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I know that a crystal oscillator has a particular ageing curve, that shall be taken into account when designing a long-term application.

But what about the RC internal to the MCUs? I did not find data related to ageing, but only precision over temperature. I am speaking about Freescale/NXP 9s12g family because is what I am studying now, but this question is also more general.

Thanks.

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Generally speaking, crystal oscillators are far more precise than RC oscillators. So if you want accurate timing then you cannot use an RC oscillator.

RC oscillators have such a bad (compared to a crystal oscillator) behavior over temperature and supply voltage that it is not even worth mentioning the aging as these external effects make the aging effect irrelevant.

To be able to measure the aging of an RC oscillator would be a challenge as you would have to recreate the exact conditions (temperature, supply voltage) at each measurement. A crystal oscillator is by far less sensitive to these external parameters so there it is much easier and practical to measure and specify aging.

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