DS18B20

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DS18B20 BLOCK DIAGRAM Figure 1

PARASITE POWER

The block diagram (Figure 1) shows the parasite-powered circuitry.

This circuitry “steals” power

and voltage requirements are met (see the section titled “1-Wire Bus System”).

The advantages of

parasite power are twofold: 1) by parasiting off this pin, no local power source is needed for remote

sensing of temperature, and 2) the ROM may be read in absence of normal power.

In order for the DS18B20 to be able to perform accurate temperature conversions, sufficient power must

be provided over the DQ line when a temperature conversion is taking place. Since the operating current

of the DS18B20 is up to 1.5 mA, the DQ line will not have sufficient drive due to the 5k pullup resistor.

This problem is particularly acute if several DS18B20s are on the same DQ and attempting to convert

simultaneously.

There are two ways to assure that the DS18B20 has sufficient supply current during its active conversion

cycle. The first is to provide a strong pullup on the DQ line whenever temperature conversions or copies

to the E

directly to the power supply as shown in Figure 2. The DQ line must be switched over to the strong pull-

up within 10

Another method of supplying current to the DS18B20 is through the use of an external power supply tied

DQ line, and the bus master need not be tied up holding that line high during temperature conversions.

This allows other data traffic on the 1-Wire bus during the conversion time. In addition, any number of

DS18B20s may be placed on the 1-Wire bus, and if they all use external power, they may all

simultaneously perform temperature conversions by issuing the Skip ROM command and then issuing the

Convert T command. Note that as long as the external power supply is active, the GND pin may not be

floating.

The use of parasite power is not recommended above 100

°C, since it may not be able to sustain

communications given the higher leakage currents the DS18B20 exhibits at these temperatures. For

DS18B20.

64-BIT ROM

AND

1-WIRE PORT

MEMORY AND

CONTROL LOGIC

SCRATCHPAD

8-BIT CRC

GENERATOR

TEMPERATURE SENSOR

HIGH TEMPERATURE

TRIGGER, TH

LOW TEMPERATURE

TRIGGER, TL

CONFIGURATION

REGISTER

POWER

SUPPLY

SENSE

DQ