Post #3289052

2026-06-07 17:26 UTC

@matildalove@wetdry.world @valpackett@social.treehouse.systems USB PD (power delivery) specifies how devices handshake with sources to agree on a bunch of parameters including maximum current draw. if the device does nothing at all, it is typically given no power. active negotiation can allow you to get larger current limits and higher voltages, but requires sending data down the lines, which usually needs a special USB PD sink chip. this adds cost and is often unnecessary, so there's a passive negotiation option available.

Replies (2)

@gsuberland@chaos.social 2026-06-07 17:28

@matildalove@wetdry.world @valpackett@social.treehouse.systems EDIT: this post was wrong, removing content to avoid misinformation, see the reply by @niconiconi@mk.absturztau.be below. I completely forgot how this works, siiiigh (TL;DR the 5.1k are necessary to get ANY power from compliant USB-C PD sources)

Open ##3289053
@ignaloidas@not.acu.lt 2026-06-07 18:52

@gsuberland@chaos.social @matildalove@wetdry.world @valpackett@social.treehouse.systems correction - if the device does not have anything at all (as in, no resistors on CC lines), a compliant power source will not present any voltage. This is one of the core safeties of USB C - since you can connect two sources together, no USB-C source should apply any voltage unless it is sure that the other side is a sink and will accept it (this is also part of the reason why all USB-C to USB-A cables only have plugs but not ports - a source can tolerate 5V on the input from a USB-C unaware power source, but only if it knows not to apply the power itself). Also, this lives purely on the USB-C side of things, USB PD specs only get into the active communication stuff that happens after the passive, resistor based negotiation is established.

Open ##3289066