SNIS107K – MAY 2000 – REVISED MARCH 2013
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
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.
10
Product Folder Links:
Copyright © 2000–2013, Texas Instruments Incorporated
TI [ TEXAS INSTRUMENTS ]
