DS18B20/ ,High-Precision 1-Wire Digital ThermometerFeatures®The DS18B20 digital thermometer provides 9-bit to 12-bit ● Unique 1-Wire Interface Require ..
DS18B20+ ,Programmable Resolution 1-Wire Digital ThermometerElectrical Characteristics–NV Memory(-55°C to +125°C; V = 3.0V to 5.5V)DD PARAMETER SYMBOL CONDITIO ..
DS18B20+/ ,High-Precision 1-Wire Digital ThermometerElectrical Characteristics(-55°C to +125°C; V = 3.0V to 5.5V)DD PARAMETER SYMBOL CONDITIONS MIN TYP ..
DS18B20-PAR ,1-Wire Parasite-Power Digital ThermometerELECTRICAL CHARACTERISTICS sections of this data sheet). However, when the DS18B20-PAR is performi ..
DS18B20U ,Programmable Resolution 1-Wire Digital ThermometerFeatures®The DS18B20 digital thermometer provides 9-bit to 12-bit ● Unique 1-Wire Interface Require ..
DS18B20U+T&R ,Programmable Resolution 1-Wire Digital ThermometerGeneral Description Beneits and
DS18B20-DS18B20.-DS18B20/-DS18B20+/
High-Precision 1-Wire Digital Thermometer
General DescriptionThe 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 micro-
processor. 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.
Applications●Thermostatic Controls●Industrial Systems●Consumer Products●Thermometers●Thermally Sensitive Systems
Beneits and Features●Unique 1-Wire® Interface Requires Only One Port
Pin for Communication●Reduce Component Count with Integrated emperature Sensor and EEPROMMeasures Temperatures from -55°C to +125°C
(-67°F to +257°F)±0.5°C Accuracy from -10°C to +85°CProgrammable Resolution from 9 Bits to 12 BitsNo External Components Required●Parasitic Power Mode Requires Only 2 Pins for
Operation (DQ and GND)●Simplifies Distributed Temperature-Sensing
Applications with Multidrop CapabilityEach Device Has a Unique 64-Bit Serial Code
Stored in On-Board ROM●Flexible User-Definable Nonvolatile (NV) Alarm Settings
with Alarm Search Command Identifies Devices with emperatures Outside Programmed Limits●Available in 8-Pin SO (150 mils), 8-Pin µSOP, and
3-Pin TO-92 Packages
Ordering Information appears at end of data sheet.1-Wire is a registered trademark of Maxim Integrated Products, Inc.
BOTTOM VIEW
N.C.
N.C.
VDD
N.C.
N.C.
N.C.
GND
DS18B20
SO (150 mils)
(DS18B20Z) 1
DQ
N.C.
N.C.
GND
VDD
N.C.
N.C.
N.C.
DS18B20
µSOP
(DS18B20U)
DS18B2023
GNDDQVDD
1 23
TOP VIEW
TO-92
(DS18B20)
DS18B20Programmable Resolution
1-Wire Digital Thermometer
Pin Conigurations
Voltage Range on Any Pin Relative to Ground ....-0.5V to +6.0V
Operating Temperature Range .........................-55°C to +125°C
Storage Temperature Range ............................-55°C to +125°C
Solder Temperature ...............................Refer to the IPC/JEDEC
J-STD-020 Specification.
(-55°C to +125°C; VDD = 3.0V to 5.5V)
Note 1: All voltages are referenced to ground.
Note 2: The Pullup Supply Voltage specification assumes that the pullup device is ideal, and therefore the high level of the
pullup is equal to VPU. In order to meet the VIH spec of the DS18B20, the actual supply rail for the strong pullup transis-
tor must include margin for the voltage drop across the transistor when it is turned on; thus: VPU_ACTUAL = VPU_IDEAL +
VTRANSISTOR.
Note 3: See typical performance curve in Figure 1.
Note 4: Logic-low voltages are specified at a sink current of 4mA.
Note 5: To guarantee a presence pulse under low voltage parasite power conditions, VILMAX may have to be reduced to as low as
0.5V.
Note 6: Logic-high voltages are specified at a source current of 1mA.
Note 7: Standby current specified up to +70°C. Standby current typically is 3µA at +125°C.
Note 8: To minimize IDDS, DQ should be within the following ranges: GND ≤ DQ ≤ GND + 0.3V or VDD – 0.3V ≤ DQ ≤ VDD.
Note 9: Active current refers to supply current during active temperature conversions or EEPROM writes.
Note 10: DQ line is high (“high-Z” state).
Note 11: Drift data is based on a 1000-hour stress test at +125°C with VDD = 5.5V.
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSSupply VoltageVDDLocal power (Note 1)+3.0+5.5V
Pullup Supply VoltageVPU
Parasite power
(Notes 1, 2)
+3.0+5.5Local power +3.0VDD
Thermometer ErrortERR
-10°C to +85°C
(Note 3)
±0.5-55°C to +125°C ±2
Input Logic-LowVIL(Notes 1, 4, 5)-0.3+0.8V
Input Logic-HighVIH
Local power
(Notes 1,6)
+2.2The lower
of 5.5 or
VDD + 0.3
Parasite power +3.0
Sink CurrentILVI/O = 0.4V4.0mA
Standby CurrentIDDS(Notes 7, 8)7501000nA
Active CurrentIDDVDD = 5V (Note 9)11.5mA
DQ Input CurrentIDQ(Note 10)5µA
Drift(Note 11)±0.2°C
DS18B20Programmable Resolution
1-Wire Digital Thermometer
Absolute Maximum RatingsThese are stress ratings only and functional operation of the device at these or any other conditions above those indicated in the operation sections of this specification is not implied. Exposure
to absolute maximum rating conditions for extended periods of time may affect reliability.
DC Electrical Characteristics
(-55°C to +125°C; VDD = 3.0V to 5.5V)
(-55°C to +125°C; VDD = 3.0V to 5.5V)
Note 12: See the timing diagrams in Figure 2.
Note 13: Under parasite power, if tRSTL > 960µs, a power-on reset can occur.
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSNV Write Cycle TimetWR210ms
EEPROM WritesNEEWR-55°C to +55°C50kwrites
EEPROM Data RetentiontEEDR-55°C to +55°C10years
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSTemperature Conversion TimetCONV
9-bit resolution
(Note 12)
10-bit resolution187.5
11-bit resolution375
12-bit resolution750
Time to Strong Pullup OntSPONStart convert T command issued10µs
Time SlottSLOT(Note 12)60120µs
Recovery TimetREC(Note 12)1µs
Write 0 Low TimetLOW0(Note 12)60120µs
Write 1 Low TimetLOW1(Note 12)115µs
Read Data ValidtRDV(Note 12)15µs
Reset Time HightRSTH(Note 12)480µs
Reset Time LowtRSTL(Notes 12, 13)480µs
Presence-Detect HightPDHIGH(Note 12)1560µs
Presence-Detect LowtPDLOW(Note 12)60240µs
CapacitanceCIN/OUT25pF
DS18B20 TYPICAL ERROR CURVETHERMOMETER ERROR
(°C70102030405060
TEMPERATURE (°C)
+3s ERROR
MEAN ERROR
-3s ERROR
DS18B20Programmable Resolution
1-Wire Digital Thermometer
AC Electrical Characteristics–NV Memory
AC Electrical Characteristics
Figure 2. Timing Diagrams
PIN
NAMEFUNCTIONSOµSOPTO-921, 2, 6,
7, 8
2, 3, 5,
6, 7—N.C.No Connection83VDDOptional VDD. VDD must be grounded for operation in parasite power mode.12DQData 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.) 41GNDGround
START OF NEXT CYCLE
1-WIRE WRITE ZERO TIME SLOTtREC
tSLOT
tLOW0
1-WIRE READ ZERO TIME SLOTtREC
tSLOTSTART OF NEXT CYCLE
tRDV
1-WIRE RESET PULSE
1-WIRE PRESENCE DETECTtRSTLtRSTH
tPDIH
PRESENCE DETECT
tPDLOW
RESET PULSE FROM HOST
DS18B20Programmable Resolution
1-Wire Digital Thermometer
Pin Description
OverviewFigure 3 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 (TH and
TL) and the 1-byte configuration register. The configura-
tion register allows the user to set the resolution of the
temperature-to-digital conversion to 9, 10, 11, or 12 bits.
The TH, TL, 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 proto-
col 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.
Another feature of the DS18B20 is the ability to oper-
ate without an external power supply. Power is instead
supplied through the 1-Wire pullup resistor through the
DQ pin when the bus is high. The high bus signal also
charges an internal capacitor (CPP), 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 “para-
site power.” As an alternative, the DS18B20 may also be
powered by an external supply on VDD.
Operation—Measuring TemperatureThe core functionality of the DS18B20 is its direct-to-
digital temperature sensor. The resolution of the tempera-
ture 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 sup-
ply, 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 tem-
perature conversion is in progress and 1 when the con-
version 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.
TEMPERATURE
SENSOR
SCRATCHPAD
MEMORY
CONTROL LOGIC
64-BIT ROM
AND 1-Wire
PORT
PARASITE POWER CIRCUIT
POWER-
SUPPLY SENSE
INTERNAL VDDGND
VPU
4.7kΩ
CONFIGURATION
REGISTER (EEPROM)
8-BIT CRC
GENERATOR
VDD
CPP
DS18B20ALARM LOW TRIGGER (TL)
REGISTER (EEPROM)
ALARM HIGH TRIGGER (TH)
REGISTER (EEPROM)
DS18B20Programmable Resolution
1-Wire Digital Thermometer
The DS18B20 output temperature data is calibrated in
degrees Celsius; for Fahrenheit applications, a lookup
table or conversion routine must be used. The tempera-
ture data is stored as a 16-bit sign-extended two’s comple-
ment number in the temperature register (see Figure 4).
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.
Operation—Alarm SignalingAfter 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 TH and TL registers (see Figure 5). The sign bit (S)
indicates if the value is positive or negative: for positive
numbers S = 0 and for negative numbers S = 1. The TH
and TL registers are nonvolatile (EEPROM) so they will
retain data when the device is powered down. TH and TL
can be accessed through bytes 2 and 3 of the scratchpad
as explained in the Memory section.
Only bits 11 through 4 of the temperature register are
used in the TH and TL comparison since TH and TL are
8-bit registers. If the measured temperature is lower than
Figure 4. Temperature Register Format
Table 1. Temperature/Data Relationship *The power-on reset value of the temperature register is +85°C.
TEMPERATURE (°C)DIGITAL OUTPUT
(BINARY)
DIGITAL OUTPUT
(HEX)+1250000 0111 1101 000007D0h
+85*0000 0101 0101 00000550h
+25.06250000 0001 1001 00010191h
+10.1250000 0000 1010 001000A2h
+0.50000 0000 0000 10000008h0000 0000 0000 00000000h
-0.51111 1111 1111 1000FFF8h
-10.1251111 1111 0101 1110FF5Eh
-25.06251111 1110 0110 1111FE6Fh
-551111 1100 1001 0000FC90h
BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0
LS BYTE 232221202-12-22-32-4
BIT 15BIT 14BIT 13BIT 12BIT 11BIT 10BIT 9BIT 8
MS BYTESSSSS262524
S = SIGN
BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 026252423222120
DS18B20Programmable Resolution
1-Wire Digital Thermometer
or equal to TL or higher than or equal to TH, an alarm con-
dition exists and an alarm flag is set inside the DS18B20.
This flag is updated after every temperature measure-
ment; 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 TH or TL
settings have changed, another temperature conversion
should be done to validate the alarm condition.
Powering the DS18B20The DS18B20 can be powered by an external supply on
the VDD 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 applica-
tions that require remote temperature sensing or that are
very space constrained. Figure 3 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 (CPP) to provide power when the bus is low.
When the DS18B20 is used in parasite power mode, the
VDD pin must be connected to ground.
In parasite power mode, the 1-Wire bus and CPP can pro-
vide 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 CPP. 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 tak-
ing 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 6. 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 (tCONV)
or data transfer (tWR = 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
VDD pin, as shown in Figure 7. 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 tem-
perature conversion time.
The use of parasite power is not recommended for tem-
peratures above +100°C since the DS18B20 may not be
able to sustain communications due to the higher leak-
age 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 pow-
ered by external supplies. The master needs this informa-
tion 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 fol-
lowed 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.
Figure 6. Supplying the Parasite-Powered DS18B20 During
Figure 7. Powering the DS18B20 with an External Supply
VPU
4.7kΩ
VPU
1-Wire BUS
DS18B20GNDDQVDD
TO OTHER
1-Wire DEVICES
VPU
4.7kΩ
1-Wire BUS
DS18B20GNDDQVDD
TO OTHER
1-Wire DEVICES
VDD (EXTERNAL
SUPPLY)
DS18B20Programmable Resolution
1-Wire Digital Thermometer
64-BIT Lasered ROM codeEach DS18B20 contains a unique 64–bit code (see Figure
8) stored in ROM. The least significant 8 bits of the ROM
code contain the DS18B20’s 1-Wire family code: 28h. The
next 48 bits contain a unique serial number. The most
significant 8 bits contain a cyclic redundancy check (CRC)
byte that is calculated from the first 56 bits of the ROM
code. A detailed explanation of the CRC bits is provided
in the CRC Generation section. The 64-bit ROM code and
associated ROM function control logic allow the DS18B20
to operate as a 1-Wire device using the protocol detailed
in the 1-Wire Bus System section.
MemoryThe DS18B20’s memory is organized as shown in Figure
9. The memory consists of an SRAM scratchpad with
nonvolatile EEPROM storage for the high and low alarm
trigger registers (TH and TL) and configuration register.
Note that if the DS18B20 alarm function is not used,
the TH and TL registers can serve as general-purpose
memory. All memory commands are described in detail in
the DS18B20 Function Commands section.
Byte 0 and byte 1 of the scratchpad contain the LSB and
the MSB of the temperature register, respectively. These
bytes are read-only. Bytes 2 and 3 provide access to TH
and TL registers. Byte 4 contains the configuration regis-
ter data, which is explained in detail in the Configuration
Register section. Bytes 5, 6, and 7 are reserved for inter-
nal use by the device and cannot be overwritten.
Byte 8 of the scratchpad is read-only and contains the
CRC code for bytes 0 through 7 of the scratchpad.
The DS18B20 generates this CRC using the method
described in the CRC Generation section.
Data is written to bytes 2, 3, and 4 of the scratchpad using
the Write Scratchpad [4Eh] command; the data must be
transmitted to the DS18B20 starting with the least signifi-
cant bit of byte 2. To verify data integrity, the scratchpad
can be read (using the Read Scratchpad [BEh] command)
after the data is written. When reading the scratchpad,
data is transferred over the 1-Wire bus starting with the
least significant bit of byte 0. To transfer the TH, TL and
configuration data from the scratchpad to EEPROM, the
master must issue the Copy Scratchpad [48h] command.
Data in the EEPROM registers is retained when the
device is powered down; at power-up the EEPROM data
is reloaded into the corresponding scratchpad locations.
Data can also be reloaded from EEPROM to the scratch-
pad at any time using the Recall E2 [B8h] command. The
master can issue read time slots following the Recall E2
command and the DS18B20 will indicate the status of the
recall by transmitting 0 while the recall is in progress and
1 when the recall is done.
Figure 8. 64-Bit Lasered ROM Code
BYTE 0
BYTE 1
TEMPERATURE LSB (50h)
TEMPERATURE MSB (05h)(85°C)
BYTE 2
BYTE 3
TH REGISTER OR USER BYTE 1*
TL REGISTER OR USER BYTE 2*
BYTE 4
BYTE 5
CONFIGURATION REGISTER*
RESERVED (FFh)
BYTE 6
BYTE 7
RESERVED
RESERVED (10h)
BYTE 8CRC*
*POWER-UP STATE DEPENDS ON VALUE(S) STORED IN EEPROM.
TH REGISTER OR USER BYTE 1*
TL REGISTER OR USER BYTE 2*
CONFIGURATION REGISTER*
SCRATCHPAD
(POWER-UP STATE)
EEPROM8-BIT CRC48-BIT SERIAL NUMBER8-BIT FAMILY CODE (28h)
MSBLSBMSBLSBMSBLSB
DS18B20Programmable Resolution
1-Wire Digital Thermometer
Coniguration RegisterByte 4 of the scratchpad memory contains the configura-
tion register, which is organized as illustrated in Figure 10.
The user can set the conversion resolution of the DS18B20
using the R0 and R1 bits in this register as shown in Table
2. The power-up default of these bits is R0 = 1 and R1 = (12-bit resolution). Note that there is a direct tradeoff
between resolution and conversion time. Bit 7 and bits 0 to
4 in the configuration register are reserved for internal use
by the device and cannot be overwritten.
CRC GenerationCRC bytes are provided as part of the DS18B20’s 64-bit
ROM code and in the 9th byte of the scratchpad memory.
The ROM code CRC is calculated from the first 56 bits
of the ROM code and is contained in the most significant
byte of the ROM. The scratchpad CRC is calculated from
the data stored in the scratchpad, and therefore it chang-
es when the data in the scratchpad changes. The CRCs
provide the bus master with a method of data validation
when data is read from the DS18B20. To verify that data
has been read correctly, the bus master must re-calculate
the CRC from the received data and then compare this
value to either the ROM code CRC (for ROM reads) or
to the scratchpad CRC (for scratchpad reads). If the cal-
culated CRC matches the read CRC, the data has been
received error free. The comparison of CRC values and
the decision to continue with an operation are determined
entirely by the bus master. There is no circuitry inside the
DS18B20 that prevents a command sequence from pro-
ceeding if the DS18B20 CRC (ROM or scratchpad) does
not match the value generated by the bus master.
The equivalent polynomial function of the CRC (ROM or
scratchpad) is:
CRC = X8 + X5 + X4 + 1
The bus master can re-calculate the CRC and compare it
to the CRC values from the DS18B20 using the polyno-
mial generator shown in Figure 11. This circuit consists
of a shift register and XOR gates, and the shift register
bits are initialized to 0. Starting with the least significant
bit of the ROM code or the least significant bit of byte 0
in the scratchpad, one bit at a time should shifted into the
shift register. After shifting in the 56th bit from the ROM or
the most significant bit of byte 7 from the scratchpad, the
polynomial generator will contain the recalculated CRC.
Next, the 8-bit ROM code or scratchpad CRC from the
DS18B20 must be shifted into the circuit. At this point, if
the re-calculated CRC was correct, the shift register will
contain all 0s. Additional information about the Maxim
1-Wire cyclic redundancy check is available in Application
Note 27: Understanding and Using Cyclic Redundancy
Checks with Maxim iButton Products.
Figure 10. Configuration Register
Table 2. Thermometer Resolution Configuration R0RESOLUTION
(BITS)MAX CONVERSION TIME09 93.75ms(tCONV/8)110187.5ms(tCONV/4)011375ms(tCONV/2)112750ms(tCONV)
XORXORXOR
INPUT
MSBLSB
BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0R1R011111
DS18B20Programmable Resolution
1-Wire Digital Thermometer
1-Wire Bus SystemThe 1-Wire bus system uses a single bus master to con-
trol one or more slave devices. The DS18B20 is always a
slave. When there is only one slave on the bus, the sys-
tem is referred to as a “single-drop” system; the system is
“multidrop” if there are multiple slaves on the bus.
All data and commands are transmitted least significant
bit first over the 1-Wire bus.
The following discussion of the 1-Wire bus system is
broken down into three topics: hardware configuration,
transaction sequence, and 1-Wire signaling (signal types
and timing).
Hardware ConigurationThe 1-Wire bus has by definition only a single data line.
Each device (master or slave) interfaces to the data line
via an open-drain or 3-state port. This allows each device
to “release” the data line when the device is not transmit-
ting data so the bus is available for use by another device.
The 1-Wire port of the DS18B20 (the DQ pin) is open
drain with an internal circuit equivalent to that shown in
Figure 12.
The 1-Wire bus requires an external pullup resistor of approximately 5kΩ; thus, the idle state for the 1-Wire
bus is high. If for any reason a transaction needs to be
suspended, the bus MUST be left in the idle state if the
transaction is to resume. Infinite recovery time can occur
between bits so long as the 1-Wire bus is in the inactive
(high) state during the recovery period. If the bus is held
low for more than 480µs, all components on the bus will
be reset.
Transaction SequenceThe transaction sequence for accessing the DS18B20 is
as follows:
Step 1. Initialization
Step 2. ROM Command (followed by any required data
exchange)
Step 3. DS18B20 Function Command (followed by any
required data exchange)
It is very important to follow this sequence every time the
DS18B20 is accessed, as the DS18B20 will not respond
if any steps in the sequence are missing or out of order.
Exceptions to this rule are the Search ROM [F0h] and
Alarm Search [ECh] commands. After issuing either of
these ROM commands, the master must return to Step 1
in the sequence.
InitializationAll transactions on the 1-Wire bus begin with an initializa-
tion sequence. The initialization sequence consists of a
reset pulse transmitted by the bus master followed by
presence pulse(s) transmitted by the slave(s). The pres-
ence pulse lets the bus master know that slave devices
(such as the DS18B20) are on the bus and are ready
to operate. Timing for the reset and presence pulses is
detailed in the 1-Wire Signaling section.
ROM CommandsAfter the bus master has detected a presence pulse, it
can issue a ROM command. These commands operate
on the unique 64-bit ROM codes of each slave device
and allow the master to single out a specific device if
many are present on the 1-Wire bus. These commands
also allow the master to determine how many and what
types of devices are present on the bus or if any device
has experienced an alarm condition. There are five ROM
commands, and each command is 8 bits long. The master
device must issue an appropriate ROM command before
issuing a DS18B20 function command. A flowchart for
operation of the ROM commands is shown in Figure 13.
Search Rom [F0h]When a system is initially powered up, the master must
identify the ROM codes of all slave devices on the bus,
which allows the master to determine the number of
slaves and their device types. The master learns the
ROM codes through a process of elimination that requires
the master to perform a Search ROM cycle (i.e., Search
ROM command followed by data exchange) as many
Figure 12. Hardware Configuration
VPU
4.7kΩ
DS18B20
1-Wire PORT100Ω
MOSFET
5µA
TYP
1-Wire BUSRx
Rx = RECEIVE
Tx = TRANSMIT
DS18B20Programmable Resolution
1-Wire Digital Thermometer
