STR720 datasheet(273/401 Pages) STMICROELECTRONICS

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STR720 - USB SLAVE INTERFACE (USB)

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Since the USB specification states that a SETUP packet cannot be answered with a

handshake different from ACK, eventually aborting a previously issued command to start the

new one, the USB logic doesn’t allow a control endpoint to answer with a NAK or STALL

packet to a SETUP token received from the host.

When the STAT_RX bits are set to ‘01’ (STALL) or ‘10’ (NAK) and a SETUP token is received,

the USB accepts the data, performing the required data transfers and sends back an ACK

handshake. If that endpoint has a previously issued CTR_RX request not yet acknowledged

by the application (i.e. CTR_RX bit is still set from a previously completed reception), the USB

discards the SETUP transaction and does not answer with any handshake packet regardless

of its state, simulating a reception error and forcing the host to send the SETUP token again.

This is done to avoid losing the notification of a SETUP transaction addressed to the same

endpoint immediately following the transaction, which triggered the CTR_RX interrupt.

19.5.3

Double-Buffered Endpoints

All different endpoint types defined by the USB standard represent different traffic models,

and describe the typical requirements of different kind of data transfer operations. When large

portions of data are to be transferred between the host PC and the USB function, the bulk

endpoint type is the most suited model. This is because the host schedules bulk transactions

so as to fill all the available bandwidth in the frame, maximizing the actual transfer rate as long

as the USB function is ready to handle a bulk transaction addressed to it. If the USB function

is still busy with the previous transaction when the next one arrives, it will answer with a NAK

handshake and the host PC will issue the same transaction again until the USB function is

ready to handle it, reducing the actual transfer rate due to the bandwidth occupied by

re-transmissions. For this reason, a dedicated feature called ‘double-buffering’ can be used

with bulk endpoints.

When ‘double-buffering’ is activated, data toggle sequencing is used to select, which buffer is

to be used by the USB Peripheral to perform the required data transfers, using both

‘transmission’ and ‘reception’ packet memory areas to manage buffer swapping on each

successful transaction in order to always have a complete buffer to be used by the application,

while the USB Peripheral fills the other one. For example, during an OUT transaction directed

to a ‘reception’ double-buffered bulk endpoint, while one buffer is being filled with new data

coming from the USB host, the other one is available for the microcontroller software usage

(the same would happen with a ‘transmission’ double-buffered bulk endpoint and an IN

transaction).

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 uni-directional 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.

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