1.0 Functional Description

The LM74A temperature sensor incorporates a band-gap

type temperature sensor and 12-bit plus sign

∆Σ ADC (Delta-

Sigma Analog-to-Digital Converter). Compatibility of the

LM74’s three wire serial interface with SPI and MICROWIRE

allows simple communications with common microcontrol-

lers and processors. Shutdown mode can be used to opti-

mize current drain for different applications. A Manufacture’s/

Device ID register identifies the LM74A as National

Semiconductor product.

1.1 POWER UP AND POWER DOWN

The LM74A always powers up in a known state. The power

up default condition is continuous conversion mode. Imme-

diatly after power up the LM74A will output an erroneous

code until the first temperature conversion has completed.

When the supply voltage is less than about 1.6V (typical),

the LM74A is considered powered down. As the supply

voltage rises above the nominal 1.6V power up threshold,

the internal registers are reset to the power up default state

described above.

1.2 SERIAL BUS INTERFACE

The LM74A operates as a slave and is compatible with SPI

or MICROWIRE bus specifications. Data is clocked out on

the falling edge of the serial clock (SC), while data is clocked

in on the rising edge of SC. A complete transmit/receive

communication will consist of 32 serial clocks. The first 16

clocks comprise the transmit phase of communication, while

the second 16 clocks are the receive phase.

tion should be initiated by taking chip select (CS) low. This

should not be done when SC is changing from a low to high

state. Once CS is low the serial I/O pin (SI/O) will transmit

the first bit of data. The master can then read this bit with the

rising edge of SC. The remainder of the data will be clocked

out by the falling edge of SC. Once the 14 bits of data (one

sign bit, twelve temperature bits and 1 high bit) are transmit-

ted the SI/O line will go into TRI-STATE. CS can be taken

high at any time during the transmit phase. If CS is brought

low in the middle of a conversion the LM74A will complete

the conversion and the output shift register will be updated

after CS is brought back high.

The receive phase of a communication starts after 16 SC

periods. CS can remain low for 32 SC cycles. The LM74A

will read the data available on the SI/O line on the rising

edge of the serial clock. Input data is to an 8-bit shift register.

The part will detect the last eight bits shifted into the register.

The receive phase can last up to 16 SC periods. All ones

must be shifted in order to place the part into shutdown. A

zero in any location will take the LM74A out of shutdown.

The following codes should only be transmitted to the

LM74A:

•

00 hex

•

01 hex

•

03 hex

•

07 hex

•

0F hex

•

1F hex

•

3F hex

•

7F hex

•

FF hex

any others may place the part into a Test Mode. Test Modes

are used by National Semiconductor to thoroughly test the

function of the LM74A during production testing. Only eight

bits have been defined above since only the last eight trans-

mitted are detected by the LM74A, before CS is taken HIGH.

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 250 ms will have to pass for a conversion to

complete before the LM74A actually transmits temperature

data.

1.3 TEMPERATURE DATA FORMAT

Temperature data is represented by a 13-bit, two’s comple-

ment 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

0B 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 neces-

sary 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 im-

mediately take action to remedy the excessive tempera-

tures.

1.4 SHUTDOWN MODE/MANUFACTURER’S ID

Shutdown mode is enabled by writing XX FF to the LM74A

as shown in Figure 7c. The serial bus is still active when the

LM74A is in shutdown. Current draw drops to less than 10

µA between serial communications. When in shutdown

mode the LM74A always will output 1000 0000 0000 0XX.

This is the manufacturer’s/Device ID information. The first

5-bits of the field (1000 0XXX) are reserved for manufactur-

er’s ID.

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