Is it ok , if the value of resistance between extal and xtal change to 5.1K

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Is it ok , if the value of resistance between extal and xtal change to 5.1K

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zhongchenhui8
Contributor I
I have a question about s12zvc,need your help,really thank you.
Recently, when we have EMC test,at EMI,the s12zvc will be reset by internal singnal.When we change the value of resistance between extal and xtal to 5.1K,the test is ok.the PCB layout about this module is ok,shortest trace,and the ground is around of the oscillator.
there is no more information about the Rf and the gain of invert amplifier.if I change the value of resistance between extal and xtal,is it will have other problems that we don't know? can I change it to 5.1K or other value?54.bmp
 
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RadekS
NXP Employee
NXP Employee

Hi,

The external oscillator module at S12ZVC runs in loop controlled mode by default - no external Rf component necessary. The amplitude is driven dynamically.

The Rf at the picture presents internal impedance at a simplified block diagram - no real resistor. 

You may switch the external oscillator module at S12ZVC into full swing mode by OSCMOD bit set in CPMUOSC2 register. In such a case, some external Rf is expected (typically 1MOhm).

Generally, the full swing mode provides a better immunity against EMI while you pay by slightly higher power consumption.

     


I hope it helps you.

Have a great day,
Radek

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lama
NXP TechSupport
NXP TechSupport

Hi,

Rf is approximately 1M because of Pierce configuration. 5.1k is not correct value for this configuration. I would like to ask.
Have you removed any ground/Vdd plane, wire, part, everything from the place bellow the oscillator circuitry. Recommended is to leave this space clear to do not affect load capacitance and EMI. Have you used recommended osc circuitry pcb layout and correct load capacitance values. One common recommendation is as close as possible, as short wires as possible.

I do not see your design and I am very sorry EMI design is out of TIC support responsibility. The only I can do is to check you schematic design.

Best regards,

Ladislav

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