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
Famous quotes containing the words device and/or roles:
“Syntax is the study of the principles and processes by which sentences are constructed in particular languages. Syntactic investigation of a given language has as its goal the construction of a grammar that can be viewed as a device of some sort for producing the sentences of the language under analysis.”
—Noam Chomsky (b. 1928)
“Productive collaborations between family and school, therefore, will demand that parents and teachers recognize the critical importance of each other’s participation in the life of the child. This mutuality of knowledge, understanding, and empathy comes not only with a recognition of the child as the central purpose for the collaboration but also with a recognition of the need to maintain roles and relationships with children that are comprehensive, dynamic, and differentiated.”
—Sara Lawrence Lightfoot (20th century)