USB2504/USB2504A

DS000002250A-page 20

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5.3.1.1

Byte Protocols

When using the Hub SMBus Interface for byte transfers, a write will always consist of the SMBus Interface Slave

Address byte, followed by the Internal Address Register byte, then the data byte.

The normal read protocol consists of a write to the HUB with the SMBus Interface Address byte, followed by the Internal

Address Register byte. Then restart the Serial Communication with a Read consisting of the SMBus Interface Address

byte, followed by the data byte read from the Hub. This can be accomplished by using the Read Byte protocol.

Write Byte

The Write Byte protocol is used to write data to the registers. The data will only be written if the protocol shown in

Table 5-2 is performed correctly. Only one byte is transferred at a time for a Write Byte protocol.

Read Byte

The Read Byte protocol is used to read data from the registers. The data will only be read if the protocol shown in

Table 5-3 is performed correctly. Only one byte is transferred at a time for a Read Byte protocol.

5.3.2

INVALID PROTOCOL RESPONSE BEHAVIOR

Registers that are accessed with an invalid protocol are not updated. A register is only updated following a valid protocol.

The only valid protocols are Write Byte and Read Byte, which are described above.

The Hub only responds to the hardware selected Slave Address.

Attempting to communicate with the Hub over SMBus with an invalid slave address or invalid protocol results in no

response, and the SMBus Slave Interface returns to the idle state.

The only valid registers that are accessible by the SMBus slave address are the registers defined in the Registers Sec-

tion. See Section 5.3.3 for the response to undefined registers.

5.3.3

GENERAL CALL ADDRESS RESPONSE

The Hub does not respond to a general call address of 0000_000b.

5.3.4

SLAVE DEVICE TIME-OUT

According to the SMBus Specification, V1.0 devices in a transfer can abort the transfer in progress and release the bus

Note:

Data bytes are transferred MSB first (msb first).

Note:

For the following SMBus tables:

TABLE 5-2:

SMBUS WRITE BYTE PROTOCOL

Field:

Start

Slave Addr

Wr

Ack

Reg. Addr

Ack

Reg. Data

Ack

Stop

Bits:

1

71

18

1

8

1

1

TABLE 5-3:

SMBUS READ BYTE PROTOCOL

Field:

Start

Slave

Addr

Wr

Ack

Reg.

Addr

Ack

Start

Slave

Addr

Rd

Ack

Reg.

Data

Nack

Stop

Bits:

1

7

1

1

8

1

1

7

1

1

8

1

1

Denotes Master-to-Slave

Denotes Slave-to-Master