usb-otg

USB On-The-Go (USB OTG) is a specification that allows certain USB devices, such as tablets or smartphones, to function either as a host or a peripheral. This enables them to connect directly to other USB devices, such as flash drives, digital cameras, mice or keyboards. USB OTG was first introduced in late 2001.
Unlike standard USB connections, which involve a fixed host (such as a computer) and a peripheral (such as a keyboard), USB OTG allows a device to switch between these roles. For example, a smartphone can act as a host when reading files from a flash drive, but function as a peripheral when connected to a computer.
USB OTG defines two device roles: the A-device, which supplies power and initially acts as the host, and the B-device, which consumes power and begins as the peripheral. These roles can be reversed using the Host Negotiation Protocol (HNP). The initial role is determined by the wiring of a specific pin, known as the ID pin, in the USB connector. The A/B naming convention reflects earlier USB connector types: Type-A connectors were used with host devices, while Type-B connectors were used with peripherals.
In September 2019 USB Implementers Forum has stopped certifying new USB OTG products because of Introduction of USB-C standard.

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