The XEM7001 is a bus-powered device in its default configuration. That is, it takes power from the 5-V USB power and generates the voltages it needs from there. To do so, the XEM7001 has switching regulators for 3.3 V, and 1.0 V as well as a linear regulator for 1.8 V.
Computers and USB hubs sometimes have USB ports that do not provide bus power. These are called unpowered ports. In order to be operated as a bus-powered device, the XEM7001 must be connected to a USB port that provides bus power. You should check with the hub or computer manufacturer to verify that the port provides bus power.
Alternative System Power
Starting with PCB date code 20160725, the XEM7001 has a small jumper at J6. This connects the +3.3 V supply to the switching regulator connected to +5 USB. In this configuration, USB power is regulated and provides the 3.3 V for the system.
Optionally, you may remove J6 and supply +3.3 V via the expansion connectors.