DS1822

6 of 21

The use of parasite power is not recommended for temperatures above 100

°C since the DS1822 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 DS1822 be

powered by an external power supply.

In some situations the bus master may not know whether the DS1822s 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 DS1822s will pull the bus low, and externally powered

DS1822s 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 DS1822 DURING TEMPERATURE

CONVERSIONS Figure 4

POWERING THE DS1822 WITH AN EXTERNAL SUPPLY Figure 5

64-BIT LASERED ROM CODE

Each DS1822 contains a unique 64–bit code (see Figure 6) stored in ROM. The least significant 8 bits of

the ROM code contain the DS1822’s 1-Wire family code: 22h. 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

DS1822 to operate as a 1-Wire device using the protocol detailed in the 1-WIRE BUS SYSTEM section

of this data sheet.

64-BIT LASERED ROM CODE Figure 6

8-BIT CRC

48-BIT SERIAL NUMBER

8-BIT FAMILY CODE (22h)

MSB

MSB

LSB

LSB

LSB

MSB

DS1822

GND

DQ

4.7k

To Other

1-Wire Devices

1-Wire Bus

Micro-

processor

4.7k

1-Wire Bus

Micro-

processor

DS1822

GND

DQ

To Other

1-Wire Devices