ST STM32F412 Reference Manual page 1112

USB on-the-go full-speed (OTG_FS)
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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
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transmit a few maximum-packet-size packets and a short packet at the end of the
transfer, the following conditions must be met:
transfer size[EPNUM] = x × MPSIZ[EPNUM] + sp
(where x is an integer ≥ 0, and 0 ≤ sp < MPSIZ[EPNUM])
If (sp > 0), packet count[EPNUM] = x + 1
Otherwise, packet count[EPNUM] = x;
MCNT[EPNUM] = packet count[EPNUM]
The application cannot transmit a zero-length data packet at the end of a transfer.
It can transmit a single zero-length data packet by itself. To transmit a single zero-
length data packet:
transfer size[EPNUM] = 0
packet count[EPNUM] = 1
MCNT[EPNUM] = packet count[EPNUM]
(MCNT – 1) × MPSIZ ≤ XFERSIZ ≤ MCNT × MPSIZ
PKTCNT = MCNT (in OTG_DIEPTSIZx)
If XFERSIZ < MCNT × MPSIZ, the last data packet of the transfer is a short
packet.
Note that: MCNT is in OTG_DIEPTSIZx, MPSIZ is in OTG_DIEPCTLx, PKTCNT
is in OTG_DIEPTSIZx and XFERSIZ is in OTG_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
RM0402 Rev 6
RM0402