Вот нашол немного
Most AVR MCUs can use different clock sources. The optional external clock sources
are Digital Clock, RC Oscillator, Crystal or Ceramic Resonator.
The choice of Oscillator type used in an application should depend on the accuracy
required. If the application uses an asynchronous communication interface, e.g., the
UART, requirement to the accuracy of the system clock could make it necessary to
choose a ceramic resonator or even a crystal. In many applications the use of a high
accuracy system clock is not required and in these applications an external or internal
RC Oscillator could be used to reduce the total system cost.
И ещё, в инете накопал, стабильность частоты зависит от источника питпния, т.к. батарейка у меня в порядке, стабилизатор на 5В есть, да и фильтров там натыкано немало (целых два
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)........... думаю что всё будет нормально.