lm71cisdx National Semiconductor Corporation, lm71cisdx Datasheet - Page 8

• Manufacturer Part Number: lm71cisdx
• Description: Spi/microwire 13-bit Plus Sign Temperature Sensor
• Manufacturer: National Semiconductor Corporation

www.national.com
Typical Performance Characteristics
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
Static Supply Current vs. Temperature
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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.
Temperature Error
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