ST STM32F412 Reference Manual page 1098

USB on-the-go full-speed (OTG_FS)
This section describes how the core handles SETUP packets and the application's
sequence for handling SETUP transactions.
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Application requirements
1. To receive a SETUP packet, the STUPCNT field (OTG_DOEPTSIZx) in a control OUT
endpoint must be programmed to a non-zero value. When the application programs the
STUPCNT field to a non-zero value, the core receives SETUP packets and writes them
to the receive FIFO, irrespective of the NAK status and EPENA bit setting in
OTG_DOEPCTLx. The STUPCNT field is decremented every time the control endpoint
receives a SETUP packet. If the STUPCNT field is not programmed to a proper value
before receiving a SETUP packet, the core still receives the SETUP packet and
decrements the STUPCNT field, but the application may not be able to determine the
correct number of SETUP packets received in the setup stage of a control transfer.
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2. The application must always allocate some extra space in the receive data FIFO, to be
able to receive up to three SETUP packets on a control endpoint.
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3. The application must read the 2 words of the SETUP packet from the receive FIFO.
4. The application must read and discard the setup stage done word from the receive
FIFO.
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Internal data flow
1. When a SETUP packet is received, the core writes the received data to the receive
FIFO, without checking for available space in the receive FIFO and irrespective of the
endpoint's NAK and STALL bit settings.
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2. For every SETUP packet received on the USB, 3 words of data are written to the
receive FIFO, and the STUPCNT field is decremented by 1.
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3. When the setup stage changes to a data IN/OUT stage, the core writes an entry (setup
stage done word) to the receive FIFO, indicating the completion of the setup stage.
4. On the AHB side, SETUP packets are emptied by the application.
5. When the application pops the setup stage done word from the receive FIFO, the core
interrupts the application with an STUP interrupt (OTG_DOEPINTx), indicating it can
process the received SETUP packet.
6. The core clears the endpoint enable bit for control OUT endpoints.
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Application programming sequence
1098/1163
STUPCNT = 3 in OTG_DOEPTSIZx
The space to be reserved is 10 words. Three words are required for the first
SETUP packet, 1 word is required for the setup stage done word and 6 words are
required to store two extra SETUP packets among all control endpoints.
3 words per SETUP packet are required to store 8 bytes of SETUP data and 4
bytes of SETUP status (setup packet pattern). The core reserves this space in the
receive data FIFO to write SETUP data only, and never uses this space for data
packets.
The core internally sets the IN NAK and OUT NAK bits for the control IN/OUT
endpoints on which the SETUP packet was received.
The first word contains control information used internally by the core
The second word contains the first 4 bytes of the SETUP command
The third word contains the last 4 bytes of the SETUP command
RM0402 Rev 6
RM0402