• USB History: USB was released in 1996 but took time to gain widespread adoption. It had significant advantages over conventional serial, being self-configuring. Serial ports had no clear sense of ends, while USB employs a host-device architecture with specific connector standards.
• Nokia 770 Internet Tablet: Introduced in 2005, it was one of the first devices in a new niche. It had a 4" touchscreen and OMAP/ARM SoC, running the Linux-based Maemo operating system. It could act as a peripheral but faced limitations as it couldn't act as a host due to USB restrictions.
• USB On-The-Go (OTG): Added in 2001 to address issues with USB on portable devices. It decouples connector roles and power supply. A device with OTG support has a type AB socket. In common cases, the A-device supplies power and acts as the host. OTG added Host Negotiation Protocol (HNP) for role swapping.
• Android and iOS OTG: Android gained OTG support in 3.1 (2011) and it became common in higher-end devices. iOS got OTG support on the iPhone 12 (but earlier versions might have had hardware support not exposed by the OS). iPads got OTG support earlier.
• USB-C and Dual Role: USB-C cables are symmetric, and ports use resistance values to indicate type. When both ends support the host role, the Dual Role feature comes into play. Detection of host or device role is based on CC pins. USB-C doesn't support HNP for role reversal like USB-A/B.
• Current State: USB-C dual role support is ubiquitous in modern smartphones. Embedded devices often have separate host and device ports. USB's expansion with non-mandatory features has led to confusion about port capabilities.