DS18B20
Programmable Resolution
1-Wire Digital Thermometer
DESCRIPTION
The DS18B20 digital thermometer provides 9-bit
to 12-bit Celsius 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.
User-Definable Nonvolatile (NV) Alarm
Settings
Alarm Search Command Identifies and
Addresses Devices Whose Temperature is
Outside Programmed Limits (Temperature
Alarm Condition)
Available in 8-Pin SO (150 mils), 8-Pin
µSOP,
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 CONFIGURATIONS
MAXIM
18B20
1 2 3
V
DD
DQ
3
4
N.C.
N.C.
1
8
N.C.
N.C.
N.C.
GND
MAXIM
18B20
2
7
6
5
SO (150 mils)
(DS18B20Z)
FEATURES
Unique 1-Wire® Interface Requires Only One
Port Pin for Communication
Each Device has a Unique 64-Bit Serial Code
Stored in an On-Board 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)
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GND
DQ
V
DD
DQ
N.C.
N.C.
GND
1
2
3
4
8
7
6
5
V
DD
N.C.
N.C.
N.C.
1 2 3
(BOTTOM VIEW)
µSOP
(DS18B20U)
18B20
TO-92
(DS18B20)
1-Wire is a registered trademark of Maxim Integrated Products, Inc.
REV: 042208
DS18B20
ORDERING INFORMATION
PART
DS18B20
DS18B20+
DS18B20/T&R
DS18B20+T&R
DS18B20-SL/T&R
DS18B20-SL+T&R
DS18B20U
DS18B20U+
DS18B20U/T&R
DS18B20U+T&R
DS18B20Z
DS18B20Z+
DS18B20Z/T&R
DS18B20Z+T&R
TEMP RANGE
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
PIN-PACKAGE
3 TO-92
3 TO-92
3 TO-92 (2000 Piece)
3 TO-92 (2000 Piece)
3 TO-92 (2000 Piece)*
3 TO-92 (2000 Piece)*
8
µSOP
8
µSOP
8
µSOP
(3000 Piece)
8
µSOP
(3000 Piece)
8 SO
8 SO
8 SO (2500 Piece)
8 SO (2500 Piece)
TOP MARK
18B20
18B20
18B20
18B20
18B20
18B20
18B20
18B20
18B20
18B20
DS18B20
DS18B20
DS18B20
DS18B20
+Denotes
a lead-free package. A “+” will appear on the top mark of lead-free packages.
T&R = Tape and reel.
*TO-92 packages in tape and reel can be ordered with straight or formed leads. Choose “SL” for straight leads. Bulk TO-92 orders are straight
leads only.
PIN DESCRIPTION
PIN
SO
1, 2, 6,
7, 8
3
4
5
µSOP
2, 3, 5,
6, 7
8
1
4
TO-92
—
3
2
1
NAME
N.C.
V
DD
DQ
GND
No Connection
Optional V
DD
. V
DD
must be grounded for operation in
parasite power mode.
Data Input/Output. Open-drain 1-Wire interface pin. Also
provides power to the device when used in parasite power
mode (see the
Powering the DS18B20
section.)
Ground
FUNCTION
OVERVIEW
Figure 1 shows a block diagram of the DS18B20, and pin descriptions are given in the
Pin Description
table. 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 Maxim’s 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
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DS18B20
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.
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
.
Figure 1. DS18B20 Block Diagram
V
PU
PARASITE POWER
CIRCUIT
4.7k
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
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.
The DS18B20 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 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 1 gives examples of digital output data and the
corresponding temperature reading for 12-bit resolution conversions.
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DS18B20
Figure 2. Temperature Register Format
BIT 7
3
2
BIT 15
MS BYTE
S = SIGN
LS BYTE
BIT 6
2
2
BIT 14
S
BIT 5
2
1
BIT 13
S
BIT 4
2
0
BIT 12
S
BIT 3
2
-1
BIT 11
S
BIT 2
2
-2
BIT 10
2
6
BIT 1
2
-3
BIT 9
2
5
BIT 0
2
-4
BIT 8
2
4
S
Table 1. Temperature/Data Relationship
TEMPERATURE (°C)
+125
+85*
+25.0625
+10.125
+0.5
0
-0.5
-10.125
-25.0625
-55
DIGITAL OUTPUT
(BINARY)
0000 0111 1101 0000
0000 0101 0101 0000
0000 0001 1001 0001
0000 0000 1010 0010
0000 0000 0000 1000
0000 0000 0000 0000
1111 1111 1111 1000
1111 1111 0101 1110
1111 1110 0110 1111
1111 1100 1001 0000
DIGITAL OUTPUT
(HEX)
07D0h
0550h
0191h
00A2h
0008h
0000h
FFF8h
FF5Eh
FE6Fh
FC90h
*The
power-on reset value of the temperature register is +85°C.
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.
Figure 3. T
H
and T
L
Register Format
BIT 7
S
BIT 6
2
6
BIT 5
2
5
BIT 4
2
4
BIT 3
2
3
BIT 2
2
2
BIT 1
2
1
BIT 0
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 or equal to 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.
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DS18B20
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 (see the
DC Electrical
Characteristics
and
AC Electrical Characteristics).
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 10µs (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.
The use of parasite power is not recommended for temperatures above +100°C since the DS18B20 may
not be able to sustain communications due to the higher leakage currents that can exist at these
temperatures. For applications in which such temperatures are likely, it is strongly recommended that the
DS18B20 be powered by an external power supply.
In some situations the bus master may not know whether the DS18B20s on the bus are parasite powered
or powered by external supplies. The master needs this information to determine if the strong bus pullup
should be used during temperature conversions. To get this information, the master can issue a Skip ROM
[CCh] command followed by a Read Power Supply [B4h] command followed by a “read time slot”.
During the read time slot, parasite powered DS18B20s will pull the bus low, and externally powered
DS18B20s will let the bus remain high. If the bus is pulled low, the master knows that it must supply the
strong pullup on the 1-Wire bus during temperature conversions.
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