SAM3X crystal oscillator design

Discussion around product based on ARM Cortex M3 core.

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Joined: Tue Jan 06, 2015 12:53 pm

SAM3X crystal oscillator design

Tue Jan 06, 2015 1:13 pm

There's references to an oscillator design document mentioned in the SAM3X checklist document but I can't seem to find it anywhere.
I know they exist for the ATMEGA series as I've used them in the past. I could just copy the capacitor values from the reference board schematic as I'm using a 12MHz crystal but does anyone have the link to document described? I've checked both the full document and the summary but no luck.
Posts: 31
Joined: Mon Aug 25, 2014 11:20 pm

Re: SAM3X crystal oscillator design

Sat Feb 07, 2015 6:33 pm

Capacitor values should come from the crystal manufacturer's datasheet. The load capacitance (this is the figure you're looking for) will vary between crystals and manufacturers. Don't forget to double this figure as they're effectively in series, and their value is thus halved. The value isn't absolutely critical- the nearest standard value will usually do.
  For example, if the load capacitance is given as 15pF, doubling this gives 30pF. The nearest standard value is 33pF- near enough. Bigger values (within reason!) will result in a slower start-up (too big, and it'll take forever!), but also more stable oscillation once started. Smaller values will tend to instability, harmonic oscillation, or even a refusal to start.
  Keep the crystal and it's capacitors as close to the chip as possible (on a double-sided board, I put mine directly below the chip, along with the decoupling caps, with a ground plane between them and the chip (on the chip side), leaving more room for i/o traces)- parasitic capacitances and stray pick-up will have an effect on performance-  see discussions/viewtopic.php/f,24/t,23467.html (nicely put, Tsuneo)

Regards, Mike.

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