Mega32 Crystal Capacitor Selection

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I was perusing the the Mega32 datasheet for info on crystal capacitor selection in Table 4 on page 24 of the 2503G-AVR-11/04 revision (right off the Atmel web site).

The table only recommends caps for frequency ranges up to 8MHz when the part goes up to 16MHz...

What cap values should I be using for an oscillator in the 14MHz to 16MHz region? Should I just be using a 12-22 pf cap as listed for up to 8MHz?

Erick

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Hi,

I suggest you consult the data sheet of the crystal.
A more pragmatic answer is that for my 14.x MHz crystal it worked fine
with 15 pF to GND.

Cheers

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I used a 14.7456 MHz crystal (DigiKey p/n CTX089-ND) with 22pf capacitors. It starts up fine and has a good clean signal.

The capacitance is not super-critical. If it is too high or too low the frequency might be ever-so-slightly off.

Don Kinzer
ZBasic Microcontrollers
http://www.zbasic.net

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Have a look at the AVR Hardware Design Considerations application note from Atmel, it gives a basic formula to calculate the two caps with, along with a LOT of other great information:
C1 = C2 = 2(CL - 5pF)
Here C1 and C2 are the two caps (use the same value for both!), and CL is from the crystals datasheet - make sure you look at the datasheet for the exact crystal you'll be using, since it can vary quite a lot from type to type.
For instance, if the CL for your crystal is 16pF:
C1,2 = 2(16pF - 5pF)
C1,2 = 2(11pF)
C1,2 = 22pF
So your crystal should have 22pF caps on it.

The application note says that a value of 22pF - 33pF is normal. On this subject however, one crystal I'm looking at has a CL of 10pF, hence C1 and C2 would end up as 10pF also, which is quite far away from 22pF! Does anyone have any thoughts or tips on this?

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I found a good whitepaper on crystal and crystal capacitor selection on ECS's website: that shows where the equations come from and a slew more equations for various crystal circuit parameters

http://www.ecsxtal.com/pdf/quar_des.pdf

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