Gonna order several things from digikey, among these some TINY 13As, 2313s, and ATMEGA328s. I selected this crystal:
http://search.digikey.com/script...
so from what I've read on some other threads I would need 30 pf capacitors (parallel, usual setup) to use these crystals with the MCU. Correct?
http://search.digikey.com/script...
Those links don't!
Most likely not correct. The 30pF capacitors sound too big. It may still work, but most AVR datasheets seem to suggest capacitor range of 11 to 22 pF or something like that.
But you need to select capacitor value based on what the crystal needs, it just has to fit the range suggested for AVRs.
Check out this thread...
https://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=108229
The crystal is digikey part: X176-ND
and the capacitors: 490-3721-ND
The crystal has a load capacitance of 20pf. So from what I saw on here before, you assume 5 pf parasitic capcitance and so using C = 2(CL-CP), C = 2(20pf-15pf)=30pf.
One person on that thread you directed to be hobbss said
Most 12-16MHz parallel resonant crystals have a load capacitance of around 18pF, which means you would need about 30pF per cap.
Not saying that that's right or anything but yeah it does seem a little high. I've seen a lot of crystals use 22pf.
I would not use 30pf. I have done many boards based on calculations in the link I provided in the other thread. The crystal circuit on all of them perfectly the first time (i.e., no tweaking required).
Here's the link:
http://www.ecsxtal.com/store/pdf/quar_des.pdf
Well your calculations are correct. 15pF of CP stray capacitance is a bit high, normally I would use 10pF perhaps, but that is a matter of taste and 15pF might come from real hard experience.
The question now is if it a suitable crystal if it needs more capacitance than AVR datasheets recommend? Perhaps a crystal with 16pF load rating would be better? I think they are more common as well.
Sorry I didn't mean 15, I meant 5 pf. If you read that equation with 15 it doesn't make sense. (20-5)*2 = 30.
hobbs I used that equation in that sheet the CL= (CL1*CL2)/(CL1+CL2) + Cstray. Assuming Cstray is 5, using CL1 and CL2 values of 30pf would yield a CL of 20 pf (which is what CL is in my crystal datasheet).
Assuming Cstray = 5 is somewhat risky. How long are your traces? Are you hand soldering or machine placing? For a crystal that wants 20pf, I would use at most 26pf (if you can find it) or more likely 22pf.
I think you will be fine using this ECS crystal. Worst case, you can swap it out for an 18pf version (same footprint).
I got some 22pf. Haven't used any of them, but if I do, I have them
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Ok gotcha so more like 9-10 Cstray. I saw that most caps for avr's were 22pf or so but I wanted to be sure I was calculating it right. Thanks!
