I

2

C Interface

SDA

SCL

Pointer Register

(Selects register for

communication)

Address

Data

Temperature

(Read-Only)

Pointer = 00000000

(Read-Write)

Pointer = 00000011

Configuration

(Read-Write)

Pointer = 00000001

(Read-Write)

Pointer = 00000010

LM75B, LM75C

SNIS153B – JULY 2009 – REVISED MARCH 2013

www.ti.com

COMPARATOR/INTERRUPT MODE

As indicated in the O.S. Output Temperature Response Diagram, Figure 9, the events that trigger O.S. are

identical for either Comparator or Interrupt mode. The most important difference is that in Interrupt mode the O.S.

will remain set indefinitely once it has been set. To reset O.S. while in Interrupt mode, perform a read from any

register in the LM75.

O.S. OUTPUT

The O.S. output is an open-drain output and does not have an internal pull-up. A “high” level will not be observed

on this pin until pull-up current is provided 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 errors due to internal heating of the LM75. The maximum resistance of the pull

up, based on LM75 specification for High Level Output Current, to provide a 2V high level, is 30 k

Ω.

O.S. POLARITY

The O.S. output can be programmed via the configuration register to be either active low (default mode), or

active high. In active low mode the O.S. output goes low when triggered exactly as shown on the O.S. Output

Temperature Response Diagram, Figure 9. Active high simply inverts the polarity of the O.S. output.

INTERNAL REGISTER STRUCTURE

Figure 10.

There are four data registers in the LM75B and LM75C selected by the Pointer register. At power-up the Pointer

is set to “000”; the location for the Temperature Register. The Pointer register latches whatever the last location it

was set to. In Interrupt Mode, a read from the LM75, or placing the device in shutdown mode, resets the O.S.

output. All registers are read and write, except the Temperature register which is a read only.

A write to the LM75 will always include the address byte and the Pointer byte. A write to the Configuration

Reading the LM75 can take place either of two ways: If the location latched in the Pointer is correct (most of the

time it is expected that the Pointer will point to the Temperature register because it will be the data most

frequently read from the LM75), then the read can simply consist of an address byte, followed by retrieving the

corresponding number of data bytes. If the Pointer needs to be set, then an address byte, pointer byte, repeat

start, and another address byte will accomplish a read.

The first data byte is the most significant byte with most significant bit first, permitting only as much data as

necessary to be read to determine temperature condition. For instance, if the first four bits of the temperature

data indicates an overtemperature condition, the host processor could immediately take action to remedy the

excessive temperatures. At the end of a read, the LM75 can accept either Acknowledge or No Acknowledge from

the Master (No Acknowledge is typically used as a signal for the slave that the Master has read its last byte).

10

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