We’re bringing an external USB connection in as 4 wires, so we can power an LED array separately from the Photon. Since we’re powering the Photon off the GND and 3V3 PINS, can we route the D- and D+ USB wires to leads on the Photon to connect up serial access to the Photon?
Yes, there’s pads for USB D+ / D- on the bottom of the Photon you can use, they’re pads 30 & 31 documented here https://docs.particle.io/datasheets/photon-datasheet/#pin-out-diagrams
Hmm, I’m a bit reluctant to agree fully.
It is true that there are D+/D- pads on the Photon, but I’m not too sure if you’ll actually be able to establish a relyable USB connection if the Photon is not 5V powered.
There is the word of 15k pull-ups to 3.6V, so it might be a bit tight especially if your 3V3 is not quite 3.3V and is by no means allowed to drop anywhere bellow 3.0V (to have working USB).
Look at the circuit of the photon under “SCHEMATIC - POWER” at https://docs.particle.io/datasheets/photon-datasheet/. There’s no electrical connection between the 5V from USB (or the VIN pin) and the rest of the USB circuit or P0 module.
It’s pretty common for self-powered devices to leave the USB 5V pin disconnected; you only need GND, D-, and D+ for USB to work.
I believe technically it’s true that the specs say you’re supposed to have some kind of sense circuit on USB 5V if you’re not using it for power because you’re not supposed to communicate if you don’t see voltage from the host on that pin, but in practice it works fine. Consider for example these instructions on how to run a Teensy without USB 5V: https://www.pjrc.com/teensy/external_power.html. I’ve done this myself many times and never had issues.