DS18S20 - Dallas Semiconductor

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FEATURES

 Unique 1–Wire interface requires only one

port pin for communication

 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

 Zero standby power required

 Measures temperatures from –55°C to

+125°C. Fahrenheit equivalent is –67°F to

+257°F

 ±0.5°C accuracy from –10°C to +85°C

 Temperature is read as a 9–bit digital value

 Converts temperature to digital word in

750 ms (max.)

 User–definable, nonvolatile temperature

alarm settings

 Alarm search command identifies and

addresses devices whose temperature is

outside of programmed limits (temperature

alarm condition)

 Functionally compatible with DS1820 1-Wire

digital thermometer

 Applications include thermostatic controls,

industrial systems, consumer products,

thermometers, or any thermally sensitive

system

PIN ASSIGNMENT

PIN DESCRIPTION

GND - Ground

DQ - Data In/Out

VDD - Power Supply Voltage

NC - No Connect

DESCRIPTION

The DS18S20 Digital Thermometer provides 9–bit temperature readings which indicate the temperature

of the device.

Information is sent to/from the DS18S20 over a 1–Wire interface, so that only one wire (and ground)

needs to be connected from a central microprocessor to a DS18S20. Power for reading, writing, and

performing temperature conversions can be derived from the data line itself with no need for an external

power source.

Because each DS18S20 contains a unique silicon serial number, multiple DS18S20s can exist on the

same 1–Wire bus. This allows for placing temperature sensors in many different places. Applications

where this feature is useful include HVAC environmental controls, sensin g tempe ratures inside buildings,

equipment or machinery, and process monitoring and control.

DS18S20

High Precision

1-Wire®Digital Thermomete

www.dalsemi.com

PRELIMINARY

DALLAS

DS18S20

1 2 3

GND

VDD

1 2 3

BOTTOM VIEW

DS18S20 To-92

Package

GND

VDD

DS18S20Z

8-Pin SOIC (150 mil)

DS18S20

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DETAILED PIN DESCRIPTION

8-PIN SOIC PIN

TO92 SYMBOL DESCRIPTION

5 1 GND Ground.

42DQData Input/Output pin. For 1-Wire operation: Open drain.

(See “Parasite Power” section.)

33V

DD Optional VDD pin. See “Parasite Power” section for details of

connection. VDD must be grounded for operation in parasite

power mode.

DS18S20Z (8-pin SOIC): All pins not specified in this table are not to be connected.

OVERVIEW

The block diagram of Figure 1 shows the major components of the DS18S20. The DS18S20 has three

main data components: 1) 64–bit lasered ROM, 2) temperature sensor, 3) nonvolatile temperature alarm

triggers TH and TL. The device derives its power from the 1–Wire communication line by storing energy

on an internal capacitor during periods of time when the signal line is high and continues to operate off

this power source during the low times of the 1–Wire line until it returns high to replenish the parasite

(capacitor) supply. As an alternative, the DS18S20 may also be powered from an external 3 volt – 5 volt

supply.

Communication to the DS18S20 is via a 1–Wire port. With the 1–Wire port, the memory and control

functions will not be available before the ROM function protocol has been established. The master must

first provide one of five ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, 4)

Skip ROM, or 5) Alarm Search. These commands operate on the 64–bit lasered ROM portion of each

device and can single out a specific device if many are present on the 1–Wire line as well as indicate to

the bus master how many and what types of devices are present. After a ROM function sequence has

been successfully executed, the memory and control functions are accessible and the master may then

provide any one of the six memory and control function commands.

One control funct ion comm and inst ructs th e DS18S 20 to perform a t emperat u re measu rem ent. The resul t

of this measurement will be placed in the DS18S20’s scratch-pad memory, and may be read by issuing a

memory function command which reads the contents of the scratchpad memory. The temperature alarm

triggers TH and TL consist of 1-byte EEPROM each. If the alarm search command is not applied to the

DS18S20, these registers may be used as general purpose user memory. Writing TH and TL is done

using a memory function command. Read access to these registers is through the scratchpad. All data is

read and written least significant bit first.

DS18S20

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DS18S20 BLOCK DIAGRAM Figure 1

PARASITE POWER

The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power

whenever the DQ o r VDD pins are high. DQ will provide sufficient power as long as the specified timing

and voltage requirements are met (see the section titled “1–Wire Bus System”). The advantages of

parasite power are twofold: 1) by parasiting off this pin, no local power source is needed for remote

sensing of temperature, and 2) the ROM may be read in absence of normal power.

In order for the DS18S20 to be able to perform accurate temperature conversions, sufficient power must

be provided over the DQ line when a temperature conv ersion is taking place. Since the operating curre nt

of the DS18S20 is up to 1.5 mA, the DQ line will not have sufficient drive due to the 5k pullup resistor.

This problem is particularly acute if several DS18S20s are on the same DQ and attempting to convert

simultaneously.

There are two ways to assure that the DS18S20 has sufficient suppl y current during its active conversion

cycle. The first is to provide a strong pullup on the DQ line whenever temper ature conversions or copies

to the E2 memory are taking place. This may be accomplished by using a MOSFET to pull the DQ line

directly to the power supply as shown in Figure 2. The DQ line must be switched over to the strong

pull-up within 10 µs maximum after issuing any protocol that involves copying to the E2 memory or

initiates temperature conversions. When using the parasite power mode, the VDD pin must be tied to

ground.

Another method of supplying current to the DS18S20 is through the use of an ex ternal power suppl y tied

to the VDD pin, as shown in Figure 3. The advantage to this is that the strong pullup is not required on the

DQ line, and the bus master need not be tied up holding that line high during temperature conversions.

This allows other data traffic on the 1–Wire bus during the conversion time. In addition, any number of

DS18S20s may be placed on the 1–Wire bus, and if they all use external power, they may all

simultaneously perform temper ature conversions by issuing the Skip ROM command and then issuing the

Convert T command. Note that as long as the external power supply is active, the GND pin may not be

floating.

The use of parasite power is not recommended above 100°C, since it may not be able to sustain

communications given the higher leakage currents the DS18S20 exhibits at these temperatures. For

applications in which such temperatures are likel y, it is strongly recommended that VDD be applied to the

DS18S20.

POWER

SUPPLY

SENSE

64-BIT R OM

AND

1-WIRE PORT

MEMORY AND

CONTROL LOGIC

SCRATCHPAD

8-BIT CRC

GENERATOR

TEMPERATURE

SENSOR

HIGH TEMPERATURE

TRIGGER, TH

LOW TEM P E RATURE

TRIGGER, TH

VDD

INTERNAL V

DS18S20

4 of 27

For situations where the bus master does not know whether the DS18S20s on the bus are parasite

powered or supplied with external VDD, a provision is made in the DS18S20 to signal the power supply

scheme used. The bus master can determine if any DS18S20s are on the bus which require the strong

pullup by sending a Skip ROM protocol, then issuing the read power supply command. After this

command is issued, the master then issues read time slots. The DS18S20 will send back “0” on the

1-Wire bus if it is parasite powered; it will send back a “1” if it is powered from the VDD pin. If the

master receives a “0,” it knows that it must supply the strong pullup on the DQ line during temperature

conversions. See “Memory Command Functions” section for more detail on this command protocol.

STRONG PULL-UP FOR SUPPLYING DS18S20 DURING TEMPERATURE

CONVERSION Figure 2

USING VDD TO SUPPLY TEMPERATURE CONVERSION CURRENT Figure 3

+3V - +5V

DS18S20

+3V - +5V

4.7k GND VDD

I/O

µµ

+3V - +5V

4.7k

µµ

DS18S20

GND VDD

I/O

TO OTHER

1-WIRE D EVICES

EXTERNAL +3V - +5V

SUPPLY

DS18S20

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OPERATION – MEASURING TEMPER ATURE

The core functionality of the DS18S20 is its direct-to-digital temperature sensor. The direct readout of

temperature data is 9 bits wide, equating to a resolution of 0.5°C; if higher resolution is desired, an

algorithm is desc ribed later in this section to achieve that need. Following the issuance of the Convert T

[44h] command, a temperature conversion is performed and the thermal data is stored in the scratchpad

memory in a 16-bit, sign-extended two’s complement format. The temperature information can be

retrieved over the 1-Wire interface by issuing a Read Scratchpad [BEh] command once the conversion

has been performed. The data is transferred ov er the 1 -W ire bus, LSB first. The MSB of the temp eratur e

Table 2 describes the exact r elationship of output data to measured. For Fahrenheit usage, a lookup table

or conversion routine must be used.

Temperature/Data Rela ti onships Table 2

262524232221202-1 LSB

MSb (unit = °C) LSb

SSSSSSSSMSB

TEMPERATURE DIGITAL OUTPUT

(Binary) DIGITAL

OUTPUT

(Hex)

+85°C 0000 0101 0101 0000 0550h*

+125°C 0000 0000 1111 1010 00FAh

+25.0°C 0000 0000 0011 0010 0032h

+0.5°C 0000 0000 0000 0001 0001h

0°C 0000 0000 0000 0000 0000h

-0.5°C 1111 1111 1111 1111 FFFFh

-25.0°C 1111 1111 1100 1110 FFCEh

-55°C 1111 1111 1001 0010 FF92h

*The power on reset register value is +85°C

Higher resolutions may be obtained by the following procedure. First, read the temperature,

COUNT_REMAIN, and COUNT_PER_C registers from the scratchpad. Truncate the 0.5°C bit (the

LSB) from the temperature value. This value is TEMP_READ. The actual temperature may then be

calculated by using the following:

CPERCOUNT REMAINCOUNTCPERCOUNT

READTEMPETEMPERATUR __ ___

25.0_ −

+−=

O PE RAT ION – ALAR M SIGNALING

After the DS18S20 has performed a temperature conversion, the temperature value is compared to the

trigger values stored in TH and TL. Since these registers are 8–bit only, the 0.5°C bit is ignored for

comparison. The most significant bit of TH or TL directly corresponds to the sign bit of the 16–bit

temperature register. If the result of a temperature measurement is higher than TH or lower than TL, an

alarm flag inside the device is set. This flag is updated with ever y temperature me asurement. As long as

DS18S20

6 of 27

the alarm flag is set, the DS18S20 will respond to the alarm search command. This allows many

DS18S20s to be connected in parallel doing simultaneous temperature measurements. If somewhere the

temperature exceeds the limits, the alarming device(s) can be identified and read immediately without

having to read non–alarming devices.

64-BIT LASERED ROM

Each DS18S20 contains a unique ROM code that is 64–bits long. The first 8 bits are a 1–Wire family

code (DS18S20 code is 10h). The next 48 bits are a unique serial number. The last 8 bits are a CRC of

the first 56 bits. (See Fi gure 4.) The 64–bit ROM and ROM Function Control section allow the DS18S20

to operate as a 1–Wire device and follow the 1–Wire protocol detailed in the section “1–Wire Bus

System.” The functions required to control sections of the DS18S20 are not accessible until the ROM

function protocol has been satisfied. This protocol is described in the ROM function protocol flowchart

(Figure 5). The 1–Wire bus master must first provide one of five ROM function commands: 1) Read

ROM, 2) Match ROM, 3) Search ROM, 4) Skip ROM, or 5) Alarm Search. After a ROM functions

sequence has been successfully executed, the functions specific to the DS18S20 are accessible and the

bus master may then provide one of the six memory and control function commands.

CRC GENERATION

The DS18S20 has an 8–bit CRC stored in the most significant byte of the 64–bit ROM. The bus master

can compute a CRC value from the first 56–bits of the 64–bit ROM and compare it to the value stored

within the DS18S20 to determine if the ROM data has been received error–free by the bus master. The

equivalent polynomial function of this CRC is:

CRC = X8 + X5 + X4 + 1

The DS18S20 also generates an 8–bit CRC value using the same polynomial function shown above and

provides this value to the bus master to validate the transfer of data bytes. In each case where a CRC is

used for data transfer validation, the bus master must calculate a CRC value using the polynomial

function given above and compare the calculated value to either the 8–bit CRC value stored in the 64–bit

ROM portion of the DS18S20 (for ROM reads) or the 8–bit CRC value computed within the DS18S20

(which is read as a ninth byte wh en the scratchpad is read). The comparison of CRC values and decision

to continue with an operation are determined entirel y by the bus master. There is no circuitry inside the

DS18S20 that prevents a command sequence from proceeding if the CRC stored in or calculated by the

DS18S20 does not match the value generated by the bus master.

The 1–Wire CRC can be generated using a polynomial generator consisting of a shift register and XOR

gates as shown in Figure 6. Additional information about the Dallas 1–Wire Cyclic Redundancy Check is

available in Application Note 27 entitled “Understanding and Using Cyclic Redundancy Checks with

Dallas Semiconductor Touch Memory Products.”

The shift re gister bits are initialized to “0”. Then startin g with the least significant bit of the family code,

one bit at a time is shifted in. After the eighth bit of the family code has been entered, then the serial

number is entered. After th e 48th bit of the serial number has been entered, the shift register contains the

CRC value. Shifting in the 8 bits of CRC should return the shift register to all “0s”.

64-BIT LASERED ROM Figure 4

8-BIT CRC CODE 48-BIT SERIAL NUMBER 8-BIT FAMILY CODE (10h)

MSB MSBLSB LSB LSBMSB

DS18S20

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ROM FUNCTIONS FLOW CHART Figure 5

CCh

SKIP ROM

COMMAND

DS18S20 TX

FAMILY CODE

1 BYTE

DS18S20 TX

SERIAL NUMBER

6 BYTES

MASTER T X

RESET PULS E

DS18S20 TX

PRESENCE

PULSE

MASTER T X ROM

FUNCTION COMMAND

33h

READ ROM

COMMAND

55h

MATCH ROM

COMMAND

F0h

SEARCH ROM

COMMAND

ECh

ALARM

COMMAND

ALARM

CONDITION ?

MASTER T X

BIT 0

DS18S20 T

BIT 0

DS18S20 T

BIT 0

MASTER T X BIT 0

DS18S20 T

BIT 1

DS18S20 T

BIT 1

MASTER T X BIT 1

DS18S20 T

BIT 63

DS18S20 TX BIT 63

MASTER T X BIT 63

BIT 0

MATCH? BIT 0

MATCH?

DS18S20 TX

CRC BYTE MASTER T X

BIT 1

MATCH? BIT 1

MAT

BIT 63

MAT

H? BIT 63

MAT

MASTER T X

BIT 63

YYYYY

NNN

MASTER T X MEMORY OR CONTROL

FUNCTION COMMAND

DS18S20

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1-WIRE CRC CODE Figure 6

MEMORY

The DS18S20’s memory is organized as shown in Figure 7. The memor y consists of a scratchpad RAM

and a nonvolatile, electrically erasable (E2) RAM, which stores the high and low temper ature tri ggers, TH

and TL. The scratchpad helps insure data integrity when communicating over the 1–Wire bus. Data is

first written to the scratchpad where it can be read back. After the data has been verified, a copy

scratchpad command will transfer the data to the nonvolatile (E2) RAM. This process insures data

integrity when modifying the memory. The DS18S20 EEPROM is rated for a minimum of 50,000 writes

and 10 years data retention at T = +55°C.

The scratchpad is organized as 8 bytes of memory. The first 2-bytes contain the measured temperature

information. The third and fourth bytes are volatile copies of TH and TL and are refreshed with every

power–on reset. The nex t 2 bytes are not used; upon reading back, however, th ey will appear as all logic

“1s”. The seventh and eighth bytes are count registers, which may be used in obtaining higher

temperature resolution (see “Operation–measuring Temperature” section). There is a ninth byte which

may be read with a Read Scratchpad command. This byte contains a cyclic redundancy check (CRC)

byte which is the CRC over all of the 8 previous bytes. This CRC is implemented in the fashion

described in the section titled “CRC Generation."

DS18S20 MEMORY MAP Figure 7

TEMPERATURE LSB

TEMPERATURE MSB

TH/USER BYTE 1

TL/USER BYTE 2

RESERVED

COUNT REMAIN

COUNT PER °C

CRC

TH/USER BYTE 1

TL/USER BYTE 2

RESERVED

SCRATCHPAD BYTE

E2RAM

(MSB) (LSB)

XOR XOR XOR

INPUT

DS18S20

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1-WIRE BUS SYSTEM

The 1–Wire bus is a system which has a single bus master and one or more slaves. The DS18S20

behaves as a slave. The discussion of this bus system is broken down into three topics: hardware

configuration, transaction sequence, and 1–Wire signaling (signal types and timing).

HARDWARE CONFIGUR ATION

The 1–Wire bus has onl y a single line b y definition; it is i mportant that each device on t he bus be able to

drive it at the appropriate time. To facilitate this, each device atta ched to the 1–Wire bus must have op en

drain or 3–state outputs. The 1–Wire port of the DS18S20 (DQ pin) is open dr ain with an internal ci rcuit

equivalent to that shown in Figure 8. A multidrop bus consists of a 1–Wire bus with multiple slaves

attached. The 1–Wire bus requires a pullup resistor of approximately 5 kΩ.

HARDWARE CONFIGURATION Figure 8

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 this does not

occur and the bus is left low for more than 480 µs, all components on the bus will be reset.

TRANSACTION SEQUENCE

The protocol for accessing the DS18S20 via the 1–Wire port is as follows:

• Initialization

• ROM Function Command

• Memory Function Command

• Transaction/Data

+3V - +5V

4.7k

5 µA

Typ.

DS18S20 1-WIRE PORT

100Ω

ΩΩ

Ω

FET

RX = RECEIVE

TX = TRANSMIT

BUS MASTER

DS18S20

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INITIALIZATION

All transactions on the 1–Wire bus begin with an initialization 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 presence pulse l ets the bus master know that the DS18S20 is on the bus and is read y to operate. For

more details, see the “1–Wire Signaling” section.

ROM FUNCTION COMMANDS

Once the bus master has detected a presence, it can issue one of th e five ROM function commands. All

ROM function commands are 8 bits long. A list of these commands follows (refer to flowchart in

Figure 5):

Read ROM [33h]

This command allows the bus master to read the DS18S20’s 8–bit family code, unique 48–bit serial

number, and 8–bit CRC. This command can only be used if there is a single DS18S20 on the bus. If

more than one slav e is present on the bus, a data collision will occur when all slaves tr y to transmit at the

same time (open drain will produce a wired AND result).

Match ROM [55h]

The match ROM command, followed by a 64–bit ROM sequence, allows the bus master to address a

specific DS18S20 on a multidrop bus. Onl y the DS18S20 that exactly matches the 64–bit ROM sequence

will respond to the following memory function command. All slaves that do not match the 64–bit ROM

sequence will wait for a reset pulse. This command can be us ed with a single or multiple devices on the

bus.

Skip ROM [CCh]

This command can save time in a single drop bus system by allowing the bus master to access the

memory functions without providing the 64–bit ROM code. If more than one slave is present on the bus

and a Read command is issued following the Skip ROM command, data collision will occur on the bus as

multiple slaves transmit simultaneously (open drain pulldowns will produce a wired AND result).

Search ROM [F0h]

When a system is initially brought up, the bus master might not know the number of devices on the

1-Wire bus or their 64–bit ROM codes. The search ROM command allows the bus master to use a

process of elimination to identify the 64–bit ROM codes of all slave devices on the bus.

Alarm Search [ECh]

The flowchart of this command is identical to the Search ROM command. However, the DS18S20 will

respond to this command only if an alarm condition has been encountered at the last temperature

measurement. An alarm condition is defined as a temperature higher than TH or lower than TL. The

alarm condition remains set as long as the DS18S20 is powered up, or until another temperature

measurement reveals a non–alarming value. For alarming, the trigger values stored in EEPROM are

taken into account. If an alarm condition exists and the TH or TL settings are changed, another

temperature conversion should be done to validate any alarm conditions.

DS18S20

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Example of a ROM Search

The ROM search process is the repetition of a simple three–step routine: read a bit, read th e complement

of the bit, then write the desired value o f that bit. The bus master performs this simple, three–step routine

on each bit of the ROM. After one complete pass, the bus m aster knows the contents of the ROM in one

device. The remaining number of devices and their ROM codes may be identified by additional passes.

The following example of the ROM search process assumes four different devices are connected to the

same 1–Wire bus. The ROM data of the four devices is as shown:

ROM1 00110101...

ROM2 10101010...

ROM3 11110101...

ROM4 00010001...

The search process is as follows:

1. The bus master begins the initialization sequence by issuing a reset pulse. The slave devic es respond

by issuing simultaneous presence pulses.

2. The bus master will then issue the Search ROM command on the 1–Wire bus.

3. The bus master reads a bit from the 1–Wire bus. Each device will respond by pl acin g the valu e of the

first bit of their respective ROM data onto the 1–Wire bus. ROM1 and ROM4 will place a 0 onto the

1–Wire bus, i.e., pull it low. ROM2 and ROM3 will place a 1 onto the 1–Wire bus by allowing the

line to stay high. The result is the logical AND of all devices on the line, therefore the bus master

sees a 0. The bus master r eads another bit. Si nce the Search ROM data command is bein g executed,

all of the devices on the 1–Wire bus respond to this second read by placing the complement of the

first bit of their respective ROM data onto the 1–Wire bus. ROM1 and ROM4 will place a 1 onto the

1–Wire, allowing the line to stay high. ROM2 and ROM3 will place a 0 onto the 1–W ire, thus it will

be pulled low. The bus master again obse rves a 0 for the complement of the first ROM data bit. The

bus master has determined that there are some devices on the 1–Wire bus that have a 0 in the first

position and others that have a 1.

The data obtained from the two reads of the three–step routine have the following interpretations:

00 There are still devices attached which have conflicting bits in this position.

01 All devices still coupled have a 0–bit in this bit position.

10 All devices still coupled have a 1–bit in this bit position.

11 There are no devices attached to the 1–Wire bus.

4. The bus master writes a 0. This deselects ROM2 and ROM3 for the remainder of this search pass,

leaving only ROM1 and ROM4 connected to the 1–Wire bus.

5. The bus master performs two more reads and r eceiv es a 0–bit followed by a 1–bit. This indicates that

all devices still coupled to the bus have 0s as their second ROM data bit.

6. The bus master then writes a 0 to keep both ROM1 and ROM4 coupled.

7. The bus master executes two reads and re ceives two 0–bits. This indicates that both 1–bits and 0–bits

exist as the third bit of the ROM data of the attached devices.

DS18S20

12 of 27

8. The bus master writes a 0–bit. This deselects ROM1, leaving ROM4 as the only device still

connected.

9. The bus master reads the remainder of the ROM bits for ROM4 and continues to access the part if

desired. This completes the first pass and uniquely identifies one part on the 1–Wire bus.

10. The bus master starts a new ROM search sequence by repeating steps 1 through 7.

11. The bus master writes a 1–bit. This decouples ROM4, leaving only ROM1 still coupled.

12. The bus master reads the remainder of the ROM bits for ROM1 and communicates to the underlying

logic if desired. This completes the second ROM search pass, in which another of the ROMs was

found.

13. The bus master starts a new ROM search by repeating steps 1 through 3.

14. The bus master writes a 1–bit. This deselects ROM1 and ROM4 for the remainder of this search pass,

leaving only ROM2 and ROM3 coupled to the system.

15. The bus master executes two Read time slots and receives two 0s.

16. The bus master writes a 0–bit. This decouples ROM3, leaving only ROM2.

17. The bus master reads the remainder of the ROM bits for ROM2 and communicates to the underlying

logic if desired. This completes the third ROM search pass, in which another of the ROMs was

found.

18. The bus master starts a new ROM search by repeating steps 13 through 15.

19. The bus master writes a 1–bit. This decouples ROM2, leaving only ROM3.

20. The bus master reads the remainder of the ROM bits for ROM3 and communicates to the underlying

logic if desired. This completes the fourth ROM search pass, in which another of the ROMs was

found.

NOTE:

The bus master learns the unique ID number (ROM data pattern) of one 1–Wire device on each ROM

Search operation. The time required to derive the part’s unique ROM code is:

960 µs + (8 + 3 x 64) 61 µs = 13.16 ms

The bus master is therefore capable of identifying 75 different 1–Wire devices per second.

I/O SIGNALING

The DS18S20 requires strict protocols to insure data integrity. The protocol consists of several types of

signaling on one line: reset pulse, presence pulse, write 0, write 1, read 0, and read 1. All of these signals,

with the exception of the presence pulse, are initiated by the bus master.

DS18S20

13 of 27

The initialization sequence required to begin any communication with the DS18S20 is shown in Figure

10. A reset pulse followed by a presence pulse indicates the DS18S20 is ready to send or receive data

given the correct ROM command and memory function command.

The bus master transmits (TX) a reset pulse (a low signal for a minimum of 480 ms). The bus master

then releases the line and goes into a receive mode (RX). The 1–Wire bus is pul led to a hi gh state via the

5k pullup resistor. After detectin g the risin g edge on the DQ pin, the DS18S20 waits 15–60 µs and then

transmits the presence pulse (a low signal for 60–240 µs).

MEMO RY COMMAND FUNCTIO NS

The following command protocols are summarized in Table 2, and by the flowchart of Figure 9.

Write Scratchpad [4Eh]

This command writes to the scratchpad of the DS18S20, starting at the TH register. The next 2 bytes

written will be saved in scratchpad memory at address locations 2 and 3. Writing may be terminated at

any point by issuing a reset.

Read Scratchpad [BEh]

This command reads the contents of the scratchpad. Reading will commence at b yte 0, and will continue

through the scratchpad until the ninth (byte 8, CRC) byte is read. If not all locations are to be read, the

master may issue a reset to terminate reading at any time.

Copy Scratchpad [48h]

This command copies the scratchpad into the E2 memory of the DS18S20, storing the temperature tri gger

bytes in nonvolatile memory. If the bus master issues read time slots following this command, the

DS18S20 will output 0 on the bus as long as it is busy copying the scratchpad to E2; it will return a 1

when the cop y process is compl ete. If parasite-power ed, the bus master has to enable a strong pull up for

at least 10 ms immediately after issuing this command. The DS18S20 EEPROM is rated for a minimum

of 50,000 writes and 10 years data retention at T = +55°C.

Convert T [44h]

This command begins a temperature conversion. No further data is required. The temperature

conversion will be performed and then the DS18S20 will remain idle. If the bus master issues read time

slots following this command, the DS18S20 will output 0 on the bus as long as it is busy making a

temperature conversion; it will return a 1 when the temperature conversion is complete. If parasite-

powered, the bus master has to enable a strong pullup for a time period greater than tconv immediately

after issuing this command.

Recall E2 [B8h]

This command recalls the temperature trigger values stored in E2 to the scratchpad. This recall operation

happens automatically upon power–up to the DS18S20 as well, so valid data is available in the scratchpad

as soon as the device has power applied. With every read data time slot issued after this command has

been sent, the device will output its temperature converter busy flag: 0=busy, 1=ready.

Read Power Suppl y [B4h]

With every read data time slot issued after this command has been sent to the DS18S20, the device will

signal its power mode: 0=parasite power, 1=external power supply provided.

DS18S20

14 of 27

MEMORY FUNCTIONS FLOW CHART Figure 9

DS18S20 TX

SERIAL NUMBER

6 BYTES

DS18S20 SETS ADDRESS

COUNTER TO 0

BEh

READ

SCRATCHPAD

DS18S20 INCREMENTS

ADDRESS

MASTER

TX RESET

MASTER RX DATA

FROM SCRATCHPAD

MASTER

TX RESET

MASTER RX 8-BIT

CRC OF DATA

MASTER

TX RESET

MASTER T X MEMO R Y OR

CONTROL COMMAND

MASTER

TX RESET

DS18S20 SETS ADDRESS

COUNTER TO 2

4Eh

WRITE

SCRATCHPAD

MASTER T X DATA BYTE

TO SCRATCHPAD

DS18S20 INCREMENTS

ADDRESS

DS18S20 TX

PRESENCE PULSE

MASTER RX “1s”

DS18S20

15 of 27

MEMORY FUNCTIONS FLOW CHART Figure 9 (cont’d)

MASTER ENABLES

STRONG PULLUP FOR

10ms

48h

COPY

SCRATCHPAD

NPARASITE

POWER

44h

CONVERT

TEMPERATURE

PARASITE

POWER

DS18S20 BEGINS

CONVERSION

MASTER DISABLES

STRONG PULLUP

MASTER ENABLES

STRONG PULL-UP

DS18S20 CONVERTS

TEMPERATURE

MASTER DISABLES

STRONG PULL-UP

MASTER

RX “1”S

MASTER T X

RESET

DEVICE BUSY

CONVERTING

TEMPERATURE

MASTER

“0”S

MASTER

RX “1”S

MASTER T X

RESET

NONVOLATILE

MEMORY

BUSY

MASTER

“0”S

DS18S20

16 of 27

MEMORY FUNCTIONS FLOW CHART Figure 9 (cont’d)

MASTER T X

RESET

DEVICE

BUSY CONVERT ING

TEMPERATURE

MASTER

RX “1”S MASTER

“0”S

DS18S20 RECALLS

FROM E2 PROM

B8h

RECALL

MASTER T X

RESET

PARASITE

POWERED

MASTER

RX “1”S MASTER

“0”S

B4h

READ

POWERSUPPLY

MASTER

TX RESET

DS18S20

17 of 27

INITIALI ZATION PROCEDURE “RESET AND PRESENCE PULSES” Figure 10

DS18S20 COMMAND SET Table 2

INSTRUCTION DESCRIPTION PROTOCOL

1-WIRE BUS

AFTER ISSUING

PROTOCOL NOTES

TEMPERATURE CONVERSION COMMANDS

Convert T Initiates temperature

conversion. 44h <read temperature busy

status> 1

MEMORY COMMANDS

Read Scratchpad Reads bytes from

scratchpad and reads

CRC byte.

BEh <read data up to 9 bytes>

Write Scratchpad Writes bytes into

scratchpad at addresses 2

and 3 (TH and TL

temperature triggers).

4Eh <write data into 2 bytes

at addr. 2 and addr 3>

Copy Scratchpad Copies scratchpad into

nonvolatile memory

(addresses 2 and 3 only).

48h <read cop y status> 2

Recall E2Recalls values stored in

nonvolatile memory into

scratchpad (temperature

triggers).

B8h <read temperature busy

status>

Read Power Supply Signals the mode of

DS18S20 power supply

to the master.

B4h <read supply status>

LINE TYPE LEGEND

Bus master active l o w DS18S20 acti ve low

Both bus master and Resistor pul l - up

DS18S20 acti ve low

Master TX "reset pulse"

480 µ

µµ

µs minmum

960 µ

µµ

µs maximum

Master

480 µ

µµ

µs minmum

DS18S2 0 TX

waits

15-60 µ

µµ

µs

DS18S2 0 TX

"presence Pulse"

60-240 µ

µµ

µs

VCC

GND

1-WIRE

DS18S20

18 of 27

NOTES:

1. Temperature conversion takes up to 750 ms. After receiving the Convert T protocol, if the part does

not receive power from the VDD pin, the DQ line for the DS18S20 must be held high for at least

750 ms to provide power during the conversion proc ess. As such, no other activit y may take place on

the 1–Wire bus for at least this period after a Convert T command has been issued.

2. After receiving the Copy Scratchpad protocol, if the part does not receive power from the VDD pin, the

DQ line for the DS18S20 must be held high for at least 10 ms to provide power during the copy

process. As such, no other activity may take place on the 1–Wire bus for at least this period after a

Copy Scratchpad command has been issued.

READ/WRITE TIME SLOTS

DS18S20 data is read and written through the use of time slots to manipulate bits and a command word to

specify the transaction.

Write Time Slots

A write time slot is initiated when the host pulls the data line from a high logic level to a low logic level.

There are two types of write time slots: Write 1 time slots and Write 0 time slots. All write time slots

must be a minimum of 60 µs in duration with a minimum of a 1-µs recovery time between individual

write cycles.

The DS18S20 samples the DQ line in a window of 15 µs to 60 µs after the DQ line falls. If the line is

high, a Write 1 occurs. If the line is low, a Write 0 occurs (see Figure 11).

For the host to generate a Write 1 time slot, the data line must be pulled to a logic low level and then

released, allowing the data line to pull up to a high level within 15 µs after the start of the write time slot.

For the host to generate a Write 0 time slot, the data line must be pulled to a logic low level and remain

low for 60 µs.

Read Time Slots

The host generates read time slots when data is to be read from the DS18S20. A read time slot is initiated

when the host pulls the data line from a logic high level to logic low level. The data line must remain at a

low logic level for a minimum of 1 µs; output data from the DS18S20 is valid for 15 µs after the falling

edge of the read time slot. The host therefore must stop driving the DQ pin low in ord er to read its state

15 µs from the start of the read slot (see Figure 11). B y the end of the read time slot, the DQ pin will pull

back high via the external pullup resistor. All read time slots must be a minimum of 60 µs in duration

with a minimum of a 1-µs recovery time between individual read slots.

Figure 12 shows that the sum of TINIT, TRC, and TSAMPLE must be less than 15 µs. Figure 13 shows that

system timing margin is maximized by keeping TINIT and TRC as small as possible and by locating the

master sample time towards the end of the 15-µs period.

DS18S20

19 of 27

READ/WRITE TIMI NG DIAGRAM Figure 11

Master Write "0" Slot

60 µ

µµ

µs <Tx "0" < 120 µ

µµ

µs

µµ

µs < tREC <

∞

VCC

GND

1-WIRE

Master Write "1" Slot

DS18S20 SAMPLES

MIN TYP MAX

µµ

µs

µµ

µs15

µµ

µs15

µµ

µs

DS18S20 SAMPLES

MIN TYP MAX

µµ

µs 15

µµ

µs30

µµ

µs

Master Read "0" Slot

µµ

µs < tREC <

∞

VCC

GND

1-WIRE

BUS

Master Read "1" Slot

µµ

µs

µµ

µs15

µµ

µs15

µµ

µs 15

µµ

µs30

µµ

µs

LINE TYPE LEGEND

Bus master active l o w DS18S20 acti ve low

Both bus master and Resistor pul l - up

DS18S20 acti ve low

Master Samples

DS18S20

20 of 27

DETAILED MASTER READ 1 TIMING Figure 12

RECOMME NDED MASTER READ 1 TIMING Figure 13

TINIT >

µµ

µs

VCC

GND

1-WIRE

VIH OF MASTER

MASTER SAMPLES

TRC

µµ

µs

LINE TYPE LEGEND

Bus master active l o w DS18S20 acti ve low

Both bus master and Resistor pul l -up

DS18S20 acti ve low

TINIT =

SMALL

VCC

GND

1-WIRE

VIH OF MASTER

MASTER SAMPLES

µµ

µs

TRC =

SMALL

DS18S20

21 of 27

Related Appli cation Notes

The following Application Notes can be applied to the DS18S20. These notes can be obtained from the

Dallas Semiconductor “Application Note Book,” via our website at http://www.dalsemi.com/.

Application Note 27: “Understanding and Using Cyclic Redundancy Checks with Dallas Semiconductor

Touch Memory Product”

Application Note 55: “Extending the Contact Range of Touch Memories”

Application Note 74: “Reading and Writing Touch Memories via Serial Interfaces”

Application Note 104: “Minimalist Temperature Control Demo”

Application Note 105: “High Resolution Temperature Measurement with Dallas Direct–to–Direct

Temperature Sensors”

Application Note 106: “Complex MicroLANs”

Application Note 108: “MicroLAN – In the Long Run”

Sample 1–Wire subroutines that can be used in conjunction with AN74 can be downloaded from the

website or our Anonymous FTP Site.

MEMORY FUNCTI O N EXAMPLE Table 3

Example: Bus Master initiates temperature conversion, then reads temperature (parasite power assumed).

MASTER MODE DATA (LSB FIRST) COMMENTS

TX Reset Reset pulse (480-960 µs).

RX Presence Presence pulse.

TX 55h Issue “Match ROM” command.

TX <64-bit ROM code> Issue address for DS18S20.

TX 44h Issue “ Convert T” command.

TX <I/O LINE HIGH> I/O line is held high for at least 750 ms by bus master to

allow conversion to complete.

TX Reset Reset pulse.

RX Presence Presence pulse.

TX 55h Issue “Match ROM” command.

TX <64-bit ROM code> Issue address for DS18S20.

TX BEh Issue “Read Scratchpad” command.

RX <9 data bytes> Read entire scratchpad plus CRC; the master now

recalculates the CRC of the eight data bytes received

from the scratchpad, compares the CRC calculated and

the CRC read. If they match, the master continues; if

not, this read operation is repeated.

TX Reset Reset pulse.

RX Presence Presence pulse, done.

DS18S20

22 of 27

MEMORY FUNCTI O N EXAMPLE Table 4

Example: Bus Master writes memory (parasite power and only one DS18S20 assumed).

MASTER MODE DATA (LSB FIRST) COMMENTS

TX Reset Reset pulse.

RX Presence Presence pulse.

TX CCh Skip ROM command.

TX 4Eh Write Scratchpad command.

TX <2 data bytes> Writes 2 bytes to scratchpad (TH and TL).

TX Reset Reset pulse.

RX Presence Presence pulse.

TX CCh Skip ROM command.

TX BEh Read Scratchpad command.

RX <9 data bytes> Read entire scratchpad plus CRC. The master now

recalculates the CRC of the eight data bytes received

from the scratchpad, compares the CRC and the two

other bytes read back from the scratchpad. If data match,

the master continues; if not, repeat the sequence.

TX Reset Reset pulse.

RX Presence Presence pulse.

TX CCh Skip ROM command.

TX 48h Copy Scratchpad command; after issuing this command,

the master must wait 10 ms for copy operation to

complete.

TX Reset Reset pulse.

RX Presence Presence pulse, done.

DS18S20

23 of 27

MEMORY FUNCTI O N EXAMPLE Table 5

Example: Temperature conversion and interpolation (external power supply and only one DS18S20

assumed.)

MASTER MODE DATA (LSB FIRST) COMMENTS

TX Reset Reset pulse.

TR Presence Presence pulse.

TX CCh Skip ROM command.

TX 44h Convert T command.

RX <1 data byte> Read busy flag eight times. The master continues

reading 1 byte (or bit) after another until the data is FFh

(all bits 1).

TX Reset Reset pulse.

RX Presence Presence pulse.

TX CCh Skip ROM command.

TX BEh Read Scratchpad command.

RX <9 data bytes> Read entire scratchpad plus CRC. The master now

recalculates the CRC of the eight data bytes received

from the scratchpad and compares both CRCs. If the

CRCs match, the data is valid. The master saves the

temperature value and stores the contents of the count

and COUNT_PER_C, respectively.

TX Reset Reset pulse.

RX Presence Presence pulse, done.

- - CPU calculates temperature as described in the data

sheet for higher resolution.

DS18S20

24 of 27

ABSOLUTE MAXIMUM RA TINGS*

Voltage on Any Pin Relative to Ground –0.5V to +6.0V

Operating Temperature –55°C to +125°C

Storage Temperat ure –55°C to +125°C

Soldering Temperature See J-STD-020A specification

* This is a stress rating only and functional operation of the device at these or an y 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.

RECOMMENDED DC OPERATING CONDITIONS

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

Supply Voltage VDD Local Power 3.0 5.5 V 1

Data Pin DQ -0.3 VDD V1

Logic 1 VIH 2.2 VDD+0.3 V 1,2

Logic 0 VIL -0.3 +0.8 V 1,3,7

DC ELECTRICAL CHARACTERISTI CS (-55°C to +125°C; VDD= 3 .0V to 5.5V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

Thermometer Error tERR -10°C to +85°C ±½ °C

-55°C to +125°C ±2

Input Logic High V IH Local Power

Parasite Power 2.2

3.0 5.5 V

V1,2

1,2

Input Logic Low VIL -0.3 +0.8 V 1,3, 7

Sink Current ILVI/O=0.4V -4.0 mA 1

Standby Current IDDS 750 1000 nA 6,8

Active Current IDD 1 1.5 mA 4

DQ Input Load

Current IDQ 5µA5

AC ELECTRICAL CHAR ACTERI STICS: NV MEMORY

(-55°C to +125°C; VDD=3.0V to 5.5V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

NV Write Cycle Time tWR 210ms

EEPROM Writes NEEWR -55°C to +55°C 50k writes

EEPROM Data

Retention tEEDR -55°C to +55°C 10 years

DS18S20

25 of 27

AC ELECTRICAL CHARAC TERISTICS: (-55°C to +125°C; VDD=3.0 V to 5.5V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

Temperature Conversion tCONV 200 750 ms

Time

Time Slot tSLOT 60 120 µs

Recovery Time tREC 1µs

Write 0 Low Time rLOW0 60 120 µs

Write 1 Low Time tLOW1 115µs

Read Data Valid tRDV 15 µs

Reset Time High tRSTH 480 µs

Reset Time Low tRSTL 480 µs 9

Presence Detect High tPDHIGH 15 60 µs

Presence Detect Low tPDLOW 60 240 µs

Capacitance CIN/OUT 25 pF

NOTES:

1. All voltages are referenced to ground.

2. Logic one voltages are specified at a source current of 1 mA.

3. Logic zero voltages are specified at a sink current of 4 mA.

4. Active current refers to either temperature conversion or writing to the E2 memory. Writing to E2

memory consumes approximately 200 µA for up to 10 ms.

5. Input load is to ground.

6. Standby current specified up to 70°C. Standby current typically is 3 µA at 125°C.

7. To always guarantee a presence pulse under low voltage parasite power conditions, VILMAX may have

to be reduced to as much as 0.5V.

8. To minimize IDDS, DQ should be GND ≤ DQ ≤ GND + 0.3V or VDD – 0.3V ≤ DQ ≤ VDD.

9. Under parasite power, the max tRSTL before a power on reset occurs, is 960 ms.

DS18S20

26 of 27

1—WIRE WRITE ONE TIME SLOT

1—WIRE WRITE ZERO TIME SLOT

1—WIRE READ ZERO TIME SLOT

1—WIRE RESET PULSE

1-WIRE PRESENCE DETEST