UM1653 datasheet(7/45 Pages) STMICROELECTRONICS

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UM1653

STM32 NAND driver blocks

1.2

USB mass storage

The USB device is provided to the host as a particular class, which determines how the host

cross reacts with the embedded system.

In our case, the USB device must appear in the driver as a Mass Storage Class USB, which

defines that SCSI commands will be used with the protocol “bulk-only transport” (BOT).

Bulk-only-transport (BOT)

A general BOT transaction is based on a simple basic state machine. It begins with ready

state (idle state) and if a CBW is received from the host three cases can be managed:



DATA-OUT-STAGE: when direction flag is set to 0, Device shall prepare itself to receive

an amount of data indicated in dCBWDataTransferLength in the CBW block. At the end

of data transfer a CSW is returned with the remaining data length and the STATUS

field.



DATA-IN-STAGE: when direction flag is set to 1, Device shall prepare itself to send an

amount of data indicated in dCBWDataTransferLength in the CBW block. At the end of

data transfer a CSW is returned with the remaining data length and the STATUS field.



ZERO DATA: no data stage is needed so CSW block is sent immediately after CBW.

The BOT transport protocol encapsulates SCSI commands and transfers them in three

steps:

Send the command block CBW.

Transfer data.

Return the status of the block CSW.

Figure 2. BOT protocol architecture

MS33216V1

Ready

Command transport

CBW

Data In

Data Out

Status

Transport

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