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LM75AD Datasheet PDF : 18 Pages
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Philips Semiconductors
Digital temperature sensor and thermal Watchdog™
Product data sheet
LM75A
I2C serial interface
The LM75A can be connected to a compatible 2-wire serial interface
I2C-bus as a slave device under the control of a controller or master
device, using two device terminals, SCL and SDA. The controller
must provide the SCL clock signal and write/read data to/from the
device through the SDA terminal. Notice that if the I2C common
pull-up resistors have not been installed as required for I2C-bus,
then an external pull-up resistor, about 10 kΩ, is needed for each of
these two terminals. The bus communication protocols are
described in the data communication section.
Slave address
The LM75A slave address on the I2C-bus is partially defined by the
logic applied to the device address pins A2, A1 and A0. Each of
them is typically connected either to GND for logic 0, or to VCC for
logic 1. These pins represent the three LSB bits of the device 7-bit
address. The other four MSB bits of the address data are preset to
‘1001’ by hard wiring inside the LM75A. Table 1 shows the device’s
complete address and indicates that up to 8 devices can be
connected to the same bus without address conflict. Because the
input pins, SCL, SDA, A2–A0, are not internally biased, it is
important that they should not be left floating in any application.
Table 1. Address table
1 = HIGH, 0 = LOW
MSB
1
0
0
1
LSB
A2
A1
A0
Register list
The LM75A contains four data registers beside the pointer register
as listed in Table 2. The pointer value, read/write capability and
default content at power up of the registers are also shown in the
Register table.
Table 2. Register table
Register name Pointer value
Conf
01H
Temp
00H
Tos
03H
Thyst
02H
R/W
POR state
R/W
00H
Read only N/A
R/W
50 00H
R/W
4B 00H
Description
Configuration Register.
Contains a single 8-bit data byte. To set the device operating condition.
Default = 0.
Temperature Register.
Contains two 8-bit data bytes. To store the measured Temp data.
Over-temp Shutdown threshold Register.
Contains two 8-bit data bytes. To store the over-temp shut-down Tos limit.
Default = 80 °C.
Hysteresis Register.
Contains two 8-bit data bytes. To store the hysteresis Thyst limit.
Default = 75 °C.
Pointer register
The pointer register contains an 8-bit data byte of which the two
LSB bits represent the pointer value of the other four registers, and
the other 6 MSB bits are equal to 0, as shown in the Pointer register
table (Table 3) and the Pointer value table (Table 4). The pointer
register is not accessible to the user, but is used to select the data
register for write/read operation by including the pointer data byte in
the bus command.
Table 3. Pointer register table
B7 B6 B5 B4 B3
0
0
0
0
0
B2 B1 B0
0 Pointer value
Table 4. Pointer value
B1 B0 Selected register
0
0
Temperature register (Temp)
0
1
Configuration register (Conf)
1
0
Hysteresis register (Thyst)
1
1
Overtemp shut-down register (Tos)
Because the Pointer value is latched into the Pointer register when
the bus command, which includes the pointer byte, is executed, a
read from the LM75A may or may not include the pointer byte in the
statement. To read again a register, which has been recently read
and the pointer has been preset, the pointer byte does not have to
be included. To read a register, which is different with the one that
has been recently read, the pointer byte must be included. However,
a write to the LM75A must always include the pointer byte in the
statement. The bus communication protocols are described in detail
in the data communication section.
At power-up, the Pointer value is equal to 0 and the Temp register is
selected; users can then read the Temp data without specifying the
pointer byte.
2004 Oct 05
8
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