DS18B20
Programmable Resolution
1-Wire Digital Thermometer
www.maxim-ic.com
FEATURES
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Unique 1-Wire interface requires only one
port pin for communication
Each device has a unique 64-bit serial code
stored in an onboard ROM
Multidrop capability simplifies distributed
temperature sensing applications
Requires no external components
Can be powered from data line. Power supply
range is 3.0V to 5.5V
Measures temperatures from –55°C to
+125°C (–67°F to +257°F)
±0.5°C
accuracy from –10°C to +85°C
Thermometer resolution is user-selectable
from 9 to 12 bits
Converts temperature to 12-bit digital word in
750ms (max.)
User-definable nonvolatile (NV) alarm
settings
Alarm search command identifies and
addresses devices whose temperature is
outside of programmed limits (temperature
alarm condition)
Available in 8-pin SO (150mil), 8-pin
mSOP,
and 3-pin TO-92 packages
Software compatible with the DS1822
Applications include thermostatic controls,
industrial systems, consumer products,
thermometers, or any thermally sensitive
system
®
PIN ASSIGNMENT
DALLAS
18B20
1 2 3
NC
NC
V
DD
DQ
1
8
NC
NC
NC
GND
DALLAS
18B20
2
3
4
7
6
5
8-Pin 150mil SO
(DS18B20Z)
DQ
NC
NC
GND
GND
DQ
V
DD
1
2
3
4
8
7
6
5
V
DD
NC
NC
NC
18B20
1 2
3
(BOTTOM VIEW)
8-Pin
mSOP
(DS18B20U)
TO-92
(DS18B20)
PIN DESCRIPTION
GND
DQ
V
DD
NC
- Ground
- Data In/Out
- Power Supply Voltage
- No Connect
DESCRIPTION
The DS18B20 Digital Thermometer provides 9 to 12–bit centigrade temperature measurements and has
an alarm function with nonvolatile user-programmable upper and lower trigger points. The DS18B20
communicates over a 1-Wire bus that by definition requires only one data line (and ground) for
communication with a central microprocessor. It has an operating temperature range of –55°C to +125°C
and is accurate to
±0.5°C
over the range of –10°C to +85°C. In addition, the DS18B20 can derive power
directly from the data line (“parasite power”), eliminating the need for an external power supply.
Each DS18B20 has a unique 64-bit serial code, which allows multiple DS18B20s to function on the same
1–wire bus; thus, it is simple to use one microprocessor to control many DS18B20s distributed over a
large area. Applications that can benefit from this feature include HVAC environmental controls,
temperature monitoring systems inside buildings, equipment or machinery, and process monitoring and
control systems.
1-Wire is a registered trademark of Dallas Semiconductor.
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050102
DS18B20
ORDER INFORMATION
ORDERING
NUMBER
DS18B20
DS18B20/T&R
DS18B20+
DS18B20+T&R
DS18B20 in 3-pin TO92
DS18B20 in 3-pin TO92, 2000 Piece Tape-and-Reel
DS18B20 in Lead-Free 3-pin TO92
DS18B20 in Lead-Free 3-pin TO92, 2000 Piece Tape-
and-Reel
DS18B20U
18B20
DS18B20 in 8-pin uSOP
DS18B20U/T&R
18B20
DS18B20 in 8-pin uSOP, 3000 Piece Tape-and-Reel
DS18B20U+
18B20 (See Note)
DS18B20 in Lead-Free 8-pin uSOP
DS18B20U+T&R
18B20 (See Note)
DS18B20 in Lead-Free 8-pin uSOP, 3000 Piece Tape-
and-Reel
DS18B20Z
DS18B20
DS18B20 in 150 mil 8-pin SO
DS18B20Z/T&R
DS18B20
DS18B20 in 150 mil 8-pin SO, 2500 Piece Tape-and-
Reel
DS18B20Z+
DS18B20 (See Note)
DS18B20 in Lead-Free 150 mil 8-pin SO
DS18B20Z+T&R
DS18B20 (See Note)
DS18B20 in Lead-Free 150 mil 8-pin SO, 2500 Piece
Tape-and-Reel
DS18B20X
28
DS18B20 in Flip Chip, 10000 Piece Tape-and-Reel
Note: A “+” symbol will also be marked on the package.
PACKAGE
MARKING
18B20
18B20
18B20 (See Note)
18B20 (See Note)
DESCRIPTION
DETAILED PIN DESCRIPTIONS
Table 1
TO-92 SYMBOL DESCRIPTION
1
GND
Ground.
2
DQ
Data Input/Output pin.
Open-drain 1-Wire interface pin.
Also provides power to the device when used in parasite
power mode (see “Parasite Power” section.)
3
8
3
V
DD
Optional V
DD
pin.
V
DD
must be grounded for operation in
parasite power mode.
*All pins not specified in this table are “No Connect” pins.
mSOP*
4
1
SO*
5
4
OVERVIEW
Figure 1 shows a block diagram of the DS18B20, and pin descriptions are given in Table 1. The 64-bit
ROM stores the device’s unique serial code. The scratchpad memory contains the 2-byte temperature
register that stores the digital output from the temperature sensor. In addition, the scratchpad provides
access to the 1-byte upper and lower alarm trigger registers (T
H
and T
L
), and the 1-byte configuration
register. The configuration register allows the user to set the resolution of the temperature-to-digital
conversion to 9, 10, 11, or 12 bits. The T
H
, T
L
and configuration registers are nonvolatile (EEPROM), so
they will retain data when the device is powered down.
The DS18B20 uses Dallas’ exclusive 1-Wire bus protocol that implements bus communication using one
control signal. The control line requires a weak pullup resistor since all devices are linked to the bus via a
3-state or open-drain port (the DQ pin in the case of the DS18B20). In this bus system, the
microprocessor (the master device) identifies and addresses devices on the bus using each device’s unique
64-bit code. Because each device has a unique code, the number of devices that can be addressed on one
bus is virtually unlimited. The 1-Wire bus protocol, including detailed explanations of the commands and
“time slots,” is covered in the
1-WIRE BUS SYSTEM
section of this datasheet.
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DS18B20
Another feature of the DS18B20 is the ability to operate without an external power supply. Power is
instead supplied through the 1-Wire pullup resistor via the DQ pin when the bus is high. The high bus
signal also charges an internal capacitor (C
PP
), which then supplies power to the device when the bus is
low. This method of deriving power from the 1-Wire bus is referred to as “parasite power.” As an
alternative, the DS18B20 may also be powered by an external supply on V
DD
.
DS18B20 BLOCK DIAGRAM
Figure 1
V
PU
4.7k
PARASITE POWER
CIRCUIT
MEMORY CONTROL
LOGIC
DS18B20
TEMPERATURE SENSOR
DQ
INTERNAL V
DD
64-BIT ROM
AND
1-wire PORT
GND
C
PP
SCRATCHPAD
ALARM HIGH TRIGGER (T
H
)
REGISTER (EEPROM)
ALARM LOW TRIGGER (T
L
)
REGISTER (EEPROM)
V
DD
POWER
SUPPLY
SENSE
CONFIGURATION REGISTER
(EEPROM)
8-BIT CRC GENERATOR
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DS18B20
OPERATION — MEASURING TEMPERATURE
The core functionality of the DS18B20 is its direct-to-digital temperature 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-bit. The DS18B20
powers-up in a low-power idle state; to initiate a temperature measurement and A-to-D conversion, 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 scratchpad memory and the DS18B20 returns to its idle
state. If the DS18B20 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 DS18B20 will respond by
transmitting 0 while the temperature conversion is in progress and 1 when the conversion is done. If the
DS18B20 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 DS18B20
section of this datasheet.
The DS18B20 output temperature data is calibrated in degrees centigrade; 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 Figure 2). The sign bits (S) indicate if the
temperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. If the
DS18B20 is configured for 12-bit resolution, all bits in the temperature register will contain valid data.
For 11-bit resolution, bit 0 is undefined. For 10-bit resolution, bits 1 and 0 are undefined, and for 9-bit
resolution bits 2, 1 and 0 are undefined. Table 2 gives examples of digital output data and the
corresponding temperature reading for 12-bit resolution conversions.
TEMPERATURE REGISTER FORMAT
Figure 2
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
LS Byte
MS Byte
2
3
bit 15
2
2
bit 14
2
1
bit 13
2
0
bit 12
2
-1
bit 11
2
-2
bit 10
2
-3
bit 9
2
-4
bit 8
S
S
S
S
S
2
6
2
5
2
4
TEMPERATURE/DATA RELATIONSHIP
Table 2
TEMPERATURE
+125°C
+85°C*
+25.0625°C
+10.125°C
+0.5°C
0°C
-0.5°C
-10.125°C
-25.0625°C
-55°C
DIGITAL OUTPUT DIGITAL OUTPUT
(Binary)
(Hex)
0000 0111 1101 0000
07D0h
0000 0101 0101 0000
0550h
0000 0001 1001 0001
0191h
0000 0000 1010 0010
00A2h
0000 0000 0000 1000
0008h
0000 0000 0000 0000
0000h
1111 1111 1111 1000
FFF8h
1111 1111 0101 1110
FF5Eh
1111 1110 0110 1111
FE6Fh
1111 1100 1001 0000
FC90h
*The power-on reset value of the temperature register is +85°C
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DS18B20
OPERATION — ALARM SIGNALING
After the DS18B20 performs a temperature conversion, the temperature value is compared to the user-
defined two’s complement alarm trigger values stored in the 1-byte T
H
and T
L
registers (see Figure 3).
The sign bit (S) indicates if the value is positive or negative: for positive numbers S = 0 and for negative
numbers S = 1. The T
H
and T
L
registers are nonvolatile (EEPROM) so they will retain data when the
device is powered down. T
H
and T
L
can be accessed through bytes 2 and 3 of the scratchpad as explained
in the
MEMORY
section of this datasheet.
T
H
AND T
L
REGISTER FORMAT
Figure 3
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
S
2
6
2
5
2
5
2
5
2
2
2
1
2
0
Only bits 11 through 4 of the temperature register are used in the T
H
and T
L
comparison since T
H
and T
L
are 8-bit registers. If the measured temperature is lower than or equal to T
L
or higher than T
H
, an alarm
condition exists and an alarm flag is set inside the DS18B20. This flag is updated after every temperature
measurement; therefore, if the alarm condition goes away, the flag will be turned off after the next
temperature conversion.
The master device can check the alarm flag status of all DS18B20s on the bus by issuing an Alarm Search
[ECh] command. Any DS18B20s with a set alarm flag will respond to the command, so the master can
determine exactly which DS18B20s have experienced an alarm condition. If an alarm condition exists
and the T
H
or T
L
settings have changed, another temperature conversion should be done to validate the
alarm condition.
POWERING THE DS18B20
The DS18B20 can be powered by an external supply on the V
DD
pin, or it can operate in “parasite power”
mode, which allows the DS18B20 to function without a local external supply. Parasite power is very
useful for applications that require remote temperature sensing or that are very space constrained. Figure
1 shows the DS18B20’s parasite-power control circuitry, which “steals” power from the 1-Wire bus via
the DQ pin when the bus is high. The stolen charge powers the DS18B20 while the bus is high, and some
of the charge is stored on the parasite power capacitor (C
PP
) to provide power when the bus is low. When
the DS18B20 is used in parasite power mode, the V
DD
pin must be connected to ground.
In parasite power mode, the 1-Wire bus and C
PP
can provide sufficient current to the DS18B20 for most
operations as long as the specified timing and voltage requirements are met (refer to
the DC
ELECTRICAL CHARACTERISTICS
and the
AC ELECTRICAL CHARACTERISTICS
sections of this data
sheet). However, when the DS18B20 is performing temperature conversions or copying data from the
scratchpad memory to EEPROM, the operating current can be as high as 1.5mA. This current can cause
an unacceptable voltage drop across the weak 1-Wire pullup resistor and is more current than can be
supplied by C
PP
. To assure that the DS18B20 has sufficient supply current, it is necessary to provide a
strong pullup on the 1-Wire bus whenever temperature conversions are taking place or data is being
copied from the scratchpad to EEPROM. This can be accomplished by using a MOSFET to pull the bus
directly to the rail as shown in Figure 4. The 1-Wire bus must be switched to the strong pullup within
10ms (max) after a Convert T [44h] or Copy Scratchpad [48h] command is issued, and the bus must be
held high by the pullup for the duration of the conversion (t
conv
) or data transfer (t
wr
= 10ms). No other
activity can take place on the 1-Wire bus while the pullup is enabled.
The DS18B20 can also be powered by the conventional method of connecting an external power supply
to the V
DD
pin, as shown in Figure 5. The advantage of this method is that the MOSFET pullup is not
required, and the 1-Wire bus is free to carry other traffic during the temperature conversion time.
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