LM84CIMQAX datasheet(10/16 Pages) NSC

10 / 16 page

1.0 Functional Description (Continued)

1.3 SMBus INTERFACE

The LM84 operates as a slave on the SMBus, so the

SMBCLK line is an input (no clock is generated by the LM84)

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

specifications, the LM84 has a 7-bit slave address. Bit 4 (A3)

of the slave address is hard wired inside the LM84 to a 1.

The remainder of the address bits are controlled by the

address select pins ADD1 and ADD0, and are set by con-

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

CC for a high,

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

Therefore, the complete slave address is:

MSB

LSB

and is selected as follows:

Address Select Pin State

LM84 SMBus

Slave Address

ADD0

ADD1

A6:A0 binary

001 1000

TRI-LEVEL

001 1001

001 1010

TRI-LEVEL

010 1001

TRI-LEVEL

010 1010

TRI-LEVEL

010 1011

100 1100

TRI-LEVEL

100 1101

100 1110

The LM84 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 LM84.

1.4 TEMPERATURE DATA FORMAT

Temperature data can be read from the Local Temperature,

Remote Temperature, and T_CRIT setpoint registers. Tem-

perature data can only be written to the T_CRIT 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

1.5 OPEN-DRAIN OUTPUTS

SMBData and T_CRIT_A outputs are open-drain and do not

have internal pull-ups. A “high” level will not be observed on

these pins until pull-up current is provided from some exter-

nal 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 local temperature reading errors due to self

heating of the LM84. The maximum resistance of the pull-up,

based on LM84 specification for High Level Output Current,

to provide a 2V high level, is 30 k

Ω.

1.6 DIODE FAULT DETECTION

Before each remote conversion the LM84 goes through an

external diode fault detection sequence. If the D+ input is

shorted to V

CC or floating then the temperature reading will

be +127˚C, bit 2 (OPEN) of the Status Register will be set. If

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

bit 4 (RTCRIT) of the Status Register will be set which will

activate the T_CRIT_A output, if enabled. If D+ is shorted to

GND or D−, the temperature reading will be 0˚C and bit 2 of

the Status Register will not be set.

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