This API abstracts the USB peripheral controller hardware as well as offers hardware neutral routines which allow to implement USB functions (e.g.
On Linux USB peripheral mode is supported through the Gadget API. On Apalis modules a full OTG port (USBO1) is provided which makes use of the OTG ID pin using the USBO1_ID signal to determine its role and additionally the VBUS pin using the USBO1_VBUS signal to determine the actual connection status thereof allowing suspending the USB complex if not connected. On the other hand VBUS is also connected to the Colibri USBC_DET signal (usually via a diode) allowing driver software to monitor and subsequently change the role. If a USB Micro-B OTG cable is used (which connects the ID pin to ground) the circuit on the carrier board automatically enables driving +5V onto VBUS in order to provide power to USB devices as required by a host. By default the port is in peripheral mode. On Colibri modules the client port (USBC) is usually connected to a dual-role capable controller. The dual-role controllers often also support the OTG (On-The-Go) specification which defines a host negotiation protocol allowing two devices to negotiate for the role of the host. However, embedded systems often have multiple USB controller supporting host or peripheral roles or supporting both roles on a single controller (dual-role controller). Most PCs support USB host mode exclusively. The bus can only have one USB host connected all other devices need to be in USB peripheral mode. The USB (Universal Serial Bus) standard specifies two roles USB host and USB peripheral (also known as USB function, USB peripheral device or sometimes just USB device).
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