Temperature Data Format

data format is 9 bits, two’s complement, and the register

is read out in 2 bytes: an upper byte and a lower byte.

Bits D15–D7 contain the temperature data, with the LSB

representing 0.5°C and the MSB representing the sign

bit (see Table 3). The MSB is transmitted first. The last 7

bits of the lower byte, bits D6–D0, are don’t cares.

Shutdown

Set bit D0 in the Configuration register to 1 to place the

LM75 in shutdown mode and reduce supply current to

4µA. In interrupt mode, entering shutdown resets the

Configuration register remain accessible to the master.

Fault Queue

The fault queue prevents OS false tripping in noisy

environments. The number of faults set in the queue (up

to 6) must occur to trip the OS output.

Comparator/Interrupt Mode

The events that trigger OS are identical between com-

parator and interrupt modes. In comparator mode, OS

value. OS is deasserted when the temperature drops

performing a read operation.

OS Output

The OS output is an open-drain output without an inter-

Using larger resistance values reduces any tempera-

ture errors due to self heating from current entering OS.

OS Polarity

The OS polarity can be programmed for active-low or

active-high operation. In active-low operation, OS goes

low when triggered by a temperature event.

Internal Registers

The LM75’s Pointer register selects between four data

registers (see Figure 6). At power-up, the pointer is set

to read the Temperature register at address 0x00. The

Pointer register latches the last location to which it was

set. All registers are read and write, except the

Temperature register, which is read only.

Write to the Configuration register by writing an address

byte, a data pointer byte, and a data byte. If 2 data

bytes are written, the second data byte overrides the

byte, 1 pointer byte, and 2 data bytes. If only 1 data

byte is written, it is saved in bits D15–D8 of the respec-

tive register. If more than 2 data bytes are written, only

the first 2 bytes are recognized while the remaining

bytes are ignored.

Read from the LM75 in one of two ways. If the location

latched in the Pointer register is set from the previous

read, the new read consists of an address byte, fol-

lowed by retrieving the corresponding number of data

bytes. If the Pointer register needs to be set to a new

address, perform a read operation by writing an

address byte, pointer byte, repeat start, and another

address byte.

An inadvertent 8-bit read from a 16-bit register, with the

D7 bit low, can cause the device to stop in a state

where the SDA line is held low. Ordinarily, this would

prevent any further bus communication until the master

sends nine additional clock cycles or SDA goes high.

At that time, a stop condition resets the device. If the

additional clock cycles are not generated by the mas-

ter, the LM75 bus resets and unlocks after the bus time-

out period has elapsed.

Digital Temperature Sensor and Thermal

Watchdog with 2-Wire Interface

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9

SMBus is a trademark of Intel Corp.

Figure 6. Block Diagram

LM75

A2/RESET

A1

A0

SDA

SCL

OS

SMBus™

INTERFACE

BLOCK

POINTER REGISTER

(SELECTS REGISTER

FOR COMMUNICATION)

DATA

ADDRESS

REGISTER SELECT

GND

TEMPERATURE

(READ ONLY)

POINTER = 0000 0000

(READ/WRITE)

POINTER = 0000 0011

(READ/WRITE)

POINTER = 0000 0010

CONFIGURATION

(READ/WRITE)

POINTER = 0000 0001