capacitors in arduino clone

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#1 21660361 16 Dec 2011 12:25

Ezra Chamba Ezra Chamba

Anonymous

Post #1
21660361 16 Dec 2011 12:25

I am building an arduino clone and normally, coming out of pins 9 and 10 of an atmega 168, you would have a 16MHz crystal going to 2, 22pF capacitors, what would happen if I used much larger capacitors like 1uF or higher?
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#2 21660362 16 Dec 2011 12:47

Steve Lawson Steve Lawson

Anonymous

Post #2
21660362 16 Dec 2011 12:47

That simply won't work. The values of those capacitors must be within the manufacturer specified range. There usually is a range, though, but only a few picofarads (maybe 9, 10 or so).
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#3 21660363 16 Dec 2011 15:58

Todd Hayden Todd Hayden

Anonymous

Post #3
21660363 16 Dec 2011 15:58

The load capacitors you speak of should be matched to the design target of the crystal being used. The design target of the crystal includes parasitic capacitance of your physical layout and the pins/input-structure of the driving IC.

http://www.raltron.com/cust/tools/appnotes/Crystal Mini Primer - Part 1.pdf

http://www.maxim-ic.com/app-notes/index.mvp/id/726

As stated in the comments of the Maxim link "CLOAD = ((C1 x C2) / (C1 + C2)) + CSTRAY. CSTRAY is the stray capacitance in the circuit, typically 5pF."
#4 21660364 17 Dec 2011 15:47

Per Zackrisson Per Zackrisson

Anonymous

Post #4
21660364 17 Dec 2011 15:47

Dont think you should build anything if you dont have rudimentary knowledge of electronics. The crystal in this case an oscillator with two capacitor as a frequency decoupling. With a capacitor 50 times bigger than specification the oscillator wont oscillate . Then your Avr168 wont have any clock and it will not be able to do any instruction.
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