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LTC4258

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APPLICATIO S I FOR ATIO

SERIAL DIGITAL INTERFACE

The LTC4258 communicates with a host (master) using

the standard 2-wire interface as described in the SMBus

Specification Version 2.0 (available at http://smbus.org).

The SMBus is an extension of the I2C bus, and the

LTC4258 is also compatible with the I2C bus standard. The

Timing Diagrams (Figures 5 through 9) show 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, such as the

LTC1694 SMBus accelerator, are required on these lines.

If the SDA and SCL pull-ups are absent, not connected to

the same positive supply as the LTC4258’s VDD pin, or are

not activated when the power is applied to the LTC4258, it

is possible for the LTC4258 to see a START condition on

the I2C bus. The interrupt pin (INT) is only updated

between I2C transactions. Therefore if the LTC4258 sees

a START condition when it powers up because the SCL and

SDA lines were left floating, it will not assert an interrupt

(pull INT low) until it sees a STOP condition on the bus. In

a typical application the I2C bus will immediately have

traffic and the LTC4258 will see a STOP so soon after

power up that this momentary condition will go unnoticed.

Isolating the Serial Digital Interface

IEEE 802.3af requires that network segments be electri-

cally isolated from the chassis ground of each network

interface device. However, the network segments are not

required to be isolated from each other provided that the

segments are connected to devices residing within a

single building on a single power distribution system.

For simple devices such as small powered Ethernet

switches, the requirement can be met by using an iso-

lated power supply to power the entire device. This

implementation can only be used if the device has no

electrically conducting ports other than twisted-pair

Ethernet. In this case, the SDAIN and SDAOUT pins of the

LTC4258 can be connected together to act as a standard

I2C/SMBus SDA pin.

If the device is part of a larger system, contains serial

ports, or must be referenced to protective ground for

some other reason, the Power over Ethernet subsystem

including the LTC4258s must be electrically isolated

from the rest of the system. The LTC4258 includes

separate pins (SDAIN and SDAOUT) for the input and

output functions of the bidirectional data line. This eases

the use of optocouplers to isolate the data path between

the LTC4258s and the system controller. Figure 17

shows one possible implementation of an isolated inter-

face. The SDAOUT pin of the LTC4258 is designed to

drive the inputs of an optocoupler directly, but a standard

I2C device typically cannot. U1 is used to buffer I2C

signals into the optocouplers from the system controller

side. Schmitt triggers must be used to prevent extra

edges on transitions of SDA and SCL.

Bus Addresses and Protocols

The LTC4258 is a read-write slave device. The master can

communicate with the LTC4258 using the Write Byte,

Read Byte and Receive Byte protocols. The LTC4258’s

primary serial bus address is (010A3A2A1A0)b, as desig-

nated by pins AD3-AD0. All LTC4258s also respond to the

address (0110000)b, allowing the host to write the same

command into all of the LTC4258s on a bus in a single

transaction. If the LTC4258 is asserting (pulling low) the

INT pin, it will also acknowledge the Alert Response

Address (0001100)b using the receive byte protocol.

The START and STOP Conditions

When the bus is idle, both SCL and SDA must be high. A

bus master (typically the host controller) signals the

beginning of communication with a slave device (like the

LTC4258) by transmitting a START condition. A START

condition is generated by transitioning SDA from high to

low while SCL is high. A REPEATED START condition is

functionally the same as a START condition, but used to

extend the protocol for a change in data transmission

direction. A STOP condition is not used to set up a

REPEATED START condition, for this would clear any data

already latched in. When the master has finished commu-

nicating with the slave, it issues a STOP condition. A STOP

condition is generated by transitioning SDA from low to

high while SCL is high. The bus is then free for communi-

cation with another SMBus or I2C device.

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