ST STM32F405 Reference Manual page 1380

USB on-the-go full-speed (OTG_FS)
4. The OTG_FS controller continues the host role of initiating traffic, and when done,
suspends the bus by writing the Port suspend bit in the host port control and status
register.
5. In Negotiated mode, when the A-device detects a suspend, it disconnects and switches
back to the host role. The OTG_FS controller deasserts the DP pull down and DM pull
down in the PHY to indicate the assumption of the device role.
6. The application must read the current mode bit in the Core interrupt
(OTG_FS_GINTSTS) register to determine the host mode operation.
7. The OTG_FS controller connects, completing the HNP process.
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