ST STM32F207 Series Reference Manual page 1241

RM0033
Internal data flow:
1. The application must set the transfer size and packet count fields in the endpoint-
specific registers, clear the NAK bit, and enable the endpoint to receive the data.
2. Once the NAK bit is cleared, the core starts receiving data and writes it to the receive
FIFO, as long as there is space in the receive FIFO. For every data packet received on
the USB, the data packet and its status are written to the receive FIFO. Every packet
(maximum packet size or short packet) written to the receive FIFO decrements the
packet count field for that endpoint by 1.
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3. When the packet count becomes 0 or when a short packet is received on the endpoint,
the NAK bit for that endpoint is set. Once the NAK bit is set, the isochronous or
nonisochronous data packets are ignored and not written to the receive FIFO, and
nonisochronous OUT tokens receive a NAK handshake reply.
4. After the data are written to the receive FIFO, the application reads the data from the
receive FIFO and writes it to external memory, one packet at a time per endpoint.
5. At the end of every packet write on the AHB to external memory, the transfer size for
the endpoint is decremented by the size of the written packet.
6. The OUT data transfer completed pattern for an OUT endpoint is written to the receive
FIFO on one of the following conditions:
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7. When either the application pops this entry (OUT data transfer completed), a transfer
completed interrupt is generated for the endpoint and the endpoint enable is cleared.
Application programming sequence:
OUT data packets received with bad data CRC are flushed from the receive FIFO
automatically.
After sending an ACK for the packet on the USB, the core discards
nonisochronous OUT data packets that the host, which cannot detect the ACK, re-
sends. The application does not detect multiple back-to-back data OUT packets
on the same endpoint with the same data PID. In this case the packet count is not
decremented.
If there is no space in the receive FIFO, isochronous or nonisochronous data
packets are ignored and not written to the receive FIFO. Additionally,
nonisochronous OUT tokens receive a NAK handshake reply.
In all the above three cases, the packet count is not decremented because no data
are written to the receive FIFO.
The transfer size is 0 and the packet count is 0
The last OUT data packet written to the receive FIFO is a short packet
(0 ≤ packet size < maximum packet size)
USB on-the-go high-speed (OTG_HS)
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