### Crystal load capacitor value for SAMD20

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**Wed Mar 30, 2016 12:48 pm**I am using the SAMD20J18. Recently I found some of my boards have problem regarding the external 32.768kHz crystal. To investigate I generate a 8Hz PWM signal by the TC module. Its clock source is the external 32.768kHz crystal. I found that the signal will pause for a while after reset. By further investigation I found that it may be the problem of the load capacitor of the crystal.

The load capacitors are connected in parellel with the crystal (I attached a picture of the connection here).

The MCU datasheet page 596 given a formula to calculate the load capacitor value for the external 32.768KHz crystal:

CLEXT = 2(CL – CSTRAY–CSHUNT)

where CSTRAY is the capacitance of the pins and PCB, CSHUNT is the shunt capacitance of the crystal. CLEXT is the external load capacitor value.

I selected a crystal which stated that the load capacitance is 12.5pF. The MCU datasheet state the pin capacitance is 3.15pF. Assume the PCB stray capacitance is 0.5pF.

By calculation with the above formula , the capacitor value is:

CLEXT = 2(12.5pF - 3.65pF) = 18pF

However, the datasheet page 624-625 also stated that if a 12.5pF crystal is used, it does not need connect external load capacitor.

In testing, I found that I have problem (10% of my board) when using 18pF load capacitor. If I remove the load capacitor, it still have problem. By trial and error I found it is okay when I use a 8pF capacitor (on one of my board only, so I do not know whether it is good for all of my board).

So what should be the correct value of the load capacitor?

Thank you.

The load capacitors are connected in parellel with the crystal (I attached a picture of the connection here).

The MCU datasheet page 596 given a formula to calculate the load capacitor value for the external 32.768KHz crystal:

CLEXT = 2(CL – CSTRAY–CSHUNT)

where CSTRAY is the capacitance of the pins and PCB, CSHUNT is the shunt capacitance of the crystal. CLEXT is the external load capacitor value.

I selected a crystal which stated that the load capacitance is 12.5pF. The MCU datasheet state the pin capacitance is 3.15pF. Assume the PCB stray capacitance is 0.5pF.

By calculation with the above formula , the capacitor value is:

CLEXT = 2(12.5pF - 3.65pF) = 18pF

However, the datasheet page 624-625 also stated that if a 12.5pF crystal is used, it does not need connect external load capacitor.

In testing, I found that I have problem (10% of my board) when using 18pF load capacitor. If I remove the load capacitor, it still have problem. By trial and error I found it is okay when I use a 8pF capacitor (on one of my board only, so I do not know whether it is good for all of my board).

So what should be the correct value of the load capacitor?

Thank you.