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Originally Posted by Al Skierkiewicz
OK, Hold it guys!
First, Rick, you need to know your prey. the Pic you refer to does not take a 40 MHz crystal input. It has an internal 4 x multiplier PLL that you can enable which will take a 10MHz resonator to 40 MHz internal to the chip.
See p. 19 of http://www.microchip.com/download/li...xx2/39564b.pdf
As far as crystals are concerned, I think the only ones you will find that are in your price range are third overtone crystals. These crystals are cut to produce harmonic energy at intervals above their fundamental in this case three times the frequency there are cut for. By designing an oscillator that will produce significant harmonic energy, using a crystal cut for overtone use and then tuning the output for the third harmonic, you can get crystal operation at higher frequencies. The layout of the circuit is critical and the crystal manufacturers specs need to be followed closely.
I think you will be better off with a 10MHz resonator and the 4X multiplier selected. |
Thanks, i will go with a 10Mhz crystal x4 w/the PLL, like you suggested. Digikey sells 10Mhz fundamental mode crystals. Specifically i am loking at Digikey PN:X422-ND, X443-ND and XC953-ND. What is the difference? Which should i use? The X443-ND and XC953-ND specify 18pF load caps. The X422-ND just says "series" under load cap. What does that mean? Does this indicate that the crysta lis series cut rather than parallel cut? Also, if i ever want to use a third overtone crystal, do i use it like normal, or do i have to build a circuit that filters out the fundamental or something like that? Your help is GREATLY appreciated.