MaxPower Byte in Configuration Descriptors Versus HubController Power Byte in HUB Descriptors - KBA90630

Version: **

 

Question: In the 0xD4 load of HX2LP™ what is the difference between Bytes 8 and 9 and Bytes 12 through 15?

 

Answer:

In HX2LP, Bytes 8 and 9 are used to indicate the maximum power for full/high speed configurations. Bytes 12 and 13 are used to indicate HubController power for full/high speed configuration in BUS-powered mode. Bytes 14 and 15 are used to indicate HubController power for full/high speed configuration in self-powered mode.

Bytes 8 and 9 are used in the configuration descriptors. Bytes 12 through 15 are used in the HUB descriptors. The MaxPower field in the configuration descriptor is used to report to the system the maximum power the hub will draw from the VBUS when the configuration is selected. For bus-powered hubs, the reported value must not include the power for any external downstream ports. The external devices attaching to the hub will report their individual power requirements.

A compound device may power both the hub electronics and the non-removable devices from the VBUS. The entire load may be reported in the hub’s configuration descriptor with each non-removable device reporting to the Host as a self-powered device, with ‘0’ MaxPower in their respective configuration descriptors.

Bytes 12 through 15 indicate the power required for the operation of the HUB itself.

For Hubs without non-removable devices, Bytes 8 and 9 and Bytes 12 through 15 are almost the same.