USB on-the-go full-speed (OTG_FS)
RM0090
1244/1731
DocID018909 Rev 11
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.
When HNP and SRP are both disabled, the VBUS sensing pin (PA9) should not be
connected to V
BUS
. This pin can be can be used as GPIO.
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
If SRP or HNP are enabled, even if USB peripherals or B-devices can be attached at any
time, the OTG_FS will not detect any bus connection until V
BUS
is no longer sensed at a
valid level (5 V). When V
BUS
is at a valid level and a remote B-device is attached, the
OTG_FS core issues a host port interrupt triggered by the device connected bit in the host
port control and status register (PCDET bit in OTG_FS_HPRT).
When HNP and SRP are both disabled, USB peripherals or B-device are detected as soon
as they are connected. The OTG_FS core issues a host port interrupt triggered by the
device connected bit in the host port control and status (PCDET bit in OTG_FS_HPRT).
Host detection of peripheral a disconnection
The peripheral disconnection event triggers the disconnect detected interrupt (DISCINT bit
in OTG_FS_GINTSTS).
Host enumeration
After detecting a peripheral connection the host must start the enumeration process by
sending USB reset and configuration commands to the new peripheral.
Before starting to drive a USB reset, the application waits for the OTG interrupt triggered by
the debounce done bit (DBCDNE bit in OTG_FS_GOTGINT), which indicates that the bus is
stable again after the electrical debounce caused by the attachment of a pull-up resistor on
DP (FS) or DM (LS).
The application drives a USB reset signaling (single-ended zero) over the USB by keeping
the port reset bit set in the host port control and status register (PRST bit in
OTG_FS_HPRT) for a minimum of 10 ms and a maximum of 20 ms. The application takes
care of the timing count and then of clearing the port reset bit.
Once the USB reset sequence has completed, the host port interrupt is triggered by the port
enable/disable change bit (PENCHNG bit in OTG_FS_HPRT). This informs the application
that the speed of the enumerated peripheral can be read from the port speed field in the
host port control and status register (PSPD bit in OTG_FS_HPRT) and that the host is
starting to drive SOFs (FS) or Keep alives (LS). The host is now ready to complete the
peripheral enumeration by sending peripheral configuration commands.
Host suspend
The application decides to suspend the USB activity by setting the port suspend bit in the
host port control and status register (PSUSP bit in OTG_FS_HPRT). The OTG_FS core
stops sending SOFs and enters the suspended state.
