Device Roles
USB OTG defines two roles of devices: OTG A-device and OTG B-device. This terminology defines which side supplies power to the link, and which is initially the host. The OTG A-device is a power supplier, and an OTG B-device is a power consumer. The default link configuration is that A-device act as USB Host and B-device is a USB peripheral. The host and peripheral modes may be exchanged later by using HNP. Because every OTG controller supports both roles, they are often called "Dual-Role" controllers rather than "OTG controllers".
For integrated circuit (IC) designers, an attraction of USB OTG is the ability to get more USB capabilities with fewer gates. A "traditional" approach includes four controllers:
- USB high speed host controller based on EHCI (a register interface)
- Full/low speed host controller based on OHCI (another register interface)
- USB device controller, supporting both high and full speeds
- Fourth controller to switch the OTG root port between host and device controllers.
This means many gates to test and debug. Also, most gadgets must be a host only, or a device only. OTG hardware design merges all of these controllers into one dual-role controller that is somewhat more complex than an individual device controller.
Read more about this topic: USB On-The-Go
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