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With respect to the topic below, I was trying to find where in the AVR documentation does it say the 8 MHz RC internal oscillator is "disabled" when CKSEL is selected to a different source? I am working with a Atmega328p.

FCC Low Oscilation

Specifically, if CKSEL only changes the MUX input (figure 6 of atmega328 datashset), but the 8 MHz interncal oscillator is still going (just not being used) then any AVR chip in that family is not exempt under the FCC part 15 exception h. (for digital devices with clocks that produce/use frequency under 1.705 MHz powered by a battery.)

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    \$\begingroup\$ It's "MHz" and not "mhz". \$\endgroup\$
    – Andy aka
    Commented Mar 3, 2019 at 16:33
  • \$\begingroup\$ @Andyaka was about to say the same and thus fixed that. Dave, it's "Hz" with a capital H, and "M" for mega-, because "m-" is for milli, i.e. one billionth of what you meant. \$\endgroup\$
    – Marcus Müller
    Commented Mar 3, 2019 at 16:34
  • \$\begingroup\$ thx, lazy typing \$\endgroup\$
    – Dave
    Commented Mar 3, 2019 at 16:42
  • \$\begingroup\$ Look at the part of the documentation concerned with power draw in various modes, there will be information there as it consumes power when active \$\endgroup\$
    – Chris Stratton
    Commented Mar 3, 2019 at 17:00
  • \$\begingroup\$ Simply ask Microchip about it, if you enable via CKSEL the external clock source is this disable the internal oscillator or maybe you only switches the mux input. \$\endgroup\$
    – G36
    Commented Mar 3, 2019 at 17:11

3 Answers 3

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Confirmed via email confirmation with Microchip support.

When the external clock source is selected (such as for using a 1MHz crystal) by CKSEL, the internal RC oscillator is disabled.

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Datasheets like the one for the ATMEGA line http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf tend to tell you what you can do, and aren't great resources for succinctly describing what cannot be done. I don't think the chip can shut down its high-frequency oscillator. I don't see any disables and I think the chip's program memory runs off this internal oscillator so I don't imagine you can disable it and execute code.

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  • \$\begingroup\$ I agree, datasheets are an ever changing commodity based upon the willingness of the the author(s) to put energy into it. The AVR chipset does have OSCCAL, which reads values from memory, but I don't understand why flash or eeprom would use another clocking source besides system clock. I guess I should just ask Microchip. \$\endgroup\$
    – Dave
    Commented Mar 3, 2019 at 17:17
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Still going to wait for official word from Microchip, but @Andrew Macrae pointed me down the right road. It seems that the EEPROM status bits (EECR) are controlled by the calibrated internal RC oscillator; its also utilized for SPM instruction cycles as well. So it seem the internal RC oscillator is quite pervasive in the function of the chip as a whole, even if it is not used for the system clock.

EDIT: the information provided in the datasheet seems to contradict the answer provided by Microchip... but hey, they make the darn thing.

OSCCAL Description states a crictical function of internal RC Oscillator

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