LTC3871 datasheet(12/24 Pages) LINER

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LTC7106

Rev A

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OPERATION

Similarity Between PMBus, SMBus and I2C 2-Wire

Interface

The PMBus 2-wire interface is an incremental extension

of the SMBus. SMBus is built upon I2C with some minor

differences in timing, DC parameters and protocol. The

PMBus/SMBus protocols are more robust than simple I2C

bytecommandsbecausePMBus/SMBusprovidetimeouts

to prevent bus hangs and valid operation commands. In

general, a master device that can be configured for I2C

communication can be used for PMBus communication

with little or no change to hardware or firmware. Repeat

start (restart) is not supported by all I2C controllers but

is required for SMBus/PMBus reads. If a general purpose

I2C controller is used, check that repeat start is supported.

For a description of the minor extensions and exceptions

PMBus makes to SMBus, refer to PMBus Specification

Part 1, Revision 1.1: Paragraph 5: Transport.

ForadescriptionofthedifferencesbetweenSMBusandI2C,

refer to System Management Bus (SMBus) Specification

Version 2.0: Appendix B—Differences Between SMBus

and I2C.

PMBus SERIAL INTERFACE

TheLTC7106communicateswithahost(master)usingthe

standardPMBusserialbusinterface.TheTimingDiagram,

Figure 1, shows the timing relationship of the signals on

the bus. The two-bus lines, SDA and SCL, must be high

when the bus is not in use. External pull-up resistors or

current sources are required on these lines.

The LTC7106 is a slave device. The master can com-

municate with the LTC7106 using the following formats:

• Master Transmitter, Slave Receiver

• Master Receiver, Slave Transmitter

The following PMBus protocols are supported:

• Write Byte, Send Byte

• Read Byte, Read Word

• Alert Response Address

Figure 3 through Figure 6 illustrate the aforementioned

PMBus protocols. All transactions support GCP (group

command protocol).

Figure 2 is a key to the protocol diagrams in this section.

A value shown below a field in the following figures is a

mandatory value for that field.

The data formats implemented by PMBus are:

• Master transmitter transmits to slave receiver. The

transfer direction in this case is not changed.

• Master reads slave immediately after the first byte. At

the moment of the first acknowledgment (provided by

the slave receiver) the master transmitter becomes a

master receiver and the slave receiver becomes a slave

transmitter.

• Combined format. During a change of direction within

a transfer, the master repeats both a start condition and

the slave address but with the R/W bit reversed. In this

case, the master receiver terminates the transfer by

generating a NACK on the last byte of the transfer and

a STOP condition.

Examples of these formats are shown in Figure 4 and

Figure 5.

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