Serious question: Do you think Sparkfun and Adafruit have every new processor board they design certified for FCC compliance?
I guess think of free space as a transmission line, but its impedance changes depending on how much is in contact with the circuit node. Those 500 Hz PWM signals have edges, so they can have some high-frequency content. The real question is who do you want for a customer? If you want Centrylink or Comcast then they will need the FCC certification for your blinky. Perhaps Walmart can offer it, but I'm not sure on that. If you want to offer it the way Sparkfun and Adafruit do (as development boards) then there is no need to do FCC at least until someone tells you to get it certified or they won't buy it (maybe Centrylink does want it after all).As far as keeping those high-frequency edges from radiating EMI all you need to do is to slow them down e.g. perhaps drive the LED with a constant 15mA current source so that the long wires (which have capacitance) running to the LED are not slammed with a quick current flow that ends up radiating a lot of EM.
I wouldn't worry too much about the microcontroller itself...Two weeks ago I spent some days at a certification company to test a few products for the company I work for. These products was analog audio equipment controlled by AVR microcontrollers running at 8 MHz. The PCBs are four-layer boards with ground plane on top and bottomWe tested for CE approval and for maritime use. CE approval way quite simple to pass, as it does not require to measure the noise on the input power (conductive noise). The radiated emmision (radiated out from the product) was measured 15m and 3m away IIRC.Here's a few design rules that will make you probably pass a test like this:* Use decoupling capacitors for all ICs on the board. 100n should work fine.* Place the decoupling capacitors as close to the power pin as possible and use a lot of ground vias to reduce the trace inductance.* Keep your ground plane(s) whole to reduce the impedance* Switched regulators are the worst, so be careful when designing with these. We use the L5973D in most product, and the switching frequency + harmonics were dominating, even when properly decoupled see attachment). Add an inductor in series to the Vin pin if you're going to measure conductive noise (noise on the power input). this reduced the noise on one of our products by 20dBuV (dB microvolt)