2 connected to the 32 MHz crystal (nRF52-series) in our reference design, and C11 and C12 for the 32 kHz crystal. The value of these capacitors can be found using the following formula:

C1 = C2 = 2Cl - C_pcb - C_pin

Cl: Load capacitance of the crystal (found in the crystal datasheet)

C_pcb + C_pin is approximately 4 pF.

On the PCA10040 DK the 32MHz crystal's load capacitor is 8pF. The load capacitors C1 and C2 are therefore 12pF on the PCA10040 DK. (8*2 - 4 = 12pF)

See this and this post for more information.