Other Parts Discussed in Thread: TIDA-00287
I am currently designing an USB-powered device where I communicate with the host through HS data lines but need more than 500mA to operate in "full-featured" mode. As most of the target hosts will have USB3 capabilities I thought about using a bus-powered USB3 hub IC as front-end for my device and let it negotiate the desired 900mA with the host, while I do not impact the rest of the system (communication will still occur through the HS lines). Communication through HS lines is a hard requirement, as there is no SuperSpeed capable MCU in the market for my application (I need to act as a device with plenty of interrupt and isochron EPs and there is no usable USB3.0 audio driver to support my application).
For clarification: the hub will be embedded in a device and only the upstream connector will be accessible to the user. One downstream connector will be left unconnected and the other just connected to the HS lines of the rest of the device.
I based my assumption on this thread: e2e.ti.com/.../getting-900ma-for-usb-2-0-using-tusb8020
I have the following questions:
1- Will the insertion of the TUSB8020B hub suffice to get the 900mA from the host? I wanted to just populate one of the downstream ports with the rest of my device (HS) and let the hub do the power negotiation by itself. Would that work?
2- I have seen that there is a reference design with this IC acting as a bus-powered hub, the TIDA-00287. It is though not very clear to me how the IC should be configured to appear and act as a bus-powered device. How do the internal registers and pins have to be configured to achieve such behavior?
3- I wanted to be able to detect when the host is NOT USB3-capable, in order to adjust my power consumption to the maximum 500mA of USB 2.0 (non "full-featured" mode). I assume that the PWRCTLX pins will be asserted when the hub has completed a successful power negotiation with the host but, how can it be differentiated between the cases in which the power negotiation occurred through the SS lines (900mA available) or through the HS lines (500mA available)?
The device I am currently designing would require certification and the following thread caused me some concerns: e2e.ti.com/.../tusb8020b-can-power-be-provided-from-the-upstream-usb-connection
There are two issues with the hub:
4- If the hub allegedly always reports as self-powered, how does the use-case implemented in the TIDA-00287 reference design work? In the documentation it is explicitly stated that the hub operates as a bus-powered device right in the description on the first page of its design guide (www.ti.com/.../tidu428)
5- I assume that the main blocker could be the fact that the hub exceeds the power budget before enumeration. By looking at the table in the section 7.7 of the datasheet it is not clear to me that this will be the case with a USB 3.0 host and a single USB 2.0 downstream device. Can this information be confirmed? In this case, what is the point of the above mentioned reference design, if it is not spec-compliant?
Thank you very much in advance for your time,