ChibiOS Free Embedded RTOS

USB On-The-Go introduces the concept that a device can perform both the master and slave roles, and so subtly changes the terminology. With OTG, a device can be either a host when acting as the link master, or a peripheral when acting as the link slave. The choice of whether to be host or peripheral is handled entirely by which end of the cable the device is plugged into. The device connected to the "A" end of the cable at start-up, known as the "A-device", acts as the default host, while the "B" end acts as the default peripheral, known as the "B-device".

After initial startup, setup for the bus operates as it does with the normal USB standard, with the A-device setting up the B-device and managing all communications. However, when that same A-device is plugged into another USB system, or a dedicated host becomes available, it may become a slave.

USB On-The-Go does not preclude using a USB hub, but it describes host/peripheral role swapping only for the case of a one-to-one connection where two OTG devices are directly connected. Role swapping does not work through a standard hub, as one device will act as the host and the other as the peripheral until they are disconnected.

• Attach Detection Protocol (ADP)
• Session Request Protocol (SRP)
• Host Negotiation Protocol (HNP)

• OTG A-device
  
  • Power supplier
  • Default USB Host
• OTG B-device
  
  • Default USB Peripheral (may be exchanged via HNP)

• API: What should we keep and what should we throw out?
• Reference Implementations: Collect links to relevant ARM-compatible USB-OTG and USB-Host codes.
  
  • Potentially avoid the STM32 F4 USB-OTG lib, for clarity's sake