3.3V powering & debugging with serial port

I have a completed project using a Photon. To save on battery life, I’m running at 3.3V. I was cautioned by particle.io support to be cautious when using the USB port while running with 3.3V since the 3.3V circuitry doesn’t have protection.

I often use the serial port to debug firmware. what are the appropriate protocols when using the serial port and powering with 3.3V? Should I make a USB cable that doesn’t supply power - would the serial port even work without 5V power?

@harley, when power is supplied via USB, it goes through a protection diode so that if the Photon is also powered via Vin, there won’t be a “clash” between the two supplies. There is no such protection between the output of the 3.3v regulator on the Photon and the 3V3 pin. As such, it is possible to have the onboard 3.3v regulator output and your 3.3v supply clash at the 3V3 pin. In your case, either use a USB cable that doesn’t provide 5V power or, if possible, disable your external supply and let the Photon’s onboard supply do the work.

@rickkas7, it is possible to disable the Photon onboard regulator by grounding TP as per the schematics (is TP accessible)?


Yes, you can make a USB cable with DATA+, DATA- and GND and not pass through USB power and it works.

I can’t find TP. Theres a 0402 resistor next to the regulator and that’s where you could disable the regulator, but I’d just go for using the special cable, or a N-channel P-channel MOSFET pair to disconnect the external 3V3 power when VIN has power.

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