HFXO crystal

[Sergey_Kolen]

Dear Sir,

1. The HFXO crystal can be with load capacitance 12pF? or 8pF is better?

2. According to the datasheet, "17.1.1 64 MHz crystal oscillator (HFXO)"

there is calaculation of Cload , what is the value of Cpin?

thanks

[Sigurd]

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.

[Sergey_Kolen]

Hi,

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

thanks

[Sigurd]

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.

[Sergey_Kolen]

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

[Sigurd]

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.