SLAU723A October 2017 – October 2018 MSP432E401Y , MSP432E411Y
When the other device of the OTG setup wishes to start a session, it either raises VBUS above the Session Valid threshold if it is the A device, or if it is the B device, it pulses the data line then pulses VBUS. Depending on which of these actions happens, the USB controller can determine whether it is the A device or the B device in the current setup and act accordingly. If VBUS is raised above the Session Valid threshold, then the USB controller is the B device. The USB controller sets the SESSION bit in the USBDEVCTL register. When RESET signaling is detected on the bus, a RESET interrupt is signaled, which is interpreted as the start of a session.
The USB controller is in device mode as the B device is the default mode. At the end of the session, the A device turns off the power to VBUS. When VBUS drops below the Session Valid threshold, the USB controller detects this drop and clears the SESSION bit to indicate that the session has ended, causing a disconnect interrupt to be signaled. If data line and VBUS pulsing is detected, then the USB controller is the A device. The controller generates a SESSION REQUEST interrupt to indicate that the B device is requesting a session. The SESSION bit in the USBDEVCTL register must be set to start a session.