ST STM32F412 Reference Manual page 1120

USB on-the-go full-speed (OTG_FS)
and status register to indicate to the OTG_FS controller that the B-device supports
HNP.
2. When it has finished using the bus, the application suspends by writing the port
suspend bit in the host port control and status register.
3. When the B-device observes a USB suspend, it disconnects, indicating the initial
condition for HNP. The B-device initiates HNP only when it must switch to the host role;
otherwise, the bus continues to be suspended.
The OTG_FS controller sets the host negotiation detected interrupt in the OTG
interrupt status register, indicating the start of HNP.
The OTG_FS controller deasserts the DM pull down and DM pull down in the PHY to
indicate a device role. The PHY enables the OTG_DP pull-up resistor to indicate a
connect for B-device.
The application must read the current mode bit in the OTG control and status register
to determine device mode operation.
4. The B-device detects the connection, issues a USB reset, and enumerates the
OTG_FS controller for data traffic.
5. The B-device continues the host role, initiating traffic, and suspends the bus when
done.
The OTG_FS controller sets the early suspend bit in the core interrupt register after 3
ms of bus idleness. Following this, the OTG_FS controller sets the USB suspend bit in
the core interrupt register.
6. In Negotiated mode, the OTG_FS controller detects the suspend, disconnects, and
switches back to the host role. The OTG_FS controller asserts the DM pull down and
DM pull down in the PHY to indicate its assumption of the host role.
7. The OTG_FS controller sets the connector ID status change interrupt in the OTG
interrupt status register. The application must read the connector ID status in the OTG
control and status register to determine the OTG_FS controller operation as an A-
device. This indicates the completion of HNP to the application. The application must
read the Current mode bit in the OTG control and status register to determine host
mode operation.
8. The B-device connects, completing the HNP process.
B-device host negotiation protocol
HNP switches the USB host role from B-device to A-device. The application must set the
HNP-capable bit in the core USB configuration register to enable the OTG_FS controller to
perform HNP as a B-device.
1120/1163 RM0402 Rev 6
RM0402