General discussion • Re: Raspberry Pi5 USB-C PD

The Pi 5 does not "ask" as it is the sink. The source offers (advertises) available modes and the sink selects from the offered modes.

I think it's mostly an issue of semantics. It is as you describe but some would deem the selection from modes being offered as requesting what it wants or asking for that.

But yes, it starts with the supply announcing what it can provide, and the Pi replying to indicate what it would like, rather than the supply being silent until the Pi asks for what it would like.

What's not clear to me is exactly what happens when a supply doesn't offer 5V@5A, and I saw something when skimming some article which suggested the supply still doesn't have to deliver what the sink may ask for but it wasn't clear to me what the full picture is, and I guess most people don't know either.

What we ideally need is someone who is fully knowledgeable about PD who can detail exactly what does happen with respect to PD negotiation, and how the Pi regulator, power button, and down-stream USB current and software interactions are in all scenarios. That could be one or more flowcharts, state diagrams, or 'If-Then-Else' lists. That might be a lot of work but would help explain what we have and the various behaviours seen.

Statistics: Posted by bjtheone — Sun Apr 07, 2024 8:14 pm