Data Sheet

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 V

, 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 V

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 V

TO SUPPLY TEMPERATURE CONVERSION CURRENT Figure 3

µP

+3V - +5.5V

DS18B20

GND

I/O

4.7k

µP

DS18B20

I/O

+3V - +5.5V

4.7k

EXTERNAL

+3V - +5.5V

SUPPLY

TO OTHER

1-WIRE

DEVICES