LM74

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SNIS107K – MAY 2000 – REVISED MARCH 2013

The following communication can be used to determine the Manufacturer's/Device ID and then immediately place

the part into continuous conversion mode. With CS continuously low:

•

Read 16 bits of temperature data

•

Write 16 bits of data commanding shutdown

•

Read 16 bits of Manufacture's/Device ID data

•

Write 8 to 16 bits of data commanding Conversion Mode

•

Take CS HIGH.

Note that one complete temperature conversion period will have to pass before the LM74 Temperature register

will contain the new temperature data. Until then, it will contain a "stale" temperature (the data that was in the

register before going into shutdown mode).

Temperature Data Format

Temperature data is represented by a 13-bit, two's complement word with an LSB (Least Significant Bit) equal to

0.0625°C:

Temperature

Digital Output

Binary

Hex

+150°C

0100 1011 0000 0111

4B 07h

+125°C

0011 1110 1000 0111

3E 87h

+25°C

0000 1100 1000 0111

0C 87h

+0.0625°C

0000 0000 0000 1111

00 0Fh

0°C

0000 0000 0000 0111

00 07h

−0.0625°C

1111 1111 1111 1111

FF FFh

−25°C

1111 0011 1000 0111

F3 87h

−55°C

1110 0100 1000 0111

E4 87h

Note: The last two bits are TRI-STATE and depicted as one in the table.

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

excessive temperatures.

Shutdown Mode/Manufacturer's ID

Shutdown mode is enabled by writing XX FF to the LM74 as shown in Figure 16c. The serial bus is still active

when the LM74 is in shutdown. Current draw drops to less than 10

μA between serial communications. When in

shutdown mode the LM74 always will output 1000 0000 0000 00XX. This is the manufacturer's/Device ID

information. The first 5-bits of the field (1000 0XXX) are reserved for manufacturer's ID. As mentioned in Section

1.2, writing a zero to the LM74 configuration register will take it out of shutdown mode and place it in conversion

mode. In other words, any valid code listed in Section 1.2 other than XX FF will put it in conversion mode. After

leaving shutdown, but before the first temperature conversion is complete, the temperature register will contain

the last measured temperature which resided in the temperature register before entering shutdown mode. After

the completion of the first conversion, the temperature register will be updated with the new temperature data.

Internal Register Structure

The LM74 has three registers, the temperature register, the configuration register and the manufacturer's/device

identification register. The temperature and manufacturer's/device identification registers are read only. The

configuration register is write only.

Configuration Register

(Selects shutdown or continuous conversion modes):

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