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

UM1653 데이터시트(PDF) 7 Page - STMicroelectronics