lm74a-5-mda National Semiconductor Corporation, lm74a-5-mda Datasheet - Page 8

• Manufacturer Part Number: lm74a-5-mda
• Description: Spi/microwire 12-bit Plus Sign Temperature Sensor Lm74 In Die Form
• Manufacturer: National Semiconductor Corporation

www.national.com
1.0 Functional Description
The LM74A temperature sensor incorporates a band-gap
type temperature sensor and 12-bit plus sign ∆Σ 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 microcontrol-
lers and processors. Shutdown mode can be used to opti-
mize current drain for different applications. A Manufacture’s/
Device ID register identifies the LM74A as National
Semiconductor product.
1.1 POWER UP AND POWER DOWN
The LM74A always powers up in a known state. The power
up default condition is continuous conversion mode. Imme-
diatly after power up the LM74A will output an erroneous
code until the first temperature conversion has completed.
When the supply voltage is less than about 1.6V (typical),
the LM74A is considered powered down. As the supply
voltage 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 LM74A 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
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 LM74A 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 LM74A
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 LM74A out of shutdown.
The following codes should only be transmitted to the
LM74A:
• 00 hex
• 01 hex
• 03 hex
• 07 hex
• 0F hex
• 1F hex
®
. Communica-
8
• 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 LM74A during production testing. Only eight
bits have been defined above since only the last eight trans-
mitted are detected by the LM74A, 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 250 ms will have to pass for a conversion to
complete before the LM74A actually transmits temperature
data.
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
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 LM74A
as shown in Figure 7c. The serial bus is still active when the
LM74A is in shutdown. Current draw drops to less than 10
µA between serial communications. When in shutdown
mode the LM74A always will output 1000 0000 0000 0XX.
This is the manufacturer’s/Device ID information. The first
5-bits of the field (1000 0XXX) are reserved for manufactur-
er’s ID.
Temperature
+0.0625˚C
−0.0625˚C
+150˚C
+125˚C
+25˚C
−25˚C
−55˚C
0˚C
0100 1011 0000 0111
0000 1100 1000 0111
0000 0000 0000 1111
0000 0000 0000 0111
0011 1110 1000 0111
1110 0100 1000 0111
1111 0011 1000 0111
1111 1111 1111 1111
Binary
Digital Output
FF FFh
4B 07h
3E 87h
0B 87h
00 0Fh
00 07h
F3 87h
E4 87h
Hex