FM75 Fairchild Semiconductor, FM75 Datasheet - Page 5

• Manufacturer Part Number: FM75
• Description: Low Voltage 2-Wire Digital Temperature Sensor with Thermal Alarm
• Manufacturer: Fairchild Semiconductor

Basic Operation
The FM75 temperature sensing circuitry continuously
produces an analog voltage that is proportional to the device
temperature. At regular intervals the FM75 converts the
analog voltage to a two’s complement digital value, which is
placed into the temperature register.
The FM75 has an SMBus compatible digital serial interface
which allows the user to access the data in the temperature
register at any time. In addition, the serial interface gives the
user easy access to all other FM75 registers to customize
operation of the device.
The FM75 temperature-to-digital conversion can have 9,
10, 11, or 12-bit resolution as selected by the user, providing
0.5°C, 0.25°C, 0.125°C, and 0.0625°C temperature
resolution, respectively. At power-up the default conversion
resolution is 9-bits. The conversion resolution is controlled
by the R0 and R1 bits in the Configuration Register.
Table 1 gives examples of the relationship between the
output digital data and the external temperature. The 9-bit,
10-bit, 11-bit and 12-bit columns in Table 1 indicate the
right-most bit in the output data stream that can contain
temperature information for each conversion accuracy.
Since the output digital data is in two’s-complement format,
the most significant bit of the temperature is the “sign” bit.
If the sign bit is a zero, the temperature is positive and if the
sign bit is a one, the temperature is negative.
The FM75 has a Shutdown Mode that reduces the operating
current of the FM75 to 150nA. This mode is controlled by
the SD bit in the configuration register.
Power Up Default Conditions
The FM75 always powers up in the following default state:
• Thermostat mode: Comparator Mode
• O.S. polarity: active low
• Fault tolerance: 1 fault (i.e., F0 = 0 and F1 = 0 in the
• T
• T
• Register pointer: 00 (Temperature Register)
• Conversion resolution: 9 bits (i.e., R0 = 0 and R1 = 0 in
After power up these conditions can be reprogrammed via
the serial interface. Refer to the Serial Data Bus Operation
section for FM75 programming instructions.
Thermal Alarm Function
The FM75 thermal alarm function provides user program-
mable thermostat capability and allows the FM75 to function
as a stand alone thermostat without using the serial interface.
The Over-Limit Signal (O.S.) output is the alarm output. This
signal is an open drain output, and at power-up this pin is
configured with active-low polarity.
PRODUCT SPECIFICATION
REV. 1.0.4 10/3/02
Configuration Register)
the Configuration Register)
OS
HYST
= 80°C
= 75°C
Table 1. Relationship Between Temperature and
Digital Output
The O.S. polarity is controlled by the POL bit in the
Configuration Register. The user-programmable upper
trip-point temperature for the thermal alarm is stored in
the T
perature (i.e., the lower trip point) is stored in the T
Register.
The thermal alarm has two modes of operation: Comparator
Mode and Interrupt Mode. At power-up the default is
Comparator Mode. The alarm mode is controlled by the
CMP/INTR bit in the Configuration Register.
Fault Tolerance
For both Comparator and Interrupt modes, the alarm “fault
tolerance” setting plays a role in determining when the O.S.
output will be activated. Fault tolerance refers to the number
of consecutive times an error condition must be detected
before the user is notified. Higher fault tolerance settings can
help eliminate false alarms caused by noise in the system.
The alarm fault tolerance is controlled by bits F0 and F1 in
the Configuration Register. These bits can be used to set
the fault tolerance to 1, 2, 4 or 6 as shown in Table 4. At
power-up, these bits both default to 0 (fault tolerance = 1).
+
+
+
+
° 0
2 -
3 -
4 -
5 -
Temperature
T
1
1
5
1
e
C
. 0
3
5
° 5
2
0
0
2
m
2 .
0 .
° 5
0
1 .
2 .
° 5
OS
C
p
0 .
° 5
6
2
° 5
C
C
e
A
2
6
° 5
C
a r
° 5
l l
C
Register, and the user-programmable hysteresis tem-
2
C
° 5
u t
C
e r
C
s
S
0
0
0
0
0
1
1
1
1
g i
9
B -
10
0
0
c
1
0
1
1
N
1
1
1
e r
t i
b
0
0
o
1
1
1
0
1
0
0
B -
u
11
s t i
0
0
n
R
0
1
0
0
s
1
1
1
m
v
l o
t i
e
b
B -
1
e
b
u
s
y
t u
2
R
s r
0
0
1
0
1
0
r e
1
1
1
s
l o
t i
B -
o i
1
0
0
0
e
e
1
1
1
0
0
o i
t u
R
0
1
0
1
d
s
0
1
f o
0
1
0
n
t i
n
l o
0
0
0
0
0
0
e
o i
1
1
1
Digital Output
R
s
t u
n
l o
e
o i
b
s
t u
9
0
0
0
0
0
0
1
1
1
t i
l o
n
o i
t u
n
o i
b
1
0
0
0
0
0
0
0
1
1
1
t i
0
n
b
1
0
0
0
0
0
0
0
0
0
1
1
t i
1
b
1
0
0
0
0
0
0
0
0
0
0
1
1
t i
2
HYST
A
0
0
0
0
0
0
0
0
0
0
0
0
0
w l
z
0
0
0
0
0
0
0
0
0
0
0
0
0
e
0
0
0
0
0
0
0
0
0
0
0
0
0
a
o r
FM75
0
0
0
0
0
0
0
0
0
0
0
0
0
s y
5