Okay, this is pretty cool... But I'd love to know what happens when the source can't do the magic required to give 12v (or even 5A)... will it give 12v or nothing? will it still operate at a lower current?
@attie Oh, nice stuff!
PD ports don't give any voltage until a successful negotiation, so it's likely 12V or nothing there. However, non-PD sources will happily provide 5V right away, so be careful what you plug it into.
Since there's no PD on the other side, you have to check whether the supply provides enough current at 12V for your needs by yourself. 5A would just be a maximum.
@attie Since this is a smart cable, the negotiation is likely bootstrapped with Vconn - but thinking about it now, I'm actually not sure if there won't still be 5V on Vbus for a short moment, hmm...
Since this is a cable, it doesn't know how much current the end device needs, so it's up to the user to ensure you have enough power on the other end. So it can't really say "5A please". Sink PDOs specify three operating currents (min, typ and max), so I think this would be hard to screw up.
@dos Interesting, thanks! ... I clearly don't know enough about USB-C / PD yet! 🙃
@dos Don't they supply 5v with a low current limit initially (to bootstrap the negotiation)...
... and isn't the current / power negotiated? So if the cable says "12v@5A please" and the supply says "I can do 12v@3A" what happens ... does the negotiation fail, or does it fall back on the lower limit (potentially causing mysterious brownouts in the downstream device) ... I suspect this is heavily dependent on the cable's internals being implemented sensibly, but it's not clear. 😞