1.4 POWER ON RESET DEFAULT STATES

LM82 always powers up to these known default states:

1.

Command Register set to 00h

2.

Local Temperature set to 0˚C

3.

Remote Temperature set to 0˚C until the LM82 senses a

diode present between the D+ and D− input pins.

4.

Status Register set to 00h.

5.

Configuration Register set to 00h; INT enabled and all

T_CRIT setpoints enabled to activate T_CRIT_A.

6.

Local and Remote T_CRIT set to 127˚C

1.5 SMBus INTERFACE

The LM82 operates as a slave on the SMBus, so the

SMBCLK line is an input (no clock is generated by the LM82)

and the SMBData line is bi-directional. According to SMBus

specifications, the LM82 has a 7-bit slave address. Bit 4 (A3)

of the slave address is hard wired inside the LM82 to a 1.

The remainder of the address bits are controlled by the state

of the address select pins ADD1 and ADD0, and are set by

connecting these pins to ground for a low, (0) , to V

high, (1), or left floating (TRI-LEVEL).

Therefore, the complete slave address is:

A6

A5

A4

1

A2

A1

A0

MSB

LSB

and is selected as follows:

Address Select Pin State

LM82 SMBus Slave

Address

ADD0

ADD1

A6:A0 binary

0

0

001 1000

0

TRI-LEVEL

001 1001

0

1

001 1010

TRI-LEVEL

0

010 1001

TRI-LEVEL

TRI-LEVEL

010 1010

TRI-LEVEL

1

010 1011

1

0

100 1100

1

TRI-LEVEL

100 1101

1

1

100 1110

The LM82 latches the state of the address select pins during

the first read or write on the SMBus. Changing the state of

the address select pins after the first read or write to any

device on the SMBus will not change the slave address of

the LM82.

1.6 TEMPERATURE DATA FORMAT

Temperature data can be read from the Local and Remote

Temperature, T_CRIT, and HIGH setpoint registers; and writ-

ten to the T_CRIT and HIGH setpoint registers. Temperature

data is represented by an 8-bit, two’s complement byte with

an LSB (Least Significant Bit) equal to 1˚C:

Temperature

Digital Output

Binary

Hex

+125˚C

0111 1101

7Dh

+25˚C

0001 1001

19h

+1˚C

0000 0001

01h

0˚C

0000 0000

00h

−1˚C

1111 1111

FFh

−25˚C

1110 0111

E7h

−55˚C

1100 1001

C9h

1.7 OPEN-DRAIN OUTPUTS

The SMBData, INT and T_CRIT_A outputs are open-drain

outputs and do not have internal pull-ups. A “high” level will

not be observed on these pins until pull-up current is pro-

vided from some external source, typically a pull-up resistor.

Choice of resistor value depends on many system factors

but, in general, the pull-up resistor should be as large as

possible. This will minimize any internal temperature reading

errors due to internal heating of the LM82. The maximum

resistance of the pull up, based on LM82 specification for

High Level Output Current, to provide a 2.1V high level, is

30k

Ω. Care should be taken in a noisy system because a

high impedance pull-up will be more likely to couple noise

into the signal line.

1.8 DIODE FAULT DETECTION

Before each external conversion the LM82 goes through an

external diode fault detection sequence. If D+ input is

shorted to V

be +127 ˚C, and the OPEN bit in the Status Register will be

set. If the T_CRIT setpoint is set to less than +127 ˚C then

the D+ input RTCRIT bit in the Status Register will be set

which will activate the T_CRIT_A output, if enabled. If a D+

is shorted to GND or D−, its temperature reading will be 0 ˚C

and its OPEN bit in the Status Register will not be set.

www.national.com

10