Temperature Data Format
Temperature data is stored in the Temperature, T
OS
Set
Point, and T
HYST
Set Point registers. The temperature
data format is 9 bits, twos complement, and the register
is read out in 2 bytes: an upper byte and a lower byte.
Bits D15D7 contain the temperature data, with the LSB
representing 0.5癈 and the MSB representing the sign
bit (see Table 3). The MSB is transmitted first. The last 7
bits of the lower byte, bits D6D0, are dont cares.
Shutdown
Set bit D0 in the Configuration register to 1 to place the
LM75 in shutdown mode and reduce supply current to
4礎. In interrupt mode, entering shutdown resets the
OS output. While in shutdown, the I
2
C remains active
and T
OS
and T
HYST
limit registers along with the
Configuration register remain accessible to the master.
Fault Queue
The fault queue prevents OS false tripping in noisy
environments. The number of faults set in the queue (up
to 6) must occur to trip the OS output.
Comparator/Interrupt Mode
The events that trigger OS are identical between com-
parator and interrupt modes. In comparator mode, OS
is asserted when the temperature rises above the T
OS
value. OS is deasserted when the temperature drops
below the T
HYST
value. In interrupt mode, OS is assert-
ed when the temperature rises above the T
OS
value or
falls below the T
HYST
value. OS is deasserted only after
performing a read operation.
OS Output
The OS output is an open-drain output without an inter-
nal pullup. Connect a pullup resistor from OS to +V
S
.
Using larger resistance values reduces any tempera-
ture errors due to self heating from current entering OS.
OS Polarity
The OS polarity can be programmed for active-low or
active-high operation. In active-low operation, OS goes
low when triggered by a temperature event.
Internal Registers
The LM75s Pointer register selects between four data
registers (see Figure 6). At power-up, the pointer is set
to read the Temperature register at address 0x00. The
Pointer register latches the last location to which it was
set. All registers are read and write, except the
Temperature register, which is read only.
Write to the Configuration register by writing an address
byte, a data pointer byte, and a data byte. If 2 data
bytes are written, the second data byte overrides the
first. The T
OS
and T
HYST
registers require 1 address
byte, 1 pointer byte, and 2 data bytes. If only 1 data
byte is written, it is saved in bits D15D8 of the respec-
tive register. If more than 2 data bytes are written, only
the first 2 bytes are recognized while the remaining
bytes are ignored.
Read from the LM75 in one of two ways. If the location
latched in the Pointer register is set from the previous
read, the new read consists of an address byte, fol-
lowed by retrieving the corresponding number of data
bytes. If the Pointer register needs to be set to a new
address, perform a read operation by writing an
address byte, pointer byte, repeat start, and another
address byte.
An inadvertent 8-bit read from a 16-bit register, with the
D7 bit low, can cause the device to stop in a state
where the SDA line is held low. Ordinarily, this would
prevent any further bus communication until the master
sends nine additional clock cycles or SDA goes high.
At that time, a stop condition resets the device. If the
additional clock cycles are not generated by the mas-
ter, the LM75 bus resets and unlocks after the bus time-
out period has elapsed.
Digital Temperature Sensor and Thermal
Watchdog with 2-Wire Interface
_______________________________________________________________________________________ 9
SMBus is a trademark of Intel Corp.
Figure 6. Block Diagram
LM75
+V
S
A2/RESET
A1
A0
SDA
SCL
OS
SMBus"
INTERFACE
BLOCK
POINTER REGISTER
(SELECTS REGISTER
FOR COMMUNICATION)
DATA
ADDRESS
REGISTER SELECT
GND
TEMPERATURE
(READ ONLY)
POINTER = 0000 0000
T
OS
SET POINT
(READ/WRITE)
POINTER = 0000 0011
T
HYST
SET POINT
(READ/WRITE)
POINTER = 0000 0010
CONFIGURATION
(READ/WRITE)
POINTER = 0000 0001
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