LM74
1.0 Functional Description
(Continued)
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
®
. Communica-
tion 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 transmit-
ted 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
Note that one complete temperature conversion period will
have to pass before the LM74 Temperature register will
contain 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:
Temperature
+150˚C
+125˚C
+25˚C
+0.0625˚C
0˚C
−0.0625˚C
−25˚C
−55˚C
Digital Output
Binary
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
Hex
4B 07h
3E 87h
0C 87h
00 0Fh
00 07h
FF FFh
F3 87h
E4 87h
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
significant bit first, permitting only as much data as neces-
sary to be read to determine temperature condition. For
instance, if the first four bits of the temperature data indicate
an overtemperature condition, the host processor could im-
mediately take action to remedy the excessive tempera-
tures.
1.4 SHUTDOWN MODE/MANUFACTURER’S ID
Shutdown mode is enabled by writing XX FF to the LM74 as
shown in
The serial bus is still active when the
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 con-
figuration 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 tempera-
ture conversion 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 reg-
ister will be updated with the new temperature data.
•
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 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 trans-
mitted 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:
•
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.
9
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