STR720 datasheet(274/401 Pages) STMICROELECTRONICS

274 / 401 page

STR720 - USB SLAVE INTERFACE (USB)

274/401

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

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 uni-directional 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

‘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 58. 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 59. Double-Buffering Memory Buffers Usage

DTOG or SW_BUF

bit value

Packet buffer used by

USB Peripheral (DTOG) or

application software (SW_BUF)

ADDRn_TX / COUNTn_TX

buffer description table locations.

ADDRn_RX / COUNTn_RX

buffer description table locations.

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