5

FN8239.1

January 21, 2005

Functional Description

SMBus Overview

that more than one device connected to the bus is capable of

controlling it. Master devices communicate to other master

or slave devices using one clock and one data line. These

are both bidirectional.

In order to allow multimaster operation without bus

contention, it is necessary to allow each bus to be connected

to a positive supply voltage via a current-source or pull-up

resistor (see “System Diagram” on page 2). When the bus is

free, both lines are HIGH. The output stages of devices

connected to the bus must have an open-drain or open-

collector to perform the wired-AND function.

Simple pull-up resistors on the clock and data lines work well

unless there are long signal lines or many devices

connected to the bus. Then, the combined capacitance of

the bus increases the rise time on the signal to such an

extent that the communication becomes unreliable or fails to

meet the bus timing specifications. Smaller resistors can

sometimes compensate for the extra capacitance, but this

increases the current consumption when the signal lines are

pulled LOW.

FIGURE 7. BOOST PULL-UP CURRENT vs. SMBus VOLTAGE

0.50

0.55

0.60

0.65

0.70

0.45

0.40

0.35

0.30

-40

-20

0

204060

80

100

-60

TEMPERATURE (°C)

VDD=5.5V

VDD=2.7V

75

80

85

90

95

70

65

60

55

-40

-20

0

204060

80

100

-60

TEMPERATURE (°C)

VDD=5.5V

VDD=2.7V

2.0

2.5

3.0

3.5

1.5

1.0

0.5

0

12

34

5

0

SMBus VOLTAGE (V)

6

VDD=5.0V

VDD=2.7V

ISL88694