December 5, 2020

USB - USB "on the go"

The basic idea that motivated this addition to the USB protocol was the desire for mobile USB devices to communicate directly with each other. For example a digital camera could be connected directly to a smart phone if the phone supported OTG. This addition was developed in late 2001.

I'll just say that it is 2020 and I haven't seen or heard of this exploding into widespread usefulness or adoption over the 20 years, but here we are.

OTG is not peer to peer. One device acts as the host (most likely the smartphone in most scenarios). Hubs are supported. A possibility is using this to connect a keyboard to a phone.

An OTG dual role device can be either a host, or a standard USB peripheral.

HNP (host negotiation protocol) mediates the role swapping. It is not required to supply more than 8 mA of power.

An OTG peripheral only device has either a Micro-B receptacle or a tethered Micro-A plug.
An OTG dual role device may have a new Micro-AB receptacle.

Apparently OTG adds an extra pin to the connector somehow (this may be what the new Micro-AB receptable is all about) that does sensing. I have not dug deeply enough to understand this, because I intend to ingnore all this OTG stuff for the forseeable future.

SRP - session request protocol enables a host to power down the bus and save battery power when the bus in not in use.

Feedback? Questions? Drop me a line!

Tom's Computer Info / tom@mmto.org