Ultra Slow Teensy uC Clock Speed

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I am speaking specifically about the MKL26Z64 that is on the Teensy LC but I am interested answers to the same question regarding other Processors in the pjrc line, specifically:

Has anyone attempted to run the Teensy processors way below their rated clock frequencies? I am asking because my design does not require any kind of speed or accuracy, and if I can get the clock source down below 9 KHz then I the way I read the regulations my board is exempt from the FCC regulations...and that's a big deal.

I'm thinking of trying it at 8KHz although I will need to buy a little Schmitt trigger and a couple of timing components. But if anyone has already been down this road...

Thanks in advance,
Ray Russell
 
Silence so far, so let me try this from a slightly different angle: Has anyone ever tried to get a Teensy variant through FCC EMC testing?
 
I personally never tried this, but if your required process can work with 8khz, you probably don't need a teensy (sorry PJRC).
There are cheaper, smaller and easier to underclock MCU's that could probably do the process too.
EDIT:
Do you need the actual clock oscillator (crystal/resonator/R-C) to be under 9khz, or is it ok if it oscillates at a higher frequency and gets divided after?
 
True. You have a very good point regarding the part used in the Teensy. I just found it (and thus pjrc's teensy) first. But to answer the question in your last sentence as I read the FCC regs there seem to be two possibilities, both of which I am quoting directly from the Federal regulations:

1) From §15.103 "(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."

Now I can certainly reduce my clock frequency down below 1.705 MHz. However, the signal edge speeds will be significantly higher than that so I imagine that I would run afoul of the "generated" part.


2) From 47 CFR § 15.3 "(k) Digital device. (Previously defined as a computing device). An unintentional radiator (device or system) that generates and uses timing signals or pulses at a rate in excess of 9,000 pulses (cycles) per second and uses digital techniques; inclusive of telephone equipment..."


Notice that in item 2) there is no mention of "generated", it only addresses "timing signals or pulses".
 
I looked for this doc online, and Item 2) seem to be the definition of a "Digital Device" - a device with clock higher than (in excess of) 9khz.
Item 1) is the regulation for digital devices. Getting less than 1.705MHz is not a problem in MCU's like AVR/PIC. But the problem is this regulation don't allow you to connect to AC mains in any way, even through an external power supply. So you will have to charge a battery externally, and then connect it to your device. (this is how I understand these words).
I think, however, this is the time for you to take this issue to an electronic design forum like the EEVBLOG, or All About Circuits.
The users there have certainly gone through this kind of stuff, so they know how the regulation works better than me.
 
That was my assessment precisely. But the "Definition" section is the only place where I saw the 9 KHz limit so that's why i brought it up.

The "no external connection" part is not a problem for my particular device.

And finally, I think you're right about addressing the issue in a more general form. I will try that.

Thanks for taking the time to chat about it.
 
If you want advice on Teensy vs FCC EMC suggest doing a search via google of the forum (built in forum search is pretty broken) and most likely make a new post specifically asking that. People have certainly started the process with Teensy based products though forum posts normally only show up the ones that either did not make it or where edges cases were found.

edit: Some posts
https://forum.pjrc.com/threads/35257-FCC-certification-for-embedded-TeensyLC-USB-MIDI-device
midway down this
https://forum.pjrc.com/threads/28457-Using-Teensy-for-industrial-control-application?highlight=emc
 
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