DS18B20

6 of 21

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.

SUPPLYING THE PARASITE-POWERED DS18B20 DURING TEMPERATURE

CONVERSIONS Figure 4

POWERING THE DS18B20 WITH AN EXTERNAL SUPPLY Figure 5

64-BIT LASERED ROM CODE

Each DS18B20 contains a unique 64–bit code (see Figure 6) stored in ROM. The least significant 8 bits

of the ROM code contain the DS18B20’s 1-Wire family code: 28h. The next 48 bits contain a unique

serial number. The most significant 8 bits contain a cyclic redundancy check (CRC) byte that is

calculated from the first 56 bits of the ROM code. A detailed explanation of the CRC bits is provided in

the CRC GENERATION section. The 64-bit ROM code and associated ROM function control logic allow

the DS18B20 to operate as a 1-Wire device using the protocol detailed in the 1-WIRE BUS SYSTEM

section of this datasheet.

64-BIT LASERED ROM CODE Figure 6

8-BIT CRC

48-BIT SERIAL NUMBER

8-BIT FAMILY CODE (28h)

MSB

MSB

LSB

LSB

LSB

MSB

DS18B20

GND

DQ

4.7k

To Other

1-Wire Devices

1-Wire Bus

Micro-

processor

4.7k

1-Wire Bus

Micro-

processor

DS18B20

GND

DQ

To Other

1-Wire Devices