load capacitence atmega 128 or any really

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So I ordered a few external oclliators for my atmega 128.

The datasheet said load capacitence Cl should be within 6pf and 22pf

So my question: If i bought a resonator with a load capacitence to it already do i need the parallel caps in the circuit as atmel drew it?

For example I recently got
a

CTX401-ND CRYSTAL 3.6864 MHZ 20PF LOAD CAP

from digikey...and im trying to make sure I wire it up right when I get it

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Normally, when you buy a CRYSTAL, they are telling you the required load capacitance.

You figure the required C value by subtacting about 7pf from the spec'd load due to the capacitance inside the IC. Then, you double the remainder and that gives you the C at each end of the crystal to ground.

Now, if you buy a 3 terminal ceramic resonator, it needs no external C. But that CTX401 crystal is just a plain crystal. It needs external caps.

And, by the way, this is not normally considered an "external oscillator". It is a crystal. An external osciallator is a complete circuit that oscillates on its own. It has a power and ground connection plus an output pin. There may be other terminals to tweak the frequency.

Jim

 

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Go back to the DigiKey web site on your part number page. Follow the link in the “Technical/Catalog Information” row. This takes you to a DigiKey page with links to the crystal manufactures data sheets.

Read the ATMEL application note AVR042: AVR Hardware Design Considerations. This explains the load caps on the crystal:

http://www.atmel.com/dyn/resourc...

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Thanks!