Typical Performance Characteristics

Static Supply Current vs. Temperature

Temperature Error

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1.0 Functional Description

The LM71 temperature sensor incorporates a temperature

sensor and 13-bit plus sign

∆Σ ADC (Delta-Sigma Analog-to-

Digital Converter). Compatibility of the LM71’s three wire

serial interface with SPI and MICROWIRE allows simple

communications with common microcontrollers and proces-

sors. Shutdown mode can be used to optimize current drain

for different applications. A Manufacture’s/Device ID register

identifies the LM71 as National Semiconductor product.

1.1 POWER UP AND POWER DOWN

The LM71 always powers up in a known state. The power up

default condition is continuous conversion mode. Immedi-

ately after power up the LM71 will output an erroneous code

until the first temperature conversion has completed.

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

the LM71 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 LM71 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.

When CS is high SI/O will be in TRI-STATE. Communication

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. CS can be taken high at any

time during the transmit phase. If CS is brought low in the

middle of a conversion the LM71 will complete the conver-

sion 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 LM71 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. All zeros

must be shifted in order to place the LM71 into continuous

conversion mode. Only the following codes should be trans-

mitted to the LM71:

•

00 hex for continuous conversion

•

FF hex for shutdown

Another code may place the part into a test mode. Test

modes are used by National Semiconductor to thoroughly

test the function of the LM71 during production testing. Only

eight bits have been defined above since only the last eight

transmitted are detected by the LM71, 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 300 ms will have to pass for a conversion to

complete before the LM71 actually transmits temperature

data.

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