ST STM32G474 User Manual

2126 pages

To Next Page

To Previous Page

RM0440 Rev 4 2019/2126

RM0440 Universal serial bus full-speed device interface (USB)

2041

The memory buffer which is currently being used by the USB peripheral is defined by DTOG

buffer flag, while the buffer currently in use by application software is identified by SW_BUF

buffer flag. The relationship between the buffer flag value and the used packet buffer is the

same in both cases, and it is listed in the following table.

Double-buffering feature for a bulk endpoint is activated by:

• Writing EP_TYPE bit field at ‘00 in its USB_EPnR register, to define the endpoint as a

bulk, and

• Setting EP_KIND bit at ‘1 (DBL_BUF), in the same register.

The application software is responsible for DTOG and SW_BUF bits initialization according

to the first buffer to be used; this has to be done considering the special toggle-only property

that these two bits have. The end of the first transaction occurring after having set

DBL_BUF, triggers the special flow control of double-buffered bulk endpoints, which is used

for all other transactions addressed to this endpoint until DBL_BUF remain set. At the end of

each transaction the CTR_RX or CTR_TX bit of the addressed endpoint USB_EPnR

in the USB_EPnR register is hardware toggled making the USB peripheral buffer swapping

completely software independent. Unlike common transactions, and the first one after

Table 408. Double-buffering buffer flag definition

Buffer flag ‘Transmission’ endpoint ‘Reception’ endpoint

DTOG DTOG_TX (USB_EPnR bit 6) DTOG_RX (USB_EPnR bit 14)

SW_BUF USB_EPnR bit 14 USB_EPnR bit 6

Table 409. Bulk double-buffering memory buffers usage

Endpoint

type

DTOG SW_BUF

Packet buffer used by

USB peripheral

Packet buffer used by

Application Software

ADDRn_TX_0 / COUNTn_TX_0