ST STM32F101 series Reference Manual page 947

USB on-the-go full-speed (OTG_FS)
2. The application can only schedule data transfers one frame at a time.
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3. The complete data to be transmitted in the frame must be written into the transmit FIFO
by the application, before the IN token is received. Even when 1 Word of the data to be
transmitted per frame is missing in the transmit FIFO when the IN token is received, the
core behaves as when the FIFO is empty. When the transmit FIFO is empty:
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Internal data flow:
1. The application must set the transfer size and packet count fields in the endpoint-
specific registers and enable the endpoint to transmit the data.
2. The application must also write the required data to the associated transmit FIFO for
the endpoint.
3. Every time the application writes a packet to the transmit FIFO, the transfer size for that
endpoint is decremented by the packet size. The data are fetched from application
memory until the transfer size for the endpoint becomes 0.
4. When an IN token is received for a periodic endpoint, the core transmits the data in the
FIFO, if available. If the complete data payload (complete packet, in dedicated FIFO
mode) for the frame is not present in the FIFO, then the core generates an IN token
received when TxFIFO empty interrupt for the endpoint.
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5. The packet count for the endpoint is decremented by 1 under the following conditions:
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6. At the "Periodic frame Interval" (controlled by PFIVL in OTG_FS_DCFG), when the
core finds non-empty any of the isochronous IN endpoint FIFOs scheduled for the
current frame non-empty, the core generates an IISOIXFR interrupt in
OTG_FS_GINTSTS.
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(MCNT – 1) × MPSIZ ≤ XFERSIZ ≤ MCNT × MPSIZ
PKTCNT = MCNT (in OTG_FS_DIEPTSIZx)
If XFERSIZ < MCNT × MPSIZ, the last data packet of the transfer is a short
packet.
Note that: MCNT is in OTG_FS_DIEPTSIZx, MPSIZ is in OTG_FS_DIEPCTLx,
PKTCNT is in OTG_FS_DIEPTSIZx and XFERSIZ is in OTG_FS_DIEPTSIZx
A zero data length packet would be transmitted on the USB for isochronous IN
endpoints
A NAK handshake would be transmitted on the USB for interrupt IN endpoints
A zero-length data packet is transmitted on the USB for isochronous IN endpoints
A NAK handshake is transmitted on the USB for interrupt IN endpoints
For isochronous endpoints, when a zero- or non-zero-length data packet is
transmitted
For interrupt endpoints, when an ACK handshake is transmitted
When the transfer size and packet count are both 0, the transfer completed
interrupt for the endpoint is generated and the endpoint enable is cleared.
DocID13902 Rev 15
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