Android Tablet can't find USB device via USB port but can via OTG cable

4
votes

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.

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.

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?

1
androidusbhost
What do you mean by "a USB port"? Do you mean a Standard A (such as you get on a PC or laptop) or a mini-B (such as you get on a mobile phone)? Also, do you know for sure that your tablet supports USB Host mode? Check whether you have the file /system/etc/permissions/android.hardware.usb.host.xml. - kbro

1 Answers

1
votes

Well OTG cables have certain pins shorted out inorder to tell the device that it should switch to USB Host Mode (the tablet provides the power). The USB port may be OTG capable but only if you have the OTG cable. Typically the USB ports on tablets are set up as accessory ports, meaning that they draw power, not provide it.