AVR internal oscillator jitter research

If you go to AVR site and open an AVR application note AVR053 you will notice that there are different RC oscillators installed in to AVR chips during history. In table you can see tunable oscillator versions and their features.

Simply speaking each new version of oscillator introduces better features and improvements. But is it really truth? ChaN has made interesting research on these oscillator version. He simply tested output signal with fixed width and measured timing fluctuations of it. And he found out, that RC generator frequency slowly fluctuates during time. Of course RC oscillator fluctuations are not a problem as this type of clock source isn’t considered to be stable. In time critical applications it is better to use crystals. But most interesting thing is that newer versions of tunable oscillators were generating much more jitter than older ones.

In the picture you can see that chips introduced 2-3 years ago shows almost no noticeable jitter, while new ones give about 0.5% maximum. An it seems that significant jitter appeared from 4.0 RC oscillator version. It is hard to tel the reason why it is so. Probably it is a result of some kind of efficiency improvements,that resulted in bigger RC jitter. But this is only a guessing as Application notes do not tell anything about this issue.

You can read full article here. It is Japanese, so use Babelfish translator.