1.0 Functional Description

The LM70 temperature sensor incorporates a band-gap type

temperature sensor and 10-bit plus sign

∆Σ ADC (Delta-

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

LM70’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

ID register identifies the LM70 as National Semiconductor

product.

1.1 POWER UP AND POWER DOWN

The LM70 always powers up in a known state. The power up

default condition is continuous conversion mode. Immediatly

after power up the LM70 will output an erroneous code until

the first temperature conversion has completed.

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

the LM70 is considered powered down. As the supply volt-

age 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 LM70 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 com-

munication 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.

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, ten temperature bits and 3 high bits) are transmitted

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 LM70 will complete the con-

version 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 LM70 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 LM70 out of shutdown. The following

codes only should be transmitted to the LM70:

•

00 hex (normal operation)

•

01 hex (normal operation)

•

03 hex (normal operation)

•

07 hex (normal operation)

•

0F hex (normal operation)

•

1F hex (normal operation)

•

3F hex(normal operation)

•

7F hex(normal operation)

•

FF hex (Shutdown, transmit manufacturer’s ID)

.

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

are used by National Semiconductor to thoroughly test the

function of the LM70 during production testing. Only eight

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

mitted, before CS is taken HIGH, are detected by the LM70

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 LM70 actually transmits temperature

data.

1.3 TEMPERATURE DATA FORMAT

Temperature data is represented by a 11-bit, two’s comple-

ment word with an LSB (Least Significant Bit) equal to

0.25˚C:

Temperature

Digital Output

Binary

Hex

+150˚C

0100 1011 0001 1111

4B 1Fh

+125˚C

0011 1110 1001 1111

3E 9Fh

+25˚C

0000 1100 1001 1111

0B 9Fh

+0.25˚C

0000 0000 0011 1111

00 3Fh

0˚C

0000 0000 0001 1111

00 1Fh

−0.25˚C

1111 1111 1111 1111

FF FFh

−25˚C

1111 0011 1001 1111

F3 9Fh

−55˚C

1110 0100 1001 1111

E4 9Fh

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 LM70 as

shown in Figure 7c and discussed in Section 1.2. The serial

bus is still active when the LM70 is in shutdown. Current

draw drops to less than 10 µA between serial communica-

tions. When in shutdown mode the LM70 always will output

1000 0001 0000 00XX. This is the manufacturer’s ID/Device

ID information. The first 5-bits of the field (1000 0XXX) are

reserved for manufacturer’s ID.

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