UM0290 datasheet(40/76 Pages) STMICROELECTRONICS

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UM0290

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5.2

Mass storage protocol

5.2.1

Bulk Only Transfer (BOT)

The BOT protocol uses only bulk pipes to transfer command, status and data (no interrupt or

control pipes). The default pipe (pipe 0, or in other words, Endpoint 0) is only used to clear

the bulk pipe(s) status (clear STALL status) and to issue the two class specific requests:

Mass Storage reset and Get Max LUN.

Command transfer

To send a command, the host uses a specific format called Command Block Wrapper

(CBW). The CBW is a 31-byte length packet. The Figure 10 shows the different fields of a

CBW.

●

dCBWSignature: 43425355 USBC (little Endian)

●

dCBWTag: The host specifies this field for each command. The device should return

the same dCBWTag in the associated status.

●

dCBWDataTransferLength: total number of bytes to transfer (expected by the host).

●

bmCBWFlags: This field is used to specify the direction of the data transfer (if any).

The bits of this field are defined as follows:

Bit 7: Direction bit:

0 : Data Out transfer (host to device).

1 : Data In transfer (device to host).

Note:

The device must ignore this bit if the dCBWDataTransferLength field is cleared to zero.

Bits 6:0: reserved (cleared to zero).

●

bCBWLUN: concerned Logical Unit number.

●

bCBWCBLength: this field specifies the length (in bytes) of the command CBWCB.

●

CBWCB: the command block to be executed by the device.

Status transfer

To inform the host about the status of each received command, the device uses the

Command Status Wrapper (CSW). Table 12 shows the different fields of a CSW.

Table 11.

CBW packet fields

543

210

0-3

dCBWSignature

4-7

dCBWTag

8-11

dCBWDataTransferLength

bmCBWFlags

Reserved (0)

bCBWLUN

Reserved (0)

bCBWCBLength

15-30

CBWCB

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