LM74CIMX-3 National Semiconductor, LM74CIMX-3 Datasheet - Page 9

• Manufacturer Part Number: LM74CIMX-3
• Description: IC TEMP SENSOR DGTL 8-SOIC
• Manufacturer: National Semiconductor

Specifications of LM74CIMX-3

• Function: Temp Monitoring System (Sensor)
• Topology: ADC (Sigma Delta), Register Bank
• Sensor Type: Internal
• Sensing Temperature: -55°C ~ 150°C
• Output Type: MICROWIRE™, SPI™
• Output Alarm: No
• Output Fan: No
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -55°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: LM74CIMX-3TR

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
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 pe-
riods. 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:
•
•
•
•
•
•
•
•
•
any others may place the part into a Test Mode. Test Modes
are used by National Semiconductor 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.
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:
•
•
•
•
•
Note that one complete temperature conversion period will
have to pass before the LM74 Temperature register will con-
00 hex
01 hex
03 hex
07 hex
0F hex
1F hex
3F hex
7F hex
FF hex
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.
®
. Communication
9
tain the new temperature data. Until then, it will contain a
"stale" temperature (the data that was in the register before
going into shutdown mode).
1.3 TEMPERATURE DATA FORMAT
Temperature data is represented by a 13-bit, two's comple-
ment word with an LSB (Least Significant Bit) equal to 0.0625°
C:
Note: The last two bits are TRI-STATE and depicted as one
in the table.
The first data byte is the most significant byte with most sig-
nificant bit first, permitting only as much data as necessary to
be read to determine temperature condition. For instance, if
the first four bits of the temperature data indicate an overtem-
perature condition, the host processor could immediately take
action to remedy the excessive temperatures.
1.4 SHUTDOWN MODE/MANUFACTURER'S ID
Shutdown mode is enabled by writing XX FF to the LM74 as
shown in
LM74 is in shutdown. Current draw drops to less than 10 μA
between serial communications. When in shutdown mode the
LM74 always will output 1000 0000 0000 00XX. This is the
manufacturer's/Device ID information. The first 5-bits of the
field (1000 0XXX) are reserved for manufacturer's ID. As
mentioned in Section 1.2, writing a zero to the LM74 config-
uration register will take it out of shutdown mode and place it
in conversion mode. In other words, any valid code listed in
Section 1.2 other than XX FF will put it in conversion mode.
After leaving shutdown, but before the first temperature con-
version is complete, the temperature register will contain the
last measured temperature which resided in the temperature
register before entering shutdown mode. After the completion
of the first conversion, the temperature register will be updat-
ed with the new temperature data.
Temperature
+0.0625°C
−0.0625°C
+150°C
+125°C
+25°C
−25°C
−55°C
0°C
Figure
7c. The serial bus is still active when the
0100 1011 0000 0111
0011 1110 1000 0111
0000 1100 1000 0111
0000 0000 0000 1111
0000 0000 0000 0111
1111 1111 1111 1111
1111 0011 1000 0111
1110 0100 1000 0111
Binary
Digital Output
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0C 87h
FF FFh
4B 07h
3E 87h
00 0Fh
00 07h
F3 87h
E4 87h
Hex