RM0090: Reference manual - STMicroelectronics

USB on-the-go full-speed (OTG_FS) RM0090
The HNP program model is described in detail in Section 30.17: OTG_FS programming
model.
30.4.3 SRP dual role device
The SRP capable bit in the global USB configuration register (SRPCAP bit in
OTG_FS_GUSBCFG) enables the OTG_FS core to switch off the generation of VBUS for the
A-device to save power. Note that the A-device is always in charge of driving VBUS regardless of the host or peripheral role of the OTG_FS.
the SRP A/B-device program model is described in detail in Section 30.17: OTG_FS
programming model.
30.5 USB peripheral
This section gives the functional description of the OTG_FS in the USB peripheral mode.
The OTG_FS works as an USB peripheral in the following circumstances:
● OTG B-Peripheral
– OTG B-device default state if B-side of USB cable is plugged in
● OTG A-Peripheral
– OTG A-device state after the HNP switches the OTG_FS to its peripheral role
● B-device
– If the ID line is present, functional and connected to the B-side of the USB cable,
and the HNP-capable bit in the Global USB Configuration register (HNPCAP bit in
OTG_FS_GUSBCFG) is cleared (see On-The-Go Rev1.3 par. 6.8.3).
● Peripheral only (see Figure 358: USB peripheral-only connection)
– The force device mode bit in the Global USB configuration register (FDMOD in
OTG_FS_GUSBCFG) is set to 1, forcing the OTG_FS core to work as a USB
peripheral-only (see On-The-Go Rev1.3 par. 6.8.3). In this case, the ID line is
ignored even if present on the USB connector.
Note: To build a bus-powered device implementation in case of the B-device or peripheral-only
configuration, an external regulator has to be added that generates the VDD chip-supply
from VBUS. The VBUS pin can be freed by disabling the VBUS sensing option. This is done by setting the
NOVBUSSENS bit in the OTG_FS_GCCFG register. In this case the VBUS is considered
internally to be always at VBUS valid level (5 V).
1023/1416 Doc ID 018909 Rev 3