title: Android Tablet can't find USB device via USB port but can via OTG cable
body: <p>I have an Android tablet which has a mini-USB port and a USB port, and I want to write an app communicate with a USB key. I have written a demo to find out the USB key, but there is no response.</p>
  
  <p>What disturbs me is that if I run this demo on my another tablet which only has a mini-USB port but support USB OTG, it can find the key via an OTG cable. When I insert the USB key into the USB port, there is no response. I want to know what the difference is.</p>
  
  <p>Can't I use the USB Host API to find the key in USB port? And what's the difference between communication with USB device in the mini-USB port via OTG cable and in the USB port directly?</p>