ST STM32F102 Series Reference Manual page 476

USB full speed device interface (USB)
software). Data bytes subsequently received by the USB peripheral are packed in words
(the first byte received is stored as least significant byte) and then transferred to the packet
buffer starting from the address contained in the internal ADDR register while BUF_COUNT
is decremented and COUNT is incremented at each byte transfer. When the end of DATA
packet is detected, the correctness of the received CRC is tested and only if no errors
occurred during the reception, an ACK handshake packet is sent back to the transmitting
host. In case of wrong CRC or other kinds of errors (bit-stuff violations, frame errors, etc.),
data bytes are still copied in the packet memory buffer, at least until the error detection point,
but ACK packet is not sent and the ERR bit in USB_ISTR register is set. However, there is
usually no software action required in this case: the USB peripheral recovers from reception
errors and remains ready for the next transaction to come. If the addressed endpoint is not
valid, a NAK or STALL handshake packet is sent instead of the ACK, according to bits
STAT_RX in the USB_EPnR register and no data is written in the reception memory buffers.
Reception memory buffer locations are written starting from the address contained in the
ADDRn_RX for a number of bytes corresponding to the received data packet length, CRC
included (i.e. data payload length + 2), or up to the last allocated memory location, as
defined by BL_SIZE and NUM_BLOCK, whichever comes first. In this way, the USB
peripheral never writes beyond the end of the allocated reception memory buffer area. If the
length of the data packet payload (actual number of bytes used by the application) is greater
than the allocated buffer, the USB peripheral detects a buffer overrun condition. in this case,
a STALL handshake is sent instead of the usual ACK to notify the problem to the host, no
interrupt is generated and the transaction is considered failed.
When the transaction is completed correctly, by sending the ACK handshake packet, the
internal COUNT register is copied back in the COUNTn_RX location inside the buffer
description table entry, leaving unaffected BL_SIZE and NUM_BLOCK fields, which
normally do not require to be re-written, and the USB_EPnR register is updated in the
following way: DTOG_RX bit is toggled, the endpoint is made invalid by setting STAT_RX =
'10' (NAK) and bit CTR_RX is set. If the transaction has failed due to errors or buffer overrun
condition, none of the previously listed actions take place. The application software must
first identify the endpoint, which is requesting microcontroller attention by examining the
EP_ID and DIR bits in the USB_ISTR register. The CTR_RX event is serviced by first
determining the transaction type (SETUP bit in the USB_EPnR register); the application
software must clear the interrupt flag bit and get the number of received bytes reading the
COUNTn_RX location inside the buffer description table entry related to the endpoint being
processed. After the received data is processed, the application software should set the
STAT_RX bits to '11' (Valid) in the USB_EPnR, enabling further transactions. While the
STAT_RX bits are equal to '10' (NAK), any OUT request addressed to that endpoint is
NAKed, indicating a flow control condition: the USB host will retry the transaction until it
succeeds. It is mandatory to execute the sequence of operations in the above mentioned
order to avoid losing the notification of a second OUT transaction addressed to the same
endpoint following immediately the one which triggered the CTR interrupt.
Control transfers
Control transfers are made of a SETUP transaction, followed by zero or more data stages,
all of the same direction, followed by a status stage (a zero-byte transfer in the opposite
direction). SETUP transactions are handled by control endpoints only and are very similar to
OUT ones (data reception) except that the values of DTOG_TX and DTOG_RX bits of the
addressed endpoint registers are set to 1 and 0 respectively, to initialize the control transfer,
and both STAT_TX and STAT_RX are set to '10' (NAK) to let software decide if subsequent
transactions must be IN or OUT depending on the SETUP contents. A control endpoint must
check SETUP bit in the USB_EPnR register at each CTR_RX event to distinguish normal
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