RM0008 USB full speed device interface (USB)
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NUM_BLOCK fields. Unidirectional endpoints, except Isochronous and double-buffered bulk
endpoints, need to initialize only bits and registers related to the supported direction. Once
the transmission and/or reception are enabled, register USB_EPnR and locations
ADDRn_TX/ADDRn_RX, COUNTn_TX/COUNTn_RX (respectively), should not be modified
by the application software, as the hardware can change their value on the fly. When the
data transfer operation is completed, notified by a CTR interrupt event, they can be
accessed again to re-enable a new operation.
IN packets (data transmission)
When receiving an IN token packet, if the received address matches a configured and valid
endpoint one, the USB peripheral accesses the contents of ADDRn_TX and COUNTn_TX
locations inside buffer descriptor table entry related to the addressed endpoint. The content
of these locations is stored in its internal 16 bit registers ADDR and COUNT (not accessible
by software). The packet memory is accessed again to read the first word to be transmitted
(Refer to Structure and usage of packet buffers on page 472) and starts sending a DATA0 or
DATA1 PID according to USB_EPnR bit DTOG_TX. When the PID is completed, the first
byte from the word, read from buffer memory, is loaded into the output shift register to be
transmitted on the USB bus. After the last data byte is transmitted, the computed CRC is
sent. If the addressed endpoint is not valid, a NAK or STALL handshake packet is sent
instead of the data packet, according to STAT_TX bits in the USB_EPnR register.
The ADDR internal register is used as a pointer to the current buffer memory location while
COUNT is used to count the number of remaining bytes to be transmitted. Each word read
from the packet buffer memory is transmitted over the USB bus starting from the least
significant byte. Transmission buffer memory is read starting from the address pointed by
ADDRn_TX for COUNTn_TX/2 words. If a transmitted packet is composed of an odd
number of bytes, only the lower half of the last word accessed will be used.
On receiving the ACK receipt by the host, the USB_EPnR register is updated in the following
way: DTOG_TX bit is toggled, the endpoint is made invalid by setting STAT_TX=10 (NAK)
and bit CTR_TX is set. 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. Servicing of the CTR_TX event starts clearing the interrupt bit; the application
software then prepares another buffer full of data to be sent, updates the COUNTn_TX table
location with the number of byte to be transmitted during the next transfer, and finally sets
STAT_TX to ‘11’ (VALID) to re-enable transmissions. While the STAT_TX bits are equal to
‘10’ (NAK), any IN 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 IN transaction addressed to the same endpoint immediately following the one which
triggered the CTR interrupt.
OUT and SETUP packets (data reception)
These two tokens are handled by the USB peripheral more or less in the same way; the
differences in the handling of SETUP packets are detailed in the following paragraph about
control transfers. When receiving an OUT/SETUP PID, if the address matches a valid
endpoint, the USB peripheral accesses the contents of the ADDRn_RX and COUNTn_RX
locations inside the buffer descriptor table entry related to the addressed endpoint. The
content of the ADDRn_RX is stored directly in its internal register ADDR. While COUNT is
now reset and the values of BL_SIZE and NUM_BLOCK bit fields, which are read within
COUNTn_RX content are used to initialize BUF_COUNT, an internal 16 bit counter, which is
used to check the buffer overrun condition (all these internal registers are not accessible by
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