Post #3289045
2026-06-07 17:09 UTC
Replies (3)
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@HeNeArXn@chaos.social 2026-06-07 17:17
@matildalove@wetdry.world to properly signal to a USB-C powersupply "I want to be fed 5V please" you need to connect 2 pins to ground through 5.1kOhm resistors. Some device designers don't pay attention and connect those two pins together and then to ground through 1 resistor, or skip them entirely. That will work if you use a dumb power adapter that doesn't check those (e.g. an old USB-A one and a A-to-C cable), but not with a better charger.
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@jernej__s@infosec.exchange 2026-06-07 17:22
@matildalove@wetdry.world @valpackett@social.treehouse.systems USB Power Delivery (USB-C) requires 5.1kΩ resistors to signal that the device wants 5V charging. Since these cost about a cent, many cheap devices don't have them, which causes compliant USB-C PD chargers to not charge them (USB-PD can deliver between 3.3 and 48V, depending on charger, and therefore needs some way for the device to inform it which voltages are supported; USB-A chargers don't have this problem – they always provide 5V, as devices predating USB-PD always expected 5V input, even if there were some proprietary standards that could provide more).
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@gsuberland@chaos.social 2026-06-07 17:26
@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.