MPC5748G Spread Spectrum

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MPC5748G Spread Spectrum

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nextsteve
Contributor III

Hi, I'm interested in trying out the spread spectrum feature of the MPC5748G PLL.

Wondering if there are reference register values available for STEPSIZE, STEPNO, etc. I could use to test quickly while I work through the equations.

Thanks,

Steve 

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petervlna
NXP Employee
NXP Employee

Hi,

I have created example SW for you.

My STEP is 250 and modulation frequency is 20kHz.

I have verified it with spectral analyzer. You can use it as a reference.

https://community.nxp.com/docs/DOC-334867 

Peter

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nextsteve
Contributor III

That is awesome Peter, Thanks! 

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petervlna
NXP Employee
NXP Employee

Hi,

We do not have reference example for frequency modulation.

You have to calculate STEPSIZE and STEPNO.

From my past experience I recommend you after calculation to use spectral analyzer for manual fine tuning.

Peter

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nextsteve
Contributor III

Hi Peter, does STEPSIZE need to be non-zero? 

If so, then I don't see how the calc can possibly work because with a 40MHz crystal and modulation freq=10kHz --> STEPNO=2000. N would have to be >2000 for STEPSIZE to round to anything other than 0.

What am I missing?

Thanks,

Steve 

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