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So, how about that load capacitance?

Ok, here's my next thing.  I have this arduino on a breadboard experiment.  Used a sketch or two to program the ATmega328.  It works.  That is, it works when I pull the chip from my Diavolino and run it in there.  The blink sketch is running on it.  So, I put it in a breadboard with a pull-up resistor on the reset pin, and basically the breadboard looks like the one shown here, but with no other board attached.  I have an LED on what I believe to be pin 13; that is, from the picture, fifth pin from the right in the top row.  If I apply 5v directly to the wire that's connected here, the LED lights.  The pull-up resistor is probably working ok, but the behavior is the same even if I directly wire reset to 5v.

This leads me to think that something is wrong with my clock.  I have wires attached to the clock pins as shown, to plates in the breadboard, on one side of which rests each of the two crystal pins, one per wire.  On the opposite side are load capacitors.  Somewhat higher than people generally use because that's what I have, 33pf, each of which are connected directly to ground on the other side.  Basically it's as shown in the picture, but the load capacitors are not the usual 22pf ones.  It's my understanding that a larger capacitor should just slow the clock down some, but I don't know exactly how much breathing room I have here.  Is there an obvious problem with this setup?  Is there anything good to check if not?



  • I suppose I should mention that I've tried two identical crystals and two identical sets of capacitors, with the same results.

  • I'm not entirely clear on what you're trying to do, and what's not working here.  

    - Is the AVR working correctly when it is in the Diavolino, or in the breadboard, or both?

    - What's not working, or otherwise leading you to suspect a clock issue?

  • The AVR is working in the Diavolino.  It's not working in the breadboard.  It's hard to tell exactly what's not working, except that when I remove it from the board, and place it into the circuit on the breadboard, which I think is correct, it isn't blinking the LED.

    I suspect a clock issue just because it's one of few problems I can come up with that could cause the chip not to do anything.  The other ones being power (and I think it's getting power ok) and a low reset signal (which I've pulled high quite purposefully)... I might be missing something else I'm not thinking of, of course.

  • Then the crystal caps are indeed the likely culprit-- the crystal may simply not run if the caps are that far off from spec. It's likely worse than you think, because a breadboard adds quite a bit of stray capacitance, too.
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