I am trying to determine exactly what all of the fuse settings correspond to on the ATmega328P. The options for the CKSEL bits are:

  • 1111-1000: Low Power Crystal Oscillator
  • 0111-0110: Full Swing Crystal Oscillator
  • 0101-0100: Low Frequency Crystal Oscillator
  • 0011: Internal 128kHz RC Oscillator
  • 0010: Calibrated Internal RC Oscillator
  • 0000: External Clock

Clearly, the clocks labeled "internal" are part of the "guts" of the microcontroller. No external components are required to use those clock sources. The external clock is clearly an external source.

My question is this: are the low power, full swing, and low frequency sources built in to the microcontroller itself, or do they require an external crystal oscillator to be purchased and connected to the clock pins? This is not 100% clear from reading the data sheet. (Although I am under the impression that they are not internal to the device.)

1 Answer 1


If it requires a crystal, as the ones in question clearly state, then obviously you need to connect a crystal (and load capacitors). Or use a ceramic resonator.

Only those labelled as internal are internal. Everything else is external.

  • Thanks. That's what I thought, but the fact that it said "external" for one, and not the others, confused me.
    – lemontwist
    Commented Nov 1, 2017 at 20:03
  • 1
    An external clock is a specific physical thing - as is a crystal.
    – Majenko
    Commented Nov 1, 2017 at 22:22

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