DS18S20.pdf

DS18S20

High-Precision

1-Wire Digital Thermometer

www.maxim-ic.com

Unique 1-Wire ® interface requires only one

port pin for communication

PIN ASSIGNMENT

DALLAS

DS1820

Each device has a unique 64-bit serial code

stored in an onboard ROM

Multidrop capability simplifies distributed

temperature sensing applications

V DD

Requires no external components

GND

Can be powered from data line. Power supply

range is 3.0V to 5.5V

8-Pin 150mil SO

(DS18S20Z)

Measures temperatures from –55°C to

+125°C (–67°F to +257°F)

0.5

C accuracy from –10°C to +85°C

9-bit thermometer resolution

Converts temperature in 750ms (max.)

User-definable nonvolatile (NV) alarm

settings

Alarm search command identifies and

addresses devices whose temperature is

outside of programmed limits (temperature

alarm condition)

(BOTTOM VIEW)

Applications include thermostatic controls,

industrial systems, consumer products,

thermometers, or any thermally sensitive

system

TO-92

(DS18S20)

PIN DESCRIPTION

GND - Ground

DQ - Data In/Out

V DD - Power Supply Voltage

- No Connect

DESCRIPTION

The DS18S20 Digital Thermometer provides 9-bit centigrade temperature measurements and has an

alarm function with nonvolatile user-programmable upper and lower trigger 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. It has an operating temperature range of –55°C to +125°C

and is accurate to

0.5

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.

1-Wire is a registered trademark of Dallas Semiconductor.

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022103

FEATURES

C over the range of –10°C to +85°C. In addition, the DS18S20 can derive power

directly from the data line (“parasite power”), eliminating the need for an external power supply.

DS18S20

DETAILED PIN DESCRIPTIONS Table 1

8-PIN SOIC* TO-92 SYMBOL DESCRIPTION

5 1 GND Ground.

4 2 DQ Data Input/Output Pin. Open-drain 1-Wire interface pin. Also

provides power to the device when used in parasite power mode

(see “Parasite Power” section.)

3 3 V DD Optional V DD Pin. V DD must be grounded for operation in

parasite power mode.

*All pins not specified in this table are “No Connect” pins.

OVERVIEW

Figure 1 shows a block diagram of the DS18S20, and pin descriptions are given in Table 1. The 64-bit

ROM stores the device’s unique serial code. The scratchpad memory contains the 2-byte temperature

access to the 1-byte upper and lower alarm trigger registers (T H and T L ). The T H and T L registers are

nonvolatile (EEPROM), so they will retain data when the device is powered down.

The DS18S20 uses Dallas’ exclusive 1-Wire bus protocol that implements bus communication using one

control signal. The control line requires a weak pullup resistor since all devices are linked to the bus via a

3-state or open-drain port (the DQ pin in the case of the 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 of this datasheet.

Another feature of the DS18S20 is the ability to operate without an external power supply. Power is

instead supplied through the 1-Wire pullup resistor via the DQ pin when the bus is high. The high bus

signal also charges an internal capacitor (C PP ), which then supplies power to the device when the bus is

low. This method of deriving power from the 1-Wire bus is referred to as “parasite power.” As an

alternative, the DS18S20 may also be powered by an external supply on V DD .

DS18S20 BLOCK DIAGRAM Figure 1

V PU

4.7K

PARASITE POWER

CIRCUIT

MEMORY CONTROL

LOGIC

DS18S20

TEMPERATURE SENSOR

INTERNAL V DD

64-BIT ROM

AND

1-wire PORT

GND

C PP

SCRATCHPAD

ALARM HIGH TRIGGER (T H )

POWER

SUPPLY

SENSE

ALARM LOW TRIGGER (T L )

V DD

8-BIT CRC GENERATOR

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DS18S20

C steps. The DS18S20 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 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

temperature conversion is in progress and 1 when the conversion 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 of this datasheet.

The DS18S20 output data is calibrated in degrees centigrade; for Fahrenheit applications, a lookup table

or conversion routine must be used. The temperature data is stored as a 16-bit sign-extended two’s

complement number in the temperature register (see Figure 2). The sign bits (S) indicate if the

temperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. Table 2

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 2). The extended resolution temperature can then be

calculated using the following equation:

TEMPERATUR

TEMP

READ

COUNT

PER

COUNT

REMAIN

COUNT

PER

TEMPERATURE REGISTER FORMAT Figure 2

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

LS Byte

2 6

2 5

2 4

2 3

2 2

2 1

2 0

2 -1

bit 15

bit 14

bit 13

bit 12

bit 11

bit 10

bit 9

bit 8

MS Byte

TEMPERATURE/DATA RELATIONSHIP Table 2

TEMPERATURE

DIGITAL OUTPUT

(Binary)

DIGITAL OUTPUT

(Hex)

+85.0°C*

0000 0000 1010 1010

00AAh

+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

-55.0°C

1111 1111 1001 0010

FF92h

*The power-on reset value of the temperature register is +85°C

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OPERATION — MEASURING TEMPERATURE

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

output has 9-bit resolution, which corresponds to 0.5

DS18S20

OPERATION — ALARM SIGNALING

After 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 T H and T L registers (see Figure 3).

The sign bit (S) indicates if the value is positive or negative: for positive numbers S = 0 and for negative

numbers S = 1. The T H and T L registers are nonvolatile (EEPROM) so they will retain data when the

device is powered down. T H and T L can be accessed through bytes 2 and 3 of the scratchpad as explained

in the MEMORY section of this datasheet.

T H AND T L REGISTER FORMAT Figure 3

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

2 6

2 5

2 2

2 1

2 0

Only bits 8 through 1 of the temperature register are used in the T H and T L comparison since T H and T L

are 8-bit registers. If the measured temperature is lower than or equal to T L or higher than T H , an alarm

condition exists and an alarm flag is set inside the 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.

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 T H or T L settings have changed, another temperature conversion should be done to validate the alarm

condition.

POWERING THE DS18S20

The DS18S20 can be powered by an external supply on the V DD 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 applications that require remote temperature sensing or that are very space constrained. Figure

1 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 (C PP ) to provide power when the bus is low. When

the DS18S20 is used in parasite power mode, the V DD pin must be connected to ground.

In parasite power mode, the 1-Wire bus and C PP can provide sufficient current to the DS18S20 for most

operations as long as the specified timing and voltage requirements are met (refer to the DC

ELECTRICAL CHARACTERISTICS and the AC ELECTRICAL CHARACTERISTICS sections of this data

sheet). However, when the 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 C PP . 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 4. The 1-Wire bus must be switched to the strong pullup within

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s (max) after a Convert T [44h] or Copy Scratchpad [48h] command is issued, and the bus must be

held high by the pullup for the duration of the conversion (t conv ) or data transfer (t wr = 10ms). No other

activity can take place on the 1-Wire bus while the pullup is enabled.

The DS18S20 can also be powered by the conventional method of connecting an external power supply to

the V DD pin, as shown in Figure 5. The advantage of this method is that the MOSFET pullup is not

required, and the 1-Wire bus is free to carry other traffic during the temperature conversion time.

DS18S20

C since the DS18S20 may not

be able to sustain communications due to the higher leakage currents that can exist at these temperatures.

For applications in which such temperatures are likely, it is strongly recommended that the DS18S20 be

In some situations the bus master may not know whether the DS18S20s on the bus are parasite powered

or powered by external supplies. The master needs this information to determine if the strong bus pullup

should be used during temperature conversions. To get this information, the master can issue a Skip ROM

[CCh] command followed by a Read Power Supply [B4h] command followed by a “read-time slot”.

During the read-time slot, parasite powered 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.

SUPPLYING THE PARASITE-POWERED DS18S20 DURING TEMPERATURE

CONVERSIONS Figure 4

V PU

DS18S20

GND

V DD

Micro-

processor

4.7k

To Other

1-Wire Devices

1-Wire Bus

POWERING THE DS18S20 WITH AN EXTERNAL SUPPLY Figure 5

V PU

DS18S20

GND

V DD (External Supply)

Micro-

processor

V DD

4.7k

To Other

1-Wire Devices

1-Wire Bus

64-BIT LASERED ROM CODE

Each DS18S20 contains a unique 64-bit code (see Figure 6) 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

of this datasheet.

64-BIT LASERED ROM CODE Figure 6

8-BIT CRC

48-BIT SERIAL NUMBER

8-BIT FAMILY CODE (10h)

MSB

LSB

MSB

LSB

MSB

LSB

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The use of parasite power is not recommended for temperatures above 100

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