Has anyone any experience of running an Atmega168 at 250KHz, any problem with the TWI, ADC, eeprom or flash writing?
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The ADC's performance is guaranteed if the ADC clock is anywhere between 50 kHz and 200 kHz. With a 250 kHz CPU clock, it is possible to derive two guaranteed legal ADC clocks: 125 kHz or 62.5 kHz.
EEPROM and Flash writing are always driven by the internal calibrated RC oscillator -- no matter what clock you choose to use for the CPU. If you use the the factory default calibration constant (or change the calibration but keep it within 10% of nominal), the Flash and EEPROM writing should be unaffected.
I have no first-hand experience to report about the TWI module.
Remember that this family of chips has an internal 128kHz oscillator as a clock option.
The first page of the datasheet (as with all AVRs AFAIK) says "Fully Static Operation". A few years ago on previous generations it was noted that there seemed to be problems at very low clock frequencies like a few Hz. I do not know whether this is still the case but I wouldn't think it would be a problem in practice.
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