LM74

SNIS107K – MAY 2000 – REVISED MARCH 2013

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FUNCTIONAL DESCRIPTION

The LM74 temperature sensor incorporates a band-gap type temperature sensor and 12-bit plus sign

ΔΣ 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 microcontrollers and processors. Shutdown mode can

be used to optimize current drain for different applications. A Manufacture's/Device ID register identifies the

LM74 as Texas Instruments product.

Power Up and Power Down

When the supply voltage is less than about 1.6V (typical), the LM74 is considered powered down. The LM74

always powers up in a known state. When the supply voltage rises above 1.6V (typical), an internal Power-On

Reset (POR) occurs and the temperature register will then contain a value of 1111 1111 0000 00XX, where XX

indicates undefined values. See Temperature Register (after power-up, before first complete temperature

conversion) diagram for contents after POR but before completion of the first temperature conversion.

The LM74 power-up default condition is continuous conversion mode. After completion of the first full

temperature conversion, the register will contain temperature measurement data in bits D15 (the temperature

data MSB) through D3 (the temperature data LSB). Bit D2 will be fixed high; bits D1 and D0 are undefined. See

Section 1.5.3 for a diagram of the Temperature Regisiter contents after the first complete temperature

conversion. Note that bit D2 represents a complete conversion flag. During POR it is low and, after the first

temperature conversion is complete, it goes high. This bit can be polled to indicate when the POR data in the

Temperature Register has been replaced with valid temperature data.

After the first conversion, and any subsequent conversions, the value in the temperature register does not

change until the completion of the next conversion, at which time the temperature register is updated with the

latest temperature value.

Serial Bus Interface

The LM74 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. Once the 14 bits of data (one sign bit, twelve temperature

bits and 1 high bit) 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 LM74 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

LM74 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

LM74 out of shutdown. The following codes should only be transmitted to the LM74:

•

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 Texas Instruments to thoroughly test

the function of the LM74 during production testing. Only eight bits have been defined above since only the last

eight transmitted are detected by the LM74, before CS is taken HIGH.

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