ST STM32F40 Series Reference Manual page 1267

USB on-the-go high-speed (OTG_HS)
SETUP transaction operates in the same way but has only one packet. The
assumptions are:
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–
–
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Normal bulk and control OUT/SETUP operations
The sequence of operations for channel 1 is as follows:
a)
b)
c)
d)
e)
f)
g)
h)
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The application is attempting to send two maximum-packet-size packets (transfer
size = 1, 024 bytes).
The nonperiodic transmit FIFO can hold two packets (128 bytes for FS).
The nonperiodic request queue depth = 4.
Initialize channel 1
Write the first packet for channel 1
Along with the last DWORD write, the core writes an entry to the nonperiodic
request queue
As soon as the nonperiodic queue becomes nonempty, the core attempts to send
an OUT token in the current frame
Write the second (last) packet for channel 1
The core generates the XFRC interrupt as soon as the last transaction is
completed successfully
In response to the XFRC interrupt, de-allocate the channel for other transfers
Handling nonACK responses
Doc ID 018909 Rev 4
RM0090