USB2422

DS00001726B-page 18

4.3

SMBus

The Microchip hub can be configured by an external processor via an SMBus interface (see Table 4-1 for details on

enabling the SMBus interface). The Microchip hub waits indefinitely for the SMBus code load to complete and only

appears as a newly connected device on USB after the code load is complete.

The hub’s SMBus acts as a slave-only SMBus device. The implementation only supports block write (Section 4.3.2.1)

and block read (Section 4.3.2.2) protocols. Reference the System Management Bus Specification [2] for additional infor-

mation.

Refer to Section 4.4, "SMBus Registers," on page 20 for details on all SMBus accessible registers.

4.3.1

SMBUS SLAVE ADDRESS

The 7-bit slave address is 0101100b. The hub will not respond to the general call address of 0000000b.

4.3.2

PROTOCOL IMPLEMENTATION

Typical block write and block read protocols are shown in figures 4-2 and 4-3. Register accesses are performed using

7-bit slave addressing, an 8-bit register address field, and an 8-bit data field. The shading shown in the figures during a

read or write indicates the hub is driving data on the SMBDATA line; otherwise, host data is on the SMBDATA line.

The SMBus slave address assigned to the hub (0101100b) allows it to be identified on the SMBus. The register address

field is the internal address of the register to be accessed. The register data field is the data that the host is attempting

to write to the register or the contents of the register that the host is attempting to read.

4.3.2.1

Block Write/Read

The block write begins with a slave address and a write condition. After the command code, the host issues a byte count

which describes how many more bytes will follow in the message. If a slave had 20 bytes to send, the first byte would

be the number 20 (14h), followed by the 20 bytes of data. The byte count may not be zero. A block write or read allows

a transfer maximum of 32 data bytes.

Note:

Data bytes are transferred MSB first.

Note:

For the following SMBus tables:

FIGURE 4-2:

BLOCK WRITE

Denotes Master-to-Slave

Denotes Slave-to-Master

S

Slave Address

Register Address

Wr

A

171

1

8

A

1

...

1

81

Byte Count = N

A

Data byte 1

A

Data byte 2

A

81

1

1

88

Data byte N

A

P