Em 17/1/2012 16:11, Nathan House escreveu:
> I'm making a board for a PIC18 microcontroller and have a few
> questions about crystal selection. Firstly, on Digi-Key, under the
> heading "Load Capacitance," there is the option "Series" and then a
> bunch of different capacitor options below that. Will a series crystal
> work, or should I get one that uses capacitors?


Series resonant crystals need a different type of circuit. MCUs always
use parallel resonant crystals. Use a crystal with a load capacitance
bigger than your MCU's pin capacitance and add external capacitors that
make the total capacitance match the crystal's capacitance.


> One other question I
> have is regarding the crystal frequency. Most PIC18 microcontrollers
> (all of the ones I've used, at least) have a PLL built in, so a
> variety of different crystals can be used with the same resulting
> system clock speed. If the PIC I'm using has a PLL that would allow me
> to use, say, a 4MHz, 8MHz, 12MHz, 20MHz, or 24MHz crystal, is there
> any reason to use a higher speed crystal as opposed to a lower speed
> one?
>
> Appreciate your advice!
>
> Nathan


Lower external frequencies mean less EMI radiation, but the PLL will add
jitter to your clock.


Best regards,

Isaac

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