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Guest kenamt

33pF Cap Question

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Hello AllMy question is I do not have any 33pF Caps and have a wholedraw full of 22pF and 47pF.Question is - can I use these or does it need to be 33pF?Thanks all - my FSBUS system is running faster then ever aftera couple of good posts on how to fix it.Ken

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Hi, Ken.Without seeing the schematic it is difficult to say for sure. It sounds like it may me a timing cap? If true you will have to use the recommended value, if not you can use 22 of 47. Post a portion of your schematic for better feedback. TV

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If it's related to FSBUS, I'm almost sure this capacitor is used with the 4mhz crystal...C'yaPaulo

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Hi Ken, I'm not really sure if it will matter, you can just try it out, you won't blow anything up.Funny thing is I built a Serial PIC interface just recently that interfaces my R/C controller to my computer for use with simulators, the circuit is based on the 16F84A & also has a 4Mhz crystal, the caps they used in this circuit are 15pF around the crystal so I have a feeling they may not be that critical, but as mentioned earlier if it's to do with timing that may cause problems with FSBus.Cheers Glenn.

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The two caps used with the crystal are called "loading capacitors". Their function is to help match the electrical impedance of the crystal to the circuitry inside the PIC. If the values are extremely wrong, the PIC oscillator simply won't work. Their value has only the smallest effect on the frequency. Bottom line: just try it.Mikewww.mikesflightdeck.com

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From what I understand, the capacitance in the caps will change the stabilizing time for the crystal. lesser capacitance=shorter startup pause for the PIC is needed, but the quality of the square wave is lower. So bigger cap=longer startup pause for PIC but better square wave from crystal....Microchip has a recommendation of (taken right out of my bad memory) 15pF up to 100pF....right?Well... nuff jibberish, do as Mike said, try it out... think it'll work.Cheers!/ Olle Ramsberg

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