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Project Help and Ideas » Oscillator Speed

April 25, 2012
by Pew446
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Hey guys, how can I find out what speed my oscillator needs to be for USB? I know the kits use a 14.7456MHz oscillator, but how did you find out you needed it to be this speed? Thanks :)

April 25, 2012
by JimFrederickson
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Hello Pew446,

Look at this link...

I think it will explain most of what you need are far was "how did you find out you needed it to be this speed"...

Crystal, timing source, selection is a matter of several elements. (Many elements that are part of the physical design of the AVR Processor are covered in detail in the Datasheet...)

Probably, part count, cost/availability, power usage, and personal likes are the most important considerations.

For me...

I use 2 choices currently.

1 - 20mhz for general projects running at 5v. 2 - 8mhz Internal RC Oscillator for low part count less demanding projects running at 5v.

NOTE: If you change the Frequency then the Nerd Kit Serial Bootstrap Loader will need to be modified if you want to continue to use that. (Read in the link above for associated pitfalls.) Also some other programming method must be used if the Internal RC Oscillator is used as well.

As far as your question "how can I find out what speed my oscillator needs to be for USB", I am not sure entirely what you are asking?

As shipped the Nerd Kits AVR does not communicate directly on the USB Bus. There is a USB to Serial Adapter, so the AVR is ultimately communicating to the Computer VIA a Serial Interface.

If you want to enable your AVR Project to communicate directly on the USB Bus as a USB Device then much more is necessary. (A few external components, and a software layer as well.) While this is possible with the AVR ATmega168 and 328 Processors there AVR Processors that are directly enabled for this use.

I have seen AVR ATmega168's designed/programmed as a USB Devince with Oscillator speeds as low as 8mhz.

For Serial Communications, depending on your final baud rate, the range is much greater.

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