USB-OTG
USB-OTG
From: http://www.usb.org
3.2 On-The-Go Device
In addition to being a fully compliant USB 2.0 peripheral, an On-The-Go device must include the following features and characteristics:
a limited Host capability
full-speed operation as a peripheral (high-speed optional)
full-speed support as a host (low-speed and high-speed optional)
Targeted Peripheral List
Session Request Protocol
Host Negotiation Protocol
one, and only one connection: a Micro-AB receptacle.
minimum IA_VBUS_OUT output on VBUS
means for communicating messages to the user
3.6 Session Request Protocol
The Session Request Protocol (SRP) allows a B-device to request the A-device to turn on VBUS and start a session. This protocol allows the A-device, which may be battery powered, to conserve power by turning VBUS off when there is no bus activity while still providing a means for the B-device to initiate bus activity.
Any A-device, including a PC or laptop, is allowed to respond to SRP. Any B-device, including a standard USB peripheral, is allowed to initiate SRP. An On-The-Go device is required to be able to initiate and respond to SRP.
3.7 Host Negotiation Protocol
The Host Negotiation Protocol (HNP) allows the Host function to be transferred between two directly connected On-The-Go devices and eliminates the need for a user to switch the cable connections in order to allow a change in control of communications between the devices. HNP will typically be initiated in response to input from the user or an Application on the On-The-Go B-device. HNP may only be implemented through the Micro-AB receptacle on a device.