Electronic – Does anyone know of a microcontroller/DSP with no clock speeds or any oscillators in excess of 1.705 MHz


I am planning on making a simple electronic device which I might eventually try to market if it works out O.K. Before taking the plunge and trying to mass produce it, I would first try to sell a few on the internet to see if anyone even wants it.

It turns out, however, that to sell anything in the United States you need to pass FCC testing, or meet the criteria that makes it exempt. From what I hear FCC testing costs upward of $10,000, which I am not willing to put up right now. After much searching online for the official FCC documents (which were near impossible to find) it seems like one of the conditions is that that makes you exempt from FCC testing is if there is no oscillator or frequency in your circuit which is over 1.705 MHz (please let me know if this is wrong, I asked on here before finding the documents and everyone said it was 9 kHz and closed the thread).

Here is the link to the regulation:

Title 47: Telecommunication
Subpart B—Unintentional Radiators
§ 15.103 Exempted devices.

The following devices are subject only to the general conditions of operation in §§15.5 and 15.29 and are exempt from the specific technical standards and other requirements contained in this part. The operator of the exempted device shall be required to stop operating the device upon a finding by the Commission or its representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected. Although not mandatory, it is strongly recommended that the manufacturer of an exempted device endeavor to have the device meet the specific technical standards in this part.

(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.

Does anyone know of a micro that has a clock speed and all oscillators lower than 1.705 MHz? I found some micros that have clock speeds of 1 MHz, but oscillators are 4 MHz. A clock speed over 500 kHz could probably work, but 1 MHz would be best!

Best Answer

Many microcontrollers are fully static, i.e. that the clock can be completely stopped, or that you can run it at frequencies like 0.1 Hz, for instance, for 1 instruction per 10 seconds (might be useful for debugging). Some components on the die may require a minimum clock frequency to work, however, like an ADC: the sampling capacitor will discharge if you don't complete a conversion within a certain time.

That being said, the controller's clock will cause EMI over a band much wider than just the clock's frequency. The shorter the rise/fall of a digital signal the more energy there will be in the harmonics. To decrease EMI some microcontrollers, like Freescale's MC9S08, have (switchable) slew-rate controlled I/Os.