1.9 COMMUNICATING WITH THE LM99

The data registers in the LM99 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 LM99 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 LM99 will always include the address byte and

the command byte. A write to any register requires one data

byte.

Reading the LM99 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 LM99), 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 LM99 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 LM99 31.25 ms to measure the

temperature of the remote diode and internal diode. When

retrieving all 10 bits from a previous remote diode tempera-

ture 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.

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