DS18S20 ,1-Wire Parasite-Power Digital ThermometerApplications● Thermostatic Controls● Available in 8-Pin SO (150 mils) and 3-Pin TO-92 ● Industrial ..
DS18S20 ,1-Wire Parasite-Power Digital ThermometerElectrical Characteristics(V = 3.0V to 5.5V, T = -55°C to +125°C, unless otherwise noted.)DD APARAM ..
DS18S20+ ,1-Wire Parasite-Power Digital ThermometerApplicationsbus that by definition requires only one data line (and • Measures Temperatures from -5 ..
DS18S20+ ,1-Wire Parasite-Power Digital ThermometerElectrical Characteristics—NV Memory(V = 3.0V to 5.5V, T = -55°C to +100°C, unless otherwise noted. ..
DS18S20+PAR ,1-Wire Parasite-Power Digital ThermometerFeatures®The DS18S20 digital thermometer provides 9-bit Celsius ● Unique 1-Wire Interface Requires ..
DS18S20+T&R ,1-Wire Parasite-Power Digital ThermometerApplications● Thermostatic Controls● Available in 8-Pin SO (150 mils) and 3-Pin TO-92 ● Industrial ..
DS18S20
1-Wire Parasite-Power Digital Thermometer
Pin Conigurations
General DescriptionThe DS18S20 digital thermometer provides 9-bit Celsius
temperature measurements and has an alarm function
with nonvolatile user-programmable upper and lower trig-
ger points. The DS18S20 communicates over a 1-Wire
bus that by definition requires only one data line (and
ground) for communication with a central microprocessor.
In addition, the DS18S20 can derive power directly from
the data line (“parasite power”), eliminating the need for
an external power supply.
Each DS18S20 has a unique 64-bit serial code, which
allows multiple DS18S20s to function on the same 1-Wire
bus. Thus, it is simple to use one microprocessor to
control many DS18S20s 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●Maximize System Accuracy in Broad Range of
Thermal Management ApplicationsMeasures Temperatures from -55°C to +125°C
(-67°F to +257°F)±0.5°C Accuracy from -10°C to +85°C9-Bit ResolutionNo External Components Required●Parasite 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) 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
DS18S20
SO (150 mils)
(DS18S20Z) 1
DS18S2023
GNDDQVDD
1 23
TOP VIEW
TO-92
(DS18S20)
DS18S20High-Precision 1-Wire Digital Thermometer
Voltage Range on Any Pin Relative to Ground ....-0.5V to +6.0V
Continuous Power Dissipation (TA = +70°C)
8-Pin SO (derate 5.9mW/°C above +70°C) ..............470.6mW
3-Pin TO-92 (derate 6.3mW/°C above +70°C) ............500mW
Operating Temperature Range .........................-55°C to +125°C
Storage Temperature Range ............................-55°C to +125°C
Lead Temperature (soldering, 10s) .................................+260°C
Soldering Temperature (reflow)
Lead(Pb)-free...............................................................+260°C
Containing lead(Pb) .....................................................+240°C
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 pul-
lup is equal to VPU. In order to meet the VIH spec of the DS18S20, the actual supply rail for the strong pullup transistor
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.(VDD = 3.0V to 5.5V, TA = -55°C to +125°C, unless otherwise noted.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSSupply VoltageVDDLocal Power (Note 1)+3.0+5.5V
Pullup Supply VoltageVPUParasite Power(Note 1, 2)+3.0+5.5V
Local Power+3.0VDD
Thermometer ErrortERR-10°C to +85°C(Note 3)±0.5°C-55°C to +125°C±2
Input Logic-LowVIL(Note 1, 4, 5)-0.3+0.8V
Input Logic-HighVIH
Local Power
(Note 1, 6)
+2.2The lower of
5.5 or VDD
+ 0.3
Parasite Power+3.0
Sink CurrentILVI/O = 0.4V (Note 1)4.0mA
Standby CurrentIDDS(Note 7, 8)7501000nA
Active CurrentIDDVDD = 5V (Note 9)11.5mA
DQ Input CurrentIDQ(Note 10)5µA
Drift(Note 11)±0.2°C
DS18S20High-Precision 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
(VDD = 3.0V to 5.5V, TA = -55°C to +100°C, unless otherwise noted.)
(VDD = 3.0V to 5.5V; TA = -55°C to +125°C, unless otherwise noted.)
Note 12: See the timing diagrams in Figure 2.Note 13: Under parasite power, if tRSTL > 960µs, a power-on reset may occur.
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSNV Write Cycle TimetWR210ms
EEPROM WritesNEEWR-55°C to +55°C50kwrites
EEPROM Data RetentiontEEDR-55°C to +55°C10years
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSTemperature Conversion TimetCONV(Note 12)750ms
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(Note 12, 13)480µs
Presence-Detect HightPDHIGH(Note 12)1560µs
Presence-Detect LowtPDLOW(Note 12)60240µs
CapacitanceCIN/OUT25pF
Figure 1. Typical Performance Curve
DS18S20 TYPICAL ERROR CURVETHERMOMETER ERROR (°C)70102030405060
TEMPERATURE (°C)
+3s ERROR
MEAN ERROR
-3s ERROR
DS18S20High-Precision 1-Wire Digital Thermometer
AC Electrical Characteristics—NV Memory
AC Electrical Characteristics
PIN
NAMEFUNCTIONTO-92SO5GNDGround4DQData Input/Output. Open-drain 1-Wire interface pin. Also provides power to the
device when used in parasite power mode (see the Powering the DS18S20 section.)3VDDOptional VDD. VDD must be grounded for operation in parasite power mode.1, 2, 6, 7, 8N.C.No Connection
Figure 2. Timing Diagrams
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
tPDHIGH
PRESENCE DETECT
tPDLOW
RESET PULSE FROM HOST
DS18S20High-Precision 1-Wire Digital Thermometer
Pin Description
OverviewFigure 3 shows a block diagram of the DS18S20, 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). The TH and TL registers are nonvolatile (EEPROM),
so they will retain data when the device is powered down.
The DS18S20 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 DS18S20).
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 DS18S20 is the ability to operate
without an external power supply. Power is instead sup-
plied through the 1-Wire pullup resistor via 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 “parasite power.” As
an alternative, the DS18S20 may also be powered by an
external supply on VDD.
Operation—Measuring TemperatureThe core functionality of the DS18S20 is its direct-to-dig-
ital temperature sensor. The temperature sensor output
has 9-bit resolution, which corresponds to 0.5°C steps.
The DS18S20 powers-up in a low-power idle state; to
initiate a temperature measurement and A-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 DS18S20 returns to its idle state.
If the DS18S20 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
DS18S20 will respond by transmitting 0 while the tem-
perature conversion is in progress and 1 when the con-
version is done. If the DS18S20 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
DS18S20 section.
Figure 3. DS18S20 Block Diagram
VPU
64-BIT ROM
AND
1-Wire PORT
VDD
INTERNAL VDD
CPP
PARASITE POWER CIRCUITMEMORY
CONTROL LOGIC
SCRATCHPAD
8-BIT CRC
GENERATOR
TEMPERATURE
SENSOR
ALARM HIGH TRIGGER (TH)
REGISTER (EEPROM)
ALARM LOW TRIGGER (TL)
REGISTER (EEPROM)
GND
DS18S204.7kΩ
POWER-
SUPPLY SENSE
DS18S20High-Precision 1-Wire Digital Thermometer
The DS18S20 output data is calibrated in degrees cen-
tigrade; 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 num-
ber 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. Table 1 gives examples of digital output data and the
corresponding temperature reading.
Resolutions greater than 9 bits can be calculated using the
data from the temperature, COUNT REMAIN and COUNT
PER °C registers in the scratchpad. Note that the COUNT
PER °C register is hard-wired to 16 (10h). After reading
the scratchpad, the TEMP_READ value is obtained by
truncating the 0.5°C bit (bit 0) from the temperature data
(see Figure 4). The extended resolution temperature can
then be calculated using the following equation:
TEMPERATURETEMP_READ0.25
COUNT_PER_CCOUNT_REMAIN
COUNT_PER_C−−+
Operation—Alarm SignalingAfter the DS18S20 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 8 through 1 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 or
equal to TL or higher than TH, an alarm condition exists
and an alarm flag is set inside the DS18S20. 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.
Table 1. Temperature/Data Relationship
TEMPERATURE (°C)DIGITAL OUTPUT (BINARY)DIGITAL OUTPUT (HEX)+85.0*0000 0000 1010 101000AAh
+25.00000 0000 0011 00100032h
+0.50000 0000 0000 00010001h0000 0000 0000 00000000h
-0.51111 1111 1111 1111FFFFh
-25.01111 1111 1100 1110FFCEh
Figure 4. Temperature Register Format
Figure 5. TH and TL Register Format
BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0
LS BYTE 262524232221202-1
BIT 15BIT 14BIT 13BIT 12BIT 11BIT 10BIT 9BIT 8
MS BYTESSSSSSSS
S = SIGN
BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 026252525222120
DS18S20High-Precision 1-Wire Digital Thermometer
The master device can check the alarm flag status of
all DS18S20s on the bus by issuing an Alarm Search
[ECh] command. Any DS18S20s with a set alarm flag will
respond to the command, so the master can determine
exactly which DS18S20s 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 DS18S20The DS18S20 can be powered by an external supply on
the VDD pin, or it can operate in “parasite power” mode,
which allows the DS18S20 to function without a local
external supply. Parasite power is very useful for applica-
tions that require remote temperature sensing or those
with space constraints. Figure 3 shows the DS18S20’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 DS18S20 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 DS18S20 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
provide sufficient current to the DS18S20 for most opera-
tions as long as the specified timing and voltage require-
ments are met (see the DC Electrical Characteristics
and the AC Electrical Characteristics). However, when
the DS18S20 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 DS18S20
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 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 DS18S20 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 DS18S20 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 DS18S20 be powered by
an external power supply.
In some situations the bus master may not know whether
the DS18S20s 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 DS18S20s will pull the bus low, and externally
powered DS18S20s 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 DS18S20 During
Temperature Conversions
Figure 7. Powering the DS18S20 with an External Supply
VPU
4.7kΩ
VPU
1-Wire BUS
DS18S20GNDDQVDD
TO OTHER
1-Wire DEVICES
VDD
(EXTERNAL SUPPLY)
VPU
4.7kΩ
1-Wire BUS
DS18S20GNDDQVDD
TO OTHER
1-Wire DEVICES
DS18S20High-Precision 1-Wire Digital Thermometer
64-Bit Lasered ROM CodeEach DS18S20 contains a unique 64-bit code (see
Figure 8) stored in ROM. The least significant 8 bits of the
ROM code contain the DS18S20’s 1-Wire family code:
10h. 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 DS18S20 to operate as a 1-Wire device using
the protocol detailed in the 1-Wire Bus System section.
MemoryThe DS18S20’s memory is organized as shown in
Figure 9. The memory consists of an SRAM scratch-
pad with nonvolatile EEPROM storage for the high and
low alarm trigger registers (TH and TL). Note that if the
DS18S20 alarm function is not used, the TH and TL reg-
isters can serve as general-purpose memory. All mem-
ory commands are described in detail in the DS18S20
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. Bytes 4 and 5 are reserved for internal
use by the device and cannot be overwritten; these bytes
will return all 1s when read. Bytes 6 and 7 contain the
COUNT REMAIN and COUNT PER ºC registers, which
can be used to calculate extended resolution results as
explained in the Operation—Measuring Temperature
section.
Byte 8 of the scratchpad is read-only and contains the
CRC code for bytes 0 through 7 of the scratchpad.
The DS18S20 generates this CRC using the method
described in the CRC Generation section.
Data is written to bytes 2 and 3 of the scratchpad using
the Write Scratchpad [4Eh] command; the data must be
transmitted to the DS18S20 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 and TL
data from the scratchpad to EEPROM, the master must
issue the Copy Scratchpad [48h] command.
Figure 8. 64-Bit Lasered ROM Code
8-BIT CRC48-BIT SERIAL NUMBER8-BIT FAMILY CODE (10h)
MSBLSBMSBLSBMSBLSB
BYTE 0
BYTE 1
TEMPERATURE LSB (AAh)
TEMPERATURE MSB (00h)(85°C)
BYTE 2
BYTE 3
TH REGISTER OR USER BYTE 1*
TL REGISTER OR USER BYTE 2*
BYTE 4
BYTE 5
RESERVED (FFh)
RESERVED (FFh)
BYTE 6
BYTE 7
COUNT REMAIN (0Ch)
COUNT PER °C (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
SCRATCHPAD
(POWER-UP STATE)
EEPROMDS18S20High-Precision 1-Wire Digital Thermometer
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” (see the 1-Wire
Bus System section) following the Recall E2 command
and the DS18S20 will indicate the status of the recall by
transmitting 0 while the recall is in progress and 1 when
the recall is done.
CRC GenerationCRC bytes are provided as part of the DS18S20’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 DS18S20. 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
DS18S20 that prevents a command sequence from pro-
ceeding if the DS18S20 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 DS18S20 using the polyno-
mial generator shown in Figure 10. 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 re-calculated CRC.
Next, the 8-bit ROM code or scratchpad CRC from the
DS18S20 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 ịButton Products.
1-Wire Bus SystemThe 1-Wire bus system uses a single bus master to con-
trol one or more slave devices. The DS18S20 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).
Figure 10. CRC Generator
(MSB) (LSB)
XOR XOR XOR
INPUT
DS18S20High-Precision 1-Wire Digital Thermometer
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 transmitting
data so the bus is available for use by another device. The
1-Wire port of the DS18S20 (the DQ pin) is open drain with
an internal circuit equivalent to that shown in Figure 11.
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 DS18S20 is
as follows:
Step 1. Initialization
Step 2. ROM Command (followed by any required data
exchange)
Step 3. DS18S20 Function Command (followed by any
required data exchange)
It is very important to follow this sequence every time the
DS18S20 is accessed, as the DS18S20 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 DS18S20) 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 com-
mands, and each command is 8 bits long. The master
device must issue an appropriate ROM command before
issuing a DS18S20 function command. A flowchart for
operation of the ROM commands is shown in Figure 16.
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 times as
necessary to identify all of the slave devices. If there is
only one slave on the bus, the simpler Read ROM com-
mand (see below) can be used in place of the Search
ROM process. For a detailed explanation of the Search
ROM procedure, refer to the ịButton® Book of Standards
at www.maximintegrated.com/ibuttonbook. After every
Search ROM cycle, the bus master must return to Step 1
(Initialization) in the transaction sequence.
Read ROM [33h]This command can only be used when there is one slave
on the bus. It allows the bus master to read the slave’s
64-bit ROM code without using the Search ROM proce-
dure. If this command is used when there is more than
one slave present on the bus, a data collision will occur
when all the slaves attempt to respond at the same time.
Figure 11. Hardware Configuration
VPU
4.7kΩ
DS18S20
1-Wire PORT100Ω
MOSFET
5µA
TYP
1-Wire BUSRx
Rx = RECEIVE
Tx = TRANSMIT
DS18S20High-Precision 1-Wire Digital Thermometer
Note 1: For parasite-powered DS18S20s, the master must enable a strong pullup on the 1-Wire bus during temperature conver-
sions and copies from the scratchpad to EEPROM. No other bus activity may take place during this time.
Note 2: The master can interrupt the transmission of data at any time by issuing a reset.
Note 3: Both bytes must be written before a reset is issued.
Match ROM [55h]The match ROM command followed by a 64-bit ROM
code sequence allows the bus master to address a
specific slave device on a multidrop or single-drop bus.
Only the slave that exactly matches the 64-bit ROM code
sequence will respond to the function command issued
by the master; all other slaves on the bus will wait for a
reset pulse.
Skip ROM [CCh]The master can use this command to address all devices
on the bus simultaneously without sending out any ROM
code information. For example, the master can make all
DS18S20s on the bus perform simultaneous temperature
conversions by issuing a Skip ROM command followed by
a Convert T [44h] command.
Note that the Read Scratchpad [BEh] command can fol-
low the Skip ROM command only if there is a single slave
device on the bus. In this case, time is saved by allowing
the master to read from the slave without sending the
device’s 64-bit ROM code. A Skip ROM command followed
by a Read Scratchpad command will cause a data collision
on the bus if there is more than one slave since multiple
devices will attempt to transmit data simultaneously.
Alarm Search [ECh]The operation of this command is identical to the operation
of the Search ROM command except that only slaves with
a set alarm flag will respond. This command allows the
master device to determine if any DS18S20s experienced
an alarm condition during the most recent temperature
conversion. After every Alarm Search cycle (i.e., Alarm
Search command followed by data exchange), the bus
master must return to Step 1 (Initialization) in the transac-
tion sequence. See the Operation—Alarm Signaling sec-
tion for an explanation of alarm flag operation.
DS18S20 Function CommandsAfter the bus master has used a ROM command to
address the DS18S20 with which it wishes to communi-
cate, the master can issue one of the DS18S20 function
commands. These commands allow the master to write
to and read from the DS18S20’s scratchpad memory,
initiate temperature conversions and determine the power
supply mode. The DS18S20 function commands, which
are described below, are summarized in Table 2 and illus-
trated by the flowchart in Figure 17.
Table 2. DS18S20 Function Command Set
COMMANDDESCRIPTIONPROTOCOL1-Wire BUS ACTIVITY AFTER COMMAND IS ISSUEDNOTES
TEMPERATURE CONVERSION COMMANDSConvert TInitiates temperature
conversion.44h
DS18S20 transmits conversion status
to master (not applicable for parasite-
powered DS18S20s).
MEMORY COMMANDSRead ScratchpadReads the entire scratchpad
including the CRC byte.BEhDS18S20 transmits up to 9 data
bytes to master.2
Write ScratchpadWrites data into scratchpad
bytes 2 and 3 (TH and TL).4EhMaster transmits 2 data bytes to
DS18S20.3
Copy ScratchpadCopies TH and TL data from the
scratchpad to EEPROM.48hNone1
Recall E2Recalls TH and TL data from
EEPROM to the scratchpad.B8hDS18S20 transmits recall status to
master.
Read Power
Supply
Signals DS18S20 power supply
mode to the master.B4hDS18S20 transmits supply status to
master.
DS18S20High-Precision 1-Wire Digital Thermometer
