To Next Page

To Previous Page

DocID13902 Rev 15 624/1128

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

644

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

DBL_BUF setting, STAT bit pair is not affected by the transaction termination and its value

remains ‘11 (Valid). However, as the token packet of a new transaction is received, the

actual endpoint status will be masked as ‘10 (NAK) when a buffer conflict between the USB

peripheral and the application software is detected (this condition is identified by DTOG and

SW_BUF having the same value, see Table 170 on page 624). The application software

responds to the CTR event notification by clearing the interrupt flag and starting any

required handling of the completed transaction. When the application packet buffer usage is

over, the software toggles the SW_BUF bit, writing ‘1 to it, to notify the USB peripheral about

the availability of that buffer. In this way, the number of NAKed transactions is limited only by

the application elaboration time of a transaction data: if the elaboration time is shorter than

the time required to complete a transaction on the USB bus, no re-transmissions due to flow

control will take place and the actual transfer rate will be limited only by the host PC.

Table 170. 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