
USB on-the-go full-speed (OTG_FS)
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47.5.3
SRP dual role device
The SRP capable bit in the global USB configuration register (SRPCAP bit in
OTG_GUSBCFG) enables the OTG_FS core to switch off the generation of V
BUS
for the A-
device to save power. Note that the A-device is always in charge of driving V
BUS
regardless
of the host or peripheral role of the OTG_FS.
The SRP A/B-device program model is described in detail in
.
47.6 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_GUSBCFG) is cleared.
•
Figure 519: USB_FS peripheral-only connection
–
The force device mode bit (FDMOD) in the
configuration register (OTG_GUSBCFG)
is set to 1, forcing the OTG_FS core to
work as an USB peripheral-only. In this case, the ID line is ignored even if it is
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 V
DD
chip-supply
from V
BUS
.