18B20 datasheet, PDF - EEWORLD Datasheet

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18B20: Description1-Wire Digital Thermometer; File Size228KB，22 Pages; ManufacturerMaxim; Websitehttps://www.maximintegrated.com/en.html

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18B20 Overview

1-Wire Digital Thermometer

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DS18B20

Programmable Resolution

1-Wire Digital Thermometer

DESCRIPTION

The DS18B20 digital thermometer provides 9-bit

to 12-bit Celsius temperature measurements and

has an alarm function with nonvolatile user-

programmable upper and lower trigger points.

The DS18B20 communicates over a 1-Wire bus

that by definition requires only one data line (and

ground) for communication with a central

microprocessor. It has an operating temperature

range of -55°C to +125°C and is accurate to

±0.5°C

over the range of -10°C to +85°C. In

addition, the DS18B20 can derive power directly

from the data line (“parasite power”), eliminating

the need for an external power supply.

Each DS18B20 has a unique 64-bit serial code,

which allows multiple DS18B20s to function on

the same 1-Wire bus. Thus, it is simple to use one

microprocessor to control many DS18B20s

distributed over a large area. Applications that

can benefit from this feature include HVAC

environmental controls, temperature monitoring

systems inside buildings, equipment, or

machinery, and process monitoring and control

systems.



User-Definable Nonvolatile (NV) Alarm

Settings

Alarm Search Command Identifies and

Addresses Devices Whose Temperature is

Outside Programmed Limits (Temperature

Alarm Condition)

Available in 8-Pin SO (150 mils), 8-Pin

µSOP,

and 3-Pin TO-92 Packages

Software Compatible with the DS1822

Applications Include Thermostatic Controls,

Industrial Systems, Consumer Products,

Thermometers, or Any Thermally Sensitive

System



PIN CONFIGURATIONS

MAXIM

18B20

1 2 3

N.C.

GND

MAXIM

18B20

SO (150 mils)

(DS18B20Z)

FEATURES



Unique 1-Wire® Interface Requires Only One

Port Pin for Communication

Each Device has a Unique 64-Bit Serial Code

Stored in an On-Board ROM

Multidrop Capability Simplifies Distributed

Temperature-Sensing Applications

Requires No External Components

Can Be Powered from Data Line; Power Supply

Range is 3.0V to 5.5V

Measures Temperatures from -55°C to +125°C

(-67°F to +257°F)

±0.5°C

Accuracy from -10°C to +85°C

Thermometer Resolution is User Selectable

from 9 to 12 Bits

Converts Temperature to 12-Bit Digital Word in

750ms (Max)

1 of 22

GND

N.C.

GND

N.C.

1 2 3

(BOTTOM VIEW)

µSOP

(DS18B20U)

18B20

TO-92

(DS18B20)

1-Wire is a registered trademark of Maxim Integrated Products, Inc.

REV: 042208

DS18B20

bus is virtually unlimited. The 1-Wire bus protocol, including detailed explanations of the commands and

“time slots,” is covered in the

1-Wire Bus System

section.

Another feature of the DS18B20 is the ability to operate without an external power supply. Power is

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

signal also charges an internal capacitor (C

), which then supplies power to the device when the bus is

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

alternative, the DS18B20 may also be powered by an external supply on V

Figure 1. DS18B20 Block Diagram

PARASITE POWER

CIRCUIT

4.7k

MEMORY CONTROL

LOGIC

DS18B20

TEMPERATURE SENSOR

INTERNAL V

64-BIT ROM

AND

1-Wire PORT

GND

SCRATCHPAD

ALARM HIGH TRIGGER (T

)

ALARM LOW TRIGGER (T

)

POWER-

SUPPLY

SENSE

CONFIGURATION REGISTER

(EEPROM)

8-BIT CRC GENERATOR

OPERATION—MEASURING TEMPERATURE

The core functionality of the DS18B20 is its direct-to-digital temperature sensor. The resolution of the

temperature sensor is user-configurable to 9, 10, 11, or 12 bits, corresponding to increments of 0.5°C,

0.25°C, 0.125°C, and 0.0625°C, respectively. The default resolution at power-up is 12-bit. The DS18B20

powers up in a low-power idle state. To initiate a temperature measurement and A-to-D conversion, the

master must issue a Convert T [44h] command. Following the conversion, the resulting thermal data is

stored in the 2-byte temperature register in the scratchpad memory and the DS18B20 returns to its idle

state. If the DS18B20 is powered by an external supply, the master can issue “read time slots” (see the

1-Wire Bus System

section) after the Convert T command and the DS18B20 will respond by transmitting

0 while the temperature conversion is in progress and 1 when the conversion is done. If the DS18B20 is

powered with parasite power, this notification technique cannot be used since the bus must be pulled high

by a strong pullup during the entire temperature conversion. The bus requirements for parasite power are

explained in detail in the

Powering the DS18B20

section.

The DS18B20 output temperature data is calibrated in degrees Celsius; for Fahrenheit applications, a

lookup table or conversion routine must be used. The temperature data is stored as a 16-bit sign-extended

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

temperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. If the

DS18B20 is configured for 12-bit resolution, all bits in the temperature register will contain valid data.

For 11-bit resolution, bit 0 is undefined. For 10-bit resolution, bits 1 and 0 are undefined, and for 9-bit

resolution bits 2, 1, and 0 are undefined. Table 1 gives examples of digital output data and the

corresponding temperature reading for 12-bit resolution conversions.

3 of 22

DS18B20

The master device can check the alarm flag status of all DS18B20s on the bus by issuing an Alarm Search

[ECh] command. Any DS18B20s with a set alarm flag will respond to the command, so the master can

determine exactly which DS18B20s have experienced an alarm condition. If an alarm condition exists

and the T

or T

settings have changed, another temperature conversion should be done to validate the

alarm condition.

POWERING THE DS18B20

The DS18B20 can be powered by an external supply on the V

pin, or it can operate in “parasite power”

mode, which allows the DS18B20 to function without a local external supply. Parasite power is very

useful for applications that require remote temperature sensing or that are very space constrained.

Figure 1 shows the DS18B20’s parasite-power control circuitry, which “steals” power from the 1-Wire

bus via the DQ pin when the bus is high. The stolen charge powers the DS18B20 while the bus is high,

and some of the charge is stored on the parasite power capacitor (C

) to provide power when the bus is

low. When the DS18B20 is used in parasite power mode, the V

pin must be connected to ground.

In parasite power mode, the 1-Wire bus and C

can provide sufficient current to the DS18B20 for most

operations as long as the specified timing and voltage requirements are met (see the

DC Electrical

Characteristics

and

AC Electrical Characteristics).

However, when the DS18B20 is performing

temperature conversions or copying data from the scratchpad memory to EEPROM, the operating current

can be as high as 1.5mA. This current can cause an unacceptable voltage drop across the weak 1-Wire

pullup resistor and is more current than can be supplied by C

. To assure that the DS18B20 has sufficient

supply current, it is necessary to provide a strong pullup on the 1-Wire bus whenever temperature

conversions are taking place or data is being copied from the scratchpad to EEPROM. This can be

accomplished by using a MOSFET to pull the bus directly to the rail as shown in Figure 4. The 1-Wire

bus must be switched to the strong pullup within 10µs (max) after a Convert T [44h] or Copy Scratchpad

[48h] command is issued, and the bus must be held high by the pullup for the duration of the conversion

CONV

) or data transfer (t

= 10ms). No other activity can take place on the 1-Wire bus while the pullup

is enabled.

The DS18B20 can also be powered by the conventional method of connecting an external power supply

to the V

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

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

The use of parasite power is not recommended for temperatures above +100°C since the DS18B20 may

not be able to sustain communications due to the higher leakage currents that can exist at these

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

DS18B20 be powered by an external power supply.

In some situations the bus master may not know whether the DS18B20s on the bus are parasite powered

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

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

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

During the read time slot, parasite powered DS18B20s will pull the bus low, and externally powered

DS18B20s will let the bus remain high. If the bus is pulled low, the master knows that it must supply the

strong pullup on the 1-Wire bus during temperature conversions.

5 of 22

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