Rainbow Electronics LM70 - Manual (Page 9)

1.0 Functional Description
The LM70 temperature sensor incorporates a band-gap type
temperature sensor and 10-bit plus sign
∆Σ ADC (Delta-
Sigma Analog-to-Digital Converter). Compatibility of the
LM70’s three wire serial interface with SPI and MICROWIRE
allows simple communications with common microcontrol-
lers and processors. Shutdown mode can be used to opti-
mize current drain for different applications. A manufacture’s
ID register identifies the LM70 as National Semiconductor
product.
1.1 POWER UP AND POWER DOWN
The LM70 always powers up in a known state. The power up
default condition is continuous conversion mode. Immediatly
after power up the LM70 will output an erroneous code until
the first temperature conversion has completed.
When the supply voltage is less than about 1.6V (typical),
the LM70 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 LM70 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 TRISTATE ® . 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, ten temperature bits and 3 high bits) 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 LM70 will complete the con-
version 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 LM70 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 LM70 out of shutdown. The following
codes only should be transmitted to the LM70:
• 00 hex (normal operation)
• 01 hex (normal operation)
• 03 hex (normal operation)
• 07 hex (normal operation)
• 0F hex (normal operation)
• 1F hex (normal operation)
• 3F hex(normal operation)
• 7F hex(normal operation)
• FF hex (Shutdown, transmit manufacturer’s ID)
.
any others may place the part into a Test Mode. Test Modes
are used by National Semiconductor to thoroughly test the
function of the LM70 during production testing. Only eight
bits have been defined above since only the last eight trans-
mitted, before CS is taken HIGH, are detected by the LM70
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 250 ms will have to pass for a conversion to
complete before the LM70 actually transmits temperature
data.
1.3 TEMPERATURE DATA FORMAT
Temperature data is represented by a 11-bit, two’s comple-
ment word with an LSB (Least Significant Bit) equal to
0.25˚C:
Temperature Digital Output
Binary Hex
+150˚C 0100 1011 0001 1111 4B 1Fh
+125˚C 0011 1110 1001 1111 3E 9Fh
+25˚C 0000 1100 1001 1111 0B 9Fh
+0.25˚C 0000 0000 0011 1111 00 3Fh
0˚C 0000 0000 0001 1111 00 1Fh
−0.25˚C 1111 1111 1111 1111 FF FFh
−25˚C 1111 0011 1001 1111 F3 9Fh
−55˚C 1110 0100 1001 1111 E4 9Fh
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 LM70 as
shown in Figure 7c and discussed in Section 1.2. The serial
bus is still active when the LM70 is in shutdown. Current
draw drops to less than 10 µA between serial communica-
tions. When in shutdown mode the LM70 always will output
1000 0001 0000 00XX. This is the manufacturer’s ID/Device
ID information. The first 5-bits of the field (1000 0XXX) are
reserved for manufacturer’s ID.
LM70
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