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atmega168 instead of 2 22pf caps?
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atmega168 instead of 2 22pf caps?

by mtbf0 on Sun Sep 13, 2009 11:11 am

i've recently returned to my two year old boarduino clock project. there's a picture of it somewhere in this thread.

like the icetube it is built around an atmega168 and a 32.768KHz watch crystal. there's no cool vfd display and unlike the icetube there are no little caps from the crystal leads to ground but it was chugging along keeping pretty good time until...

i became concerned that the crystal would short out a couple of leads on the mcu and tilted it away from the chip. suddenly, (well 24 hours later anyway), i'm getting errors of 3 or 4 minutes a day. this went on for a couple of days until i bent the crystal back. seems to be doing much better now.

i'll be happy to accept that this may just be a figment of my imagination unless someone out there will tell me that it makes sense.
"i want to lead a dissipate existence, play scratchy records and enjoy my decline" - iggy pop, i need more
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Re: atmega168 instead of 2 22pf caps?

by adafruit on Sun Sep 13, 2009 1:45 pm

yes the capacitance makes a big difference! even 2pf or so will add enough error. breadboards have a lot of capacitance maybe 6pf

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Re: atmega168 instead of 2 22pf caps?

by mtbf0 on Sun Sep 13, 2009 2:20 pm

so i guess the crystal is working like a little theremin, sensing it's own distance from the mega168.

cool.

sort of.

i guess.
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Re: atmega168 instead of 2 22pf caps?

by len17 on Sun Sep 13, 2009 8:46 pm

mtbf0 wrote:cool.

sort of.

i guess.

It's extremely cool, because you can adjust the clock speed by minute adjustments of the crystal, until you get it to be more accurate than ladyada's Ice Tube! :)

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