*The power-on-reset value of the temperature register is +85°C.
the die temperature, but the sensor’s package does not
conduct heat as well as the leads, so board temperature
has the greatest influence on the measured temperature.
The device’s TO-92 package allows the sensor die to be
positioned above the board. The leads still conduct some
heat from the board, but because there is significant lead
area in contact with air, their temperature is also strongly
affectedbyairtemperature.Followtheguidelinesbelowto
getthebestresultswhenmeasuringambienttemperature:
• If air is moving (e.g., due to cooling fans), place the
sensor in the path of the air stream. This causes the
ambient temperature to influence the sensor tempera-
ture more strongly.
• If the board contains components that will heat it,
mount the sensor as far as possible from those com-
ponents. This makes the temperature in the vicinity of
the sensor closer to the temperature of the ambient air.
• PCB traces and ground planes conduct heat from
other components to the sensor. As much as practical,
avoid copper in the vicinity of the sensor.
The device’s core functionality is its direct-to-digital tem-
perature sensor. The resolution of the temperature sensor
is user-configurable to 9, 10, 11, or 12 bits, corresponding
to increments of 0.5°C, 0.25°C, 0.125°C, and 0.0625°C,
respectively. The default resolution at power-up is 12
bits. The device powers up in a low-power idle state. To
initiate a temperature measurement andA-to-D conver-
sion, the master must issue a Convert T [44h] command.
Following the conversion, the resulting thermal data is
stored in the 2-byte temperature register in the scratch-
pad memory and the device returns to its idle state. If the
device is powered by an external supply, the master can
issue read time slots (see the 1-Wire Bus System section)
after the Convert T command, and the device responds
by transmitting 0 while the temperature conversion is in
progress and 1 when the conversion is done. If the device
is powered with parasite power, this notification technique
cannot be used since the bus must be pulled high by a
strong pullup during the entire temperature conversion.
The bus requirements for parasite power are explained in
detail in the Powering the MAX31820 section.
The output temperature data is calibrated in degrees
Celsius; for Fahrenheit applications, a lookup table or
conversion routine must be used. The temperature data
is stored as a 16-bit sign-extended two’s complement
number in the temperature register (see the Temperature
Register Format).Thesignbits(S)indicateifthetempera-
tureispositiveornegative:forpositivenumbersS=0and
fornegativenumbersS=1.Ifthedeviceisconfigured
for 12-bit resolution, all bits in the temperature register
containvaliddata.For11-bitresolution,bit0isundefined.
For10-bitresolution,bits1and0areundefined,andfor
9-bit resolution bits 2, 1, and 0 are undefined. Table 1
gives examples of digital output data and the correspond-
ing temperature reading for 12-bit resolution conversions.
Temperature Register Format
Table 1. Temperature/Data Relationship
BIT 15 BIT 14 BIT 13 BIT 12 BIT 11 BIT 10 BIT 9 BIT 8
MSB SSSSS262524
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
LSB 232221202-1 2-2 2-3 2-4
TEMPERATURE (°C) DIGITAL OUTPUT (BINARY) DIGITAL OUTPUT (HEX)
+125 0000 0111 1101 0000 07D0h
+85* 0000 0101 0101 0000 0550h
+25.0625 0000 0001 1001 0001 0191h
+10.125 0000 0000 1010 0010 00A2h
+0.5 0000 0000 0000 1000 0008h
00000 0000 0000 0000 0000h
-0.5 1111 1111 1111 1000 FFF8h
-10.125 1111 1111 0101 1110 FF5Eh
-25.0625 1111 1110 0110 1111 FE6Fh
-55 1111 1100 1001 0000 FC90h
MAX31820 1-Wire Ambient Temperature Sensor
www.maximintegrated.com Maxim Integrated
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