I have heard that if a circuit is operating at less then 1.2 MHz it does not need FCC certification.

Does an ATMega328 with with a 1Mhz External crystal, and the internal clock not used qualify for this exemption. Or does does the AVR use the internal 8Mhz clock for other stuff that I am not aware and thus not satisfactory for this exemption?

Taken from https://www.gpo.gov/fdsys/pkg/CFR-2011-title47-vol1/pdf/CFR-2011-title47-vol1-sec15-103.pdf

"(h) Digital devices in which both the highest frequency generated and the highest frequency used are less than
1.705 MHz and which do not operate from the AC power lines or contain provisions for operation while connected to the AC power lines. Digital devices that include, or make provision for the use of, battery eliminators, AC adaptors or battery chargers which permit operation while charging or that connect to the AC power lines indirectly, obtaining their power through another device which is connected to the AC power lines, do not fall under
this exemption. "

  • \$\begingroup\$ A 1MHz crystal oscillator will produce harmonics well above 1 MHz so I doubt that running a CPU at 1MHz is an adequate way to sidestep FCC requirements. \$\endgroup\$ – Brian Drummond May 18 '16 at 10:26
  • \$\begingroup\$ @BrianDrummond How can I calculate the harmonics generated? \$\endgroup\$ – DarthRubik May 18 '16 at 11:52
  • \$\begingroup\$ Frequencies? Trivial. Amplitudes? you may be able to simulate them but practically you would have to measure them - essentially the FCC certification process. \$\endgroup\$ – Brian Drummond May 18 '16 at 12:19

If the external crystal source is selected, the internal oscillator is disabled. So if you use a 1MHz external crystal, then yes, nothing in the AVR will be operating above 1MHz.

The AVR in its default configuration uses the 8MHz oscillator but internally divided by 8. So nothing outside the IC or even inside the chip after the prescaler will be running above 1.2MHz, so theoretically (though I'm not a lawyer, so don't take my word for this) that may not require certification either.

Note: The above is answering the question of whether the internal clock is enabled or not. I have in no way attempted to verify whether the 1.2MHz limit you mention in the question is correct or not. See @jp314's answer.

  • \$\begingroup\$ No I don't think that would count....but definitely going to get a ton of 1Mhz crystals now \$\endgroup\$ – DarthRubik May 18 '16 at 1:09

No - the FCC restricts devices operating above 9 kHz: http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet62/oet62rev.pdf

  • \$\begingroup\$ Page 8 has a third number, 1.705 MHz. \$\endgroup\$ – CL. May 18 '16 at 7:31
  • 1
    \$\begingroup\$ Sub 9kHz any device is exempt. Sub 1.705MHz any device operating on battery power only is exempt. \$\endgroup\$ – Joel Wigton Dec 9 '17 at 4:45

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