STR720 datasheet(275/401 Pages) STMICROELECTRONICS

275 / 401 page

STR720 - USB SLAVE INTERFACE (USB)

275/401

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 register is set,

depending on the enabled direction. At the same time, the affected DTOG bit 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). 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.

The application software can always override the special flow control implemented for

double-buffered bulk endpoints, writing an explicit status different from ‘11’ (Valid) into the

STAT bit pair of the related USB_EPnR register. In this case, the USB Peripheral will always

use the programmed endpoint status, regardless of the buffer usage condition.

STR720 数据表(PDF) 275 Page - STMicroelectronics