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Hot!Help with PIC18F25K80 and ceramic resonators

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2019/02/24 17:47:28 (permalink)

Help with PIC18F25K80 and ceramic resonators

Hi, I inherited a design , using PIC18F25K80 that originally used 4Mhz Murata resonators with 30pf tanks.  This was then changed  about a year ago to 16MHz Murata with similar tanks
What we are seeing is a small cross section of boards  fail with any about Vdd of 3.2V ( these are 5V boards) , at this point the clock fails and because the BOR is below this level ( 3.0V) it never gets triggered on a power down.  We have noticed therefore occasional corrupt EEPROM or FLASH.
Yet some users have boards that do operate to below the BOR point ( which is all we require )
Can the PIC 18F25K80 actually drive these resonators at 16Mhz , we find HS1 or HS2 has no real effect on the point where teh clocks fails
We are obviously thinking of changing to crystals , but we could do with finding a verifiable reason.
Any pointers ?

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    coffee critic
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    Re: Help with PIC18F25K80 and ceramic resonators 2019/02/25 10:30:53 (permalink)
    First make sure you are using HS not XT mode.  Then check that the amplitude of the oscillation is at least 1V to 2V. 
    Replace any series resistor with a zero ohm and measure amplitude again. You might also need or need to change the value of the(any) feedback resistor. 
    If this still doesn't work you might need to work with Murata to get a resonator with lower value load capacitors.  
    Lastly is there a reason you are using ceramic resonator vs a crystal?  Resonators generally have lower Q and higher ESR. 

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