USB controllers are available to add to virtual machines to support USB passthrough from an ESXi host or from a client computer to a virtual machine.
You can add two USB controllers to a virtual machine. The xHCI controller, available for Linux guest operating systems only, supports USB 3.0 superspeed, 2.0, and 1.1 devices. The EHCI+UHCI controller supports USB 2.0 and 1.1 devices.
The conditions for adding a controller vary, depending on the device version, the type of passthrough (host or client computer), and the guest operating system.
For Mac OS X systems, the EHCI+UHCI controller is enabled by default and is required for USB mouse and keyboard access.
For virtual machines with Linux guests, you can add one or both controllers, but 3.0 superspeed devices are not supported for passthrough from an ESXi host to a virtual machine. You cannot add two controllers of the same type.
For USB passthrough from an ESXi host to a virtual machine, the USB arbitrator can monitor a maximum of 15 USB controllers. If your system includes controllers that exceed the 15 controller limit and you connect USB devices to them, the devices are not available to the virtual machine.