S08QE8 32.768KHz external crystal hardware?

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S08QE8 32.768KHz external crystal hardware?

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iansmusical
Contributor V

Hello All,

 

Looking at the QE8 datasheet/reference manual, I see that it's possible to directly connect a 32.768KHz crystal across the XTAL pins, without capacitors providing that the RANGE and HGO bits in ICSC2 are 0. However, the chips built in capacitors are not likely to be the same as the crystal capacitor load and therefore the frequency won't be optimal?

 

To get the optimal frequency I'd have to include external capacitors and set the HGO bit to 1? How do I simply calculate the Rf and Rs resistors? I have read application notes and while it seems easy to calculate the capacitor values, the other two values seem a bit of a mystery!? ;-)

 

Thanks,

 

Ian

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vicentegomez
NXP TechSupport
NXP TechSupport

Hi Lan

On the application note an3208 you can find some recommendation

AN3208
Have a great day,
Vicente

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leslie
Contributor II

Ian,

I have a feeling that playing with capacitor values won't change the frequency much. These (4?) parts are there to ensure stable oscillation and it seems like it is taken care of internally. Why don't you add "some" capacitors and measure the frequency change?

Laszlo

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iansmusical
Contributor V

Hi Laszlo,

Thank you for your input.

Yes indeed but when the HGO and RANGE bit are set to 0 I understand that the capacitors and two resistors that one would usually place outside the chip are instead built in to the chip. My point was that the capacitors are usually chosen to match the crystals capacitive load, which of course is variable depending on the chosen crystal? If the capacitances don't match then the frequency isn't optimal and I'm requiring optimal. Rather than add capacitance I have the feeling that switching to an external circuit is better but I was struggling with the values for Rf and Rs and hence how to calculate them.

Thanks,

Ian

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vicentegomez
NXP TechSupport
NXP TechSupport

Hi Lan

On the application note an3208 you can find some recommendation

AN3208
Have a great day,
Vicente

-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------

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iansmusical
Contributor V

Hi Pedro,

Thank you for referring me to app note AN3208. I now have a better understanding of crystal parameters etc. However, for stray capacitance am I right to assume that S08 devices in general have 8pf on each XTAL pin and this value for each pin is added to the C1 and C2 values in the formula given in the app note?

Thanks,

Ian

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