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Universal serial bus full-speed device interface (USB) RM0008

623/1128 DocID13902 Rev 15

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

reception, STAT_TX if the double-buffered bulk endpoint is enabled for transmission. In

case it is required to have double-buffered bulk endpoints enabled both for reception and

transmission, two USB_EPnR registers must be used.

To exploit the double-buffering feature and reach the highest possible transfer rate, the

endpoint flow control structure, described in previous chapters, has to be modified, in order

to switch the endpoint status to NAK only when a buffer conflict occurs between the USB

peripheral and application software, instead of doing it at the end of each successful

transaction. The memory buffer which is currently being used by the USB peripheral is

defined by the DTOG bit related to the endpoint direction: DTOG_RX (bit 14 of USB_EPnR

control scheme, the USB peripheral should know which packet buffer is currently in use by

the application software, so to be aware of any conflict. Since in the USB_EPnR register,

there are two DTOG bits but only one is used by USB peripheral for data and buffer

sequencing (due to the unidirectional constraint required by double-buffering feature) the

other one can be used by the application software to show which buffer it is currently using.

This new buffer flag is called SW_BUF. In the following table the correspondence between

USB_EPnR register bits and DTOG/SW_BUF definition is explained, for the cases of

‘transmission’ and ‘reception’ double-buffered bulk endpoints.

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.

Table 169. Double-buffering buffer flag definition

Buffer flag ‘Transmission’ endpoint ‘Reception’ endpoint

DTOG DTOG_TX (USB_EPnRbit 6) DTOG_RX (USB_EPnRbit 14)

SW_BUF USB_EPnR bit 14 USB_EPnR bit 6

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