available in two 8-bit registers:

Temperature

Digital Output

Binary

Hex

+125˚C

0111 1101 0000 0000

7D00h

+25˚C

0001 1001 0000 0000

1900h

+1˚C

0000 0001 0000 0000

0100h

+0.125˚C

0000 0000 0010 0000

0020h

0˚C

0000 0000 0000 0000

0000h

−0.125˚C

1111 1111 1110 0000

FFE0h

−1˚C

1111 1111 0000 0000

FF00h

−25˚C

1110 0111 0000 0000

E700h

−55˚C

1100 1001 0000 0000

C900h

Local 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, ALERT 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

provided by some external source, typically a pull-up resis-

tor. Choice of resistor value depends on many system fac-

tors 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 LM89. The maximum

resistance of the pull-up to provide a 2.1V high level, based

on LM89 specification for High Level Output Current with the

supply voltage at 3.0V, is 82k

Ω(5%) or 88.7kΩ(1%).

1.8 DIODE FAULT DETECTION

The LM89 is equipped with operational circuitry designed to

detect fault conditions concerning the remote diode. In the

event that the D+ pin is detected as shorted to V

floating, the Remote Temperature High Byte (RTHB) register

is loaded with +127˚C, the Remote Temperature Low Byte

(RTLB) register is loaded with 0, and the OPEN bit (D2) in

the status register is set. As a result, if the Remote T_CRIT

setpoint register (RCS) is set to a value less than +127˚C the

ALERT and T_Crit output pins will be pulled low, if the Alert

Mask and T_Crit Mask are disabled. If the Remote HIGH

Setpoint High Byte Register (RHSHB) is set to a value less

than +127˚C then ALERT will be pulled low, if the Alert Mask

is disabled. The OPEN bit itself will not trigger and ALERT.

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 previ-

ously 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.

1.9 COMMUNICATING WITH THE LM89

The data registers in the LM89 are selected by the Com-

mand 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 cor-

rect (most of the time it is expected that the Command

Register will point to one of the Read Temperature Reg-

isters 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 31.25ms to measure the tem-

perature of the remote diode and internal diode. When re-

trieving all 10 bits from a previous remote diode temperature

measurement, the master must insure that all 10 bits are

from the same temperature conversion. This may be

achieved by using one-shot mode or by setting the conver-

sion rate and monitoring the busy bit such that no conversion

occurs in between reading the MSB and LSB of the last

temperature conversion.

www.national.com

10