Universal serial bus full-speed device interface (USB)
RM0365
1013/1080
DocID025202 Rev 7
While enabling the last data stage, the opposite direction should be set to NAK, so that, if
the host reverses the transfer direction (to perform the status stage) immediately, it is kept
waiting for the completion of the control operation. If the control operation completes
successfully, the software will change NAK to VALID, otherwise to STALL. At the same time,
if the status stage will be an OUT, the STATUS_OUT (EP_KIND in the USB_EPnR register)
bit should be set, so that an error is generated if a status transaction is performed with not-
zero data. When the status transaction is serviced, the application clears the STATUS_OUT
bit and sets STAT_RX to VALID (to accept a new command) and STAT_TX to NAK (to delay
a possible status stage immediately following the next setup).
Since the USB specification states that a SETUP packet cannot be answered with a
handshake different from ACK, eventually aborting a previously issued command to start
the new one, the USB logic doesn’t allow a control endpoint to answer with a NAK or STALL
packet to a SETUP token received from the host.
When the STAT_RX bits are set to ‘01 (STALL) or ‘10 (NAK) and a SETUP token is
received, the USB accepts the data, performing the required data transfers and sends back
an ACK handshake. If that endpoint has a previously issued CTR_RX request not yet
acknowledged by the application (i.e. CTR_RX bit is still set from a previously completed
reception), the USB discards the SETUP transaction and does not answer with any
handshake packet regardless of its state, simulating a reception error and forcing the host to
send the SETUP token again. This is done to avoid losing the notification of a SETUP
transaction addressed to the same endpoint immediately following the transaction, which
triggered the CTR_RX interrupt.
32.5.3 Double-buffered
endpoints
All different endpoint types defined by the USB standard represent different traffic models,
and describe the typical requirements of different kind of data transfer operations. When
large portions of data are to be transferred between the host PC and the USB function, the
bulk endpoint type is the most suited model. This is because the host schedules bulk
transactions so as to fill all the available bandwidth in the frame, maximizing the actual
transfer rate as long as the USB function is ready to handle a bulk transaction addressed to
it. If the USB function is still busy with the previous transaction when the next one arrives, it
will answer with a NAK handshake and the host PC will issue the same transaction again
until the USB function is ready to handle it, reducing the actual transfer rate due to the
bandwidth occupied by re-transmissions. For this reason, a dedicated feature called
‘double-buffering’ can be used with bulk endpoints.
When ‘double-buffering’ is activated, data toggle sequencing is used to select, which buffer
is to be used by the USB peripheral to perform the required data transfers, using both
‘transmission’ and ‘reception’ packet memory areas to manage buffer swapping on each
successful transaction in order to always have a complete buffer to be used by the
application, while the USB peripheral fills the other one. For example, during an OUT
transaction directed to a ‘reception’ double-buffered bulk endpoint, while one buffer is being
filled with new data coming from the USB host, the other one is available for the
microcontroller software usage (the same would happen with a ‘transmission’ double-
buffered bulk endpoint and an IN transaction).
Since the swapped buffer management requires the usage of all 4 buffer description table
locations hosting the address pointer and the length of the allocated memory buffers, the
USB_EPnR registers used to implement double-buffered bulk endpoints are forced to be
used as unidirectional ones. Therefore, only one STAT bit pair must be set at a value
different from ‘00 (Disabled): STAT_RX if the double-buffered bulk endpoint is enabled for