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LM71, LM71-Q1

SNIS125E – MARCH 2004 – REVISED AUGUST 2018

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Copyright © 2004–2018, Texas Instruments Incorporated

9 Functional Description

The LM71 temperature sensor incorporates a temperature sensor and 13-bit plus sign ΔΣ 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 processors. Shutdown mode can be used to

optimize current drain for different applications. A Manufacture's/Device ID register identifies the LM71 as TI

product.

9.1 POWER UP AND POWER DOWN

The LM71 always powers up in a known state. The power up default condition is continuous conversion mode.

Immediately 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

voltage rises above the nominal 1.6V power up threshold, the internal registers are reset to the power up default

state described above.

9.2 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 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.

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 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

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 transmitted 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 TI 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.