18B20 datasheet(4/27 Pages) DALLAS

4 / 27 page

DS18B20

4 of 27

For situations where the bus master does not know whether the DS18B20s on the bus are parasite

powered or supplied with external VDD, a provision is made in the DS18B20 to signal the power supply

scheme used. The bus master can determine if any DS18B20s 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 DS18B20 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 PULLUP FOR SUPPLYING DS18B20 DURING TEMPERATURE

CONVERSION Figure 2

USING VDD TO SUPPLY TEMPERATURE CONVERSION CURRENT Figure 3

µP

+3V - +5.5V

DS18B20

GND

VDD

I/O

4.7k

µP

DS18B20

VDD

I/O

+3V - +5.5V

4.7k

EXTERNAL

+3V - +5.5V

SUPPLY

TO OTHER

1-WIRE

DEVICES

18B20 数据表(PDF) 4 Page - Dallas Semiconductor