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The ATMega328P datasheet specifies a load capacitance for a resonator from 12-22pF but the only 2 Resonators that will fit in my design (from Digikey) have 10pF and 33pF internal capacitors. Which one should I use, the 10pF or the 33pF, and what are the tradeoffs?

AtMega328P http://www.atmel.com/Images/Atmel-8271-8-bit-AVR-Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet.pdf

Resonator - CSTCE8M00G52-R0 http://www.murata.com/products/catalog/pdf/p16e.pdf

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  • \$\begingroup\$ Out of curiosity why don't you just use the internal RC oscillator? I believe you should be able to get 8 MHz out of it... \$\endgroup\$ – Doov Oct 4 '13 at 8:40
  • \$\begingroup\$ Out of curiosity, I built 2 boards with each and ran my application. They both work but it leaves open the question of how reliably they work in varying conditions. (They run off a 1S Lipo with no regulator, so roughly 4.2-3V). Doov has a point in that researching the RC oscillator may have been worthwhile had I considered it. \$\endgroup\$ – BSEE Oct 5 '13 at 0:31
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I've actually had to look into something similar (slightly different part) recently and decided to just use the internal RC oscillator. It sounds like your issue is that you want to run the part at 3.3V so the 16 MHz oscillator (murata part traditionally seen on arduino boards) isn't an option (I believe that the highest frequency you can "safely" use the 328 @ 3.3V is 13.333 MHz -- see the graph in the datasheet -- you can linearly extrapolate). If 8 MHz is what you want then I'd suggest just scrapping the external resonator and using the internal RC oscillator. I believe that the part ships with fuse settings such that it defacto uses the internal RC with a clock divider of 8. You'll need to reset that the clock divider fuses to 1, but you should be fine. I found this link, which may be helpful. You don't need to use the arduino stuff, but the link and downloads provide some clues as to how to do this easily.

http://arduino.cc/en/Tutorial/ArduinoToBreadboard

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The two capacitors are effectively in series, so the one with 33 pF capacitors would be best.

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  • \$\begingroup\$ what do you mean by "the two capacitors"? I'm cautious about contradicting someone of your esteem :) \$\endgroup\$ – vicatcu Mar 22 '13 at 2:07
  • \$\begingroup\$ Two feedback capacitors are required with the crystal and ceramic resonator oscillators used with most MCUs. Ceramic resonators often have them included, as in this case. \$\endgroup\$ – Leon Heller Mar 22 '13 at 3:45
  • \$\begingroup\$ were you to add additional external capacitors to each leg of this resonator, they would be in parallel with the internal capacitors, would they not? \$\endgroup\$ – vicatcu Mar 22 '13 at 17:50
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33 pF capacitors would the best, you can pair it with 16 MHz resonator and that means your minsys can run faster rather than use 10 pF ( lower resonator freq too).

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  • \$\begingroup\$ I left out the uC is powered at 3.3V. The datasheet says 8Mhz is recommended for 3.3V, and 16Mhz is ok at 5V. \$\endgroup\$ – BSEE Oct 2 '13 at 21:50
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I've used this one from Farnell: Order Code 2470366 also from Murata. It works perfectly well for me. Digikey aren't the cheapest/quickest supplier, but if you have to use them I'd say buy one of each and trial them. We're talking about a 16MHz device, not a 1.6GHz set up, so you have a favourable margin for tolerances.

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