DS18B20X
The use of parasite power is not recommended for temperatures above 100°C since the DS18B20X 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
DS18B20X be powered by an external power supply.
In some situations the bus master may not know whether the DS18B20Xs 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 DS18B20Xs will pull the bus low, and externally
powered DS18B20Xs 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 DS18B20X DURING TEMPERATURE
CONVERSIONS Figure 4
VPU
DS18B20X
GND
DQ VDD
VPU
Micro-
processor
4.7K
To Other
1-Wire Devices
1-Wire Bus
POWERING THE DS18B20X WITH AN EXTERNAL SUPPLY Figure 5
VDD (External Supply)
DS18B20X
VPU
Micro-
GND
DQ VDD
processor
4.7K
To Other
1-Wire Devices
1-Wire Bus
64-BIT LASERED ROM CODE
Each DS18B20X contains a unique 64–bit code (see Figure 6) stored in ROM. The least significant 8 bits
of the ROM code contain the DS18B20X’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 DS18B20X 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
LSB MSB
LSB
MSB
LSB
5 of 21
DALLAS [ DALLAS SEMICONDUCTOR ]
