Detailed Description
The LM75 temperature sensor measures temperature
and converts the data into digital form using a band-
gap type temperature sensor and a 9-bit delta-sigma
ADC. An I
2
C-compatible 2-wire serial interface allows
access to conversion results. The LM75 accepts stan-
dard I
2
C commands to read the data, set the overtem-
perature alarm (OS) trip thresholds, and configure other
characteristics. While reading the Temperature register,
any changes in temperature are ignored until the read
is completed. The Temperature register is updated for
the new temperature measurement upon completion of
the read operation.
OS Output, T
OS,
and T
HYST
Limits
In comparator mode (see Figure 1), the open-drain OS
output asserts when the temperature rises above the
limit programmed into the T
OS
register, and becomes
high impedance when the temperature falls below the
limit set in the T
HYST
register. In this mode the LM75
operates as a thermostat, and the OS output can be
used to take action to reduce the temperature (e.g.,
turn on a cooling fan, reduce clock speed, or shut
down the system).
In interrupt mode, exceeding T
OS
also asserts OS. OS
remains asserted until a read operation is performed on
any of the registers. Once OS has asserted due to
crossing above T
OS
and is then reset, it is asserted
again only when the temperature drops below T
HYST
.
The output then remains asserted until it is reset by a
read. It is then asserted again if the temperature rises
above T
OS
, and so on. Putting the LM75 into shutdown
mode also resets OS.
Power-Up and Power-Down
The LM75 powers up to a known state, as indicated in
Table 2. Some of these settings are summarized as fol-
lowing:
" Comparator mode
" T
OS
= +80癈
" T
HYST
= +75癈
" OS active low
" Command byte pointer = 0x00
I
2
C-Compatible Bus Interface
From a software perspective, the LM75 appears as a set
of byte-wide registers that contain temperature data,
alarm threshold values, and control bits. A standard I
2
C-
compatible, 2-wire serial interface reads temperature
data and writes control bits and alarm threshold data.
Each device responds to its own I
2
C slave address,
which is selected using A0, A1, and A2. See Table 1.
Digital Temperature Sensor and Thermal
Watchdog with 2-Wire Interface
_______________________________________________________________________________________ 5
T
OS
T
HYST
OS OUTPUT
(COMPARATOR MODE)
OS SET ACTIVE LOW
OS OUTPUT
(INTERRUPT MODE)
OS SET ACTIVE LOW
READ
OPERATION
READ
OPERATION
READ
OPERATION
TEMPERATURE
Figure 1. OS Output Temperature Response Diagram
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
1
0
0
1
A2
A1
A0
R/W
Table 1. Slave Address
REGISTER NAME
ADDRESS
(hex)
POR STATE
(hex)
POR STATE (binary)
POR STATE (?/SPAN>C)
READ/
WRITE
Temperature
00
000X
0000 0000 0XXX XXXX
Read only
Configuration
01
00
0000 0000
R/W
T
HYST
02
4B0X
0100 1011 0XXX XXXX
75
R/W
T
OS
03
500X
0101 0000 0XXX XXXX
80
R/W
Table 2. Register Functions
X = Dont care.
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