DS18B20

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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

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

Figure 1. DS18B20 Block Diagram

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.

4.7k

POWER-

SUPPLY

SENSE

64-BIT ROM

AND

1-Wire PORT

DQ

PARASITE POWER

CIRCUIT

MEMORY CONTROL

LOGIC

SCRATCHPAD

8-BIT CRC GENERATOR

TEMPERATURE SENSOR

REGISTER (EEPROM)

REGISTER (EEPROM)

CONFIGURATION REGISTER

(EEPROM)

GND

DS18B20