SparkFun Qwiic Pro Micro - USB-C (ATmega32U4) @ 3.3V/8MHz

Development Boards Arduino
1

If I wanted to run the SparkFun Qwiic Pro Micro at 3.3V because I’m connecting to 3.3V devices (rather than using level shifters) would I need to:

  1. Changes the 5V / 3.3V jumper to 3.3V.

As noted in the Hookup Guide.

  1. Change the fuses for Internal RC Oscillator for 8MHz operation.

According to the datasheet, “Figure 29-2. Maximum Frequency vs. Vcc, ATmega16U4/ATmega32U4” the maximum frequency for 3.3V operation is about 10MHz. I’ve seen notes claiming the ATmega32U4 operate at 16MHz (but it’s out of spec). Is it safe to assume the 16MHz crystal will be ignored?

  1. Select “ATmega32U4 (3.3V, 8MHz)” in the Arduino IDE.

  2. Change the boot loader? Will the boot loader handle the changes correctly or do I need to change it?

Are there any other things I should I do to operate the Pro Micro at 3.3V?

2

Thanks for reaching out to us on this.

First, I would like to recommend using logic-level shifters as a much simpler, cost-effective option so that instrumenting changes might be easier. If that is not an option, I will do my best to answer your questions (though these topics might be best-answered by an electrical engineer, so if anyone else has better info please chime in):

  1. Changes the 5V / 3.3V jumper to 3.3V.

As noted in the Hookup Guide.

Yes

  1. Change the fuses for Internal RC Oscillator for 8MHz operation.

According to the datasheet, “Figure 29-2. Maximum Frequency vs. Vcc, ATmega16U4/ATmega32U4” the maximum frequency for 3.3V operation is about 10MHz. I’ve seen notes claiming the ATmega32U4 operate at 16MHz (but it’s out of spec). Is it safe to assume the 16MHz crystal will be ignored?

The documentation http://cdn.sparkfun.com/datasheets/Dev/ … ga32U4.pdf seems to indicate such, but I would not say it is safe to assume so. Page 32 might have the relevant info you need

  1. Select “ATmega32U4 (3.3V, 8MHz)” in the Arduino IDE.

Yes

  1. Change the boot loader? Will the boot loader handle the changes correctly or do I need to change it?

It appears you would need to change it - reference page 330 in the above datasheet

Hope this helps, and happy sparking!

3

Thanks for the reply. I have a couple more follow-up questions.

  1. Change the fuses…Page 32 might have the relevant info you need.

I don’t see the “relevant” information on page 32 you’re pointing me to. Which paragraph is relevant? Page 32 doesn’t say anything about ignoring the crystal.

  1. Change the boot loader…

Is there a ready to use boot loader for the SparkFun Qwiic Pro Micro at 3.3? Or will I have to mod an existing one? If so suggested starting point? An URL to a “how to”?

I was thinking of a Li-Ion related project and thought changing the board would be easier than adding a boost converter to get to 5V. Now I’m not so sure. Maybe I the “SparkFun LiPo Charger/Booster - 5V/1A” (PRT-14411)

4

Almost all of this delves into the realm of Electrical Engineering consulting - I would still strongly advise to just simply use logic-level converters to appropriate your voltage(s) https://www.sparkfun.com/products/12009

2. Change the fuses…Page 32 might have the relevant info you need.
I don’t see the “relevant” information on page 32 you’re pointing me to. Which paragraph is relevant? Page 32 doesn’t say anything about ignoring the crystal.

Sorry; it actually starts on page 27-42 (section for setting which clocks to use)

4. Change the boot loader…
Is there a ready to use boot loader for the SparkFun Qwiic Pro Micro at 3.3? Or will I have to mod an existing one? If so suggested starting point? An URL to a “how to”?

No, we don’t have a ready-made version. Page 330 of that datasheet begins details how to begin creating one yourself.

5

You convinced me! I’ll just use a boost converter and the logic level converter as required (an the 3.3V LDO). Thanks.