LM89

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SNIS128D – AUGUST 2002 – REVISED JUNE 2014

In the event that the D+ pin is shorted to ground or D

−, the Remote Temperature High Byte (RTHB) register is

loaded with

−128°C (1000 0000) and the OPEN bit (D2) in the status register will not be set. Since operating the

LM89 at

−128°C is beyond it's operational limits, this temperature reading represents this shorted fault condition.

If the value in the Remote Low Setpoint High Byte Register (RLSHB) is more than

−128°C and the Alert Mask is

disabled, ALERT will be pulled low.

Remote diode temperature sensors that have been previously released and are competitive with the LM89 output

a code of 0°C if the external diode is short-circuited. This change is an improvement that allows a reading of 0°C

to be truly interpreted as a genuine 0°C reading and not a fault condition.

9.3.8 Communicating With The LM89

The data registers in the LM89 are selected by the Command Register. At power-up the Command Register is

set to “00”, the location for the Read Local Temperature Register. The Command Register latches the last

location it was set to. Each data register in the LM89 falls into one of four types of user accessibility:

1. Read only

2. Write only

3. Read/Write same address

4. Read/Write different address

A Write to the LM89 will always include the address byte and the command byte. A write to any register requires

one data byte.

Reading the LM89 can take place either of two ways:

1. If the location latched in the Command Register is correct (most of the time it is expected that the Command

Register will point to one of the Read Temperature Registers because that will be the data most frequently

read from the LM89), then the read can simply consist of an address byte, followed by retrieving the data

byte.

2. If the Command Register needs to be set, then an address byte, command byte, repeat start, and another

address byte will accomplish a read.

The data byte has the most significant bit first. At the end of a read, the LM89 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). It takes the LM89 31.25 ms to measure the temperature of the remote diode and internal

diode. When retrieving all 11 bits from a previous remote diode temperature measurement, the master must

insure that all 11 bits are from the same temperature conversion. This may be achieved by reading the MSB

(most significant byte first) followed by the LSB (least significant byte). Reading the MSB first will lock the LSB,

thus synchronizing the two bytes. One-shot mode can also be used without any restrictions on the MSB and LSB

reading sequence.

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