12MHz Oscillator Substitute

General discussions about V-USB, our firmware-only implementation of a low speed USB device on Atmel's AVR microcontrollers
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tureiti
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Joined: Tue Nov 21, 2006 2:09 am

12MHz Oscillator Substitute

Post by tureiti » Tue Nov 21, 2006 2:18 am

I have a few questions regarding the 12MHz oscillator...

Is it possible to use an oscillator of frequency <12MHz?

Is it possible to use the AVR's internal oscillator?

If so, what changes would need to be made to the firmware, and are these changes easily made?

christian
Objective Development
Objective Development
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Joined: Thu Nov 09, 2006 11:46 am

Post by christian » Tue Nov 21, 2006 11:04 am

The oscillator frequency must be 12 MHz +/- 0.2%. This precision is usually only met by a crystal resonator. Even a ceramic resonator may not be precise enough.

The precision is computed as the number of bits in one transaction (~ 100 bits) and the maximum bit skew of 1/4 bit until the end of the transaction. That makes 1/400 bit times or ~ 0.25%.

In practice, the clock should be as precise as possible.

You may succeed with a calibrated internal RC oscillator, but I really doubt that you can meet the requirement above over the entire temperature range, voltage range and life time of the device.

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