
RM0008
USB on-the-go full-speed (OTG_FS)
Doc ID 13902 Rev 12
813/1096
Figure 304. USB host-only connection
1.
STMPS2141STR needed only if the application has to support a V
BUS
powered device. A basic power
switch can be used if 5 V are available on the application board.
28.6.1 SRP-capable
host
SRP support is available through the SRP capable bit in the global USB configuration
register (SRPCAP bit in OTG_FS_GUSBCFG). With the SRP feature enabled, the host can
save power by switching off the V
BUS
power while the USB session is suspended.
The SRP host mode program model is described in detail in the
) section.
28.6.2 USB
host
states
Host port power
On-chip 5 V V
BUS
generation is not supported. For this reason, a charge pump or, if 5 V are
available on the application board, a basic power switch, must be added externally to drive
the 5 V V
BUS
line. The external charge pump can be driven by any GPIO output. When the
application decides to power on V
BUS
using the chosen GPIO, it must also set the port
power bit in the host port control and status register (PPWR bit in OTG_FS_HPRT).
V
BUS
valid
The V
BUS
input ensures that valid V
BUS
levels are supplied by the charge pump during USB
operations.
Any unforeseen V
BUS
voltage drop below the V
BUS
valid threshold (4.25 V) leads to an OTG
interrupt triggered by the session end detected bit (SEDET bit in OTG_FS_GOTGINT). The
application is then required to remove the V
BUS
power and clear the port power bit. The
charge pump overcurrent flag can also be used to prevent electrical damage. Connect the
overcurrent flag output from the charge pump to any GPIO input and configure it to generate
a port interrupt on the active level. The overcurrent ISR must promptly disable the V
BUS
generation and clear the port power bit.
Host detection of a peripheral connection
Even if USB peripherals or B-devices can be attached at any time, the OTG_FS will not
detect any bus connection until the end of the V
BUS
sensing (V
BUS
over 4.75 V).
34-&XX
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