1

I have a question regarding USB2.0 power negotiation. I'm in the process of improving the design of a support equipment PCB that utilises an FT232H chip.

The layout of the current board consists of two separate Micro-USB connectors, one directly connected to the FT232H and the other directly connected to another device (essentially a pass-through).

The FT232H is used as a usb-to-serial bridge to communicate with a camera that is also powered off of this USB connection. I use the FT_Prog program to make the FT232H negotiate 500 mA of maximum power (the camera needs more than the standard 100 mA) and it all seems to work very well.

However I would like to simplify the design from two to one USB connector, meaning I'd like to use an on-board USB Hub IC. The question that I have is how this power negotiation performed by the FT232H will work when it is 'behind' a USB hub. (i.e. a Microchip USB2422) I've looked into datasheets of many hub IC's but none seem to talk about the up-stream power negotiation. Do hubs automatically request the maximum?

The only real difference I can find is self-powered vs bus-powered. However I'm unsure what the current limit is on the VBUS pin when it is self-powered.

Amdixer
  • 11
  • 2
  • You have all the power delivery concepts wrong. There is no "negotiations" for 500 mA, there is only description of device needs, passively over descriptors. That's why no USB IC will talk about "up-stream power negotiations". Hubs are no different. – Ale..chenski Mar 26 '20 at 02:54
  • Is that not what 'device enumeration' is? A method of the downstream device letting the upstream device know what kind of power it requires? From what I understand the default is 100 mA and the spec allows a device to ask for up to 500 mA, but nowhere its specified if a hub controller requests 500 mA from the Host. My setup is: Host (laptop etc.) -> Hub -> FT232 & Second Device What I don't want to happen is the Host -> Hub connection only allowing 100 mA, I need the board to get the full 500 mA. If this is not how it works, then how is the 500 mA requested through the hub? – Amdixer Mar 26 '20 at 10:55
  • for more elaborations check this https://electronics.stackexchange.com/q/429209/117785 and this https://superuser.com/a/1350801/620011 – Ale..chenski Mar 26 '20 at 18:05

0 Answers0