Instruction Manual

DS18S20

5 of 21

The use of parasite power is not recommended for temperatures above 100°C since the DS18S20 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 DS18S20 be

In some situations the bus master may not know whether the DS18S20s 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 DS18S20s will pull the bus low, and externally

powered DS18S20s 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 DS18S20 DURING TEMPERATURE

CONVERSIONS Figure 4

POWERING THE DS18S20 WITH AN EXTERNAL SUPPLY Figure 5

64-BIT LASERED ROM CODE

Each DS18S20 contains a unique 64–bit code (see Figure 6) stored in ROM. The least significant 8 bits

of the ROM code contain the DS18S20’s 1–wire family code: 10h. 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 DS18S20 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 (10h)

MSB MSB LSB LSB LSBMSB

(External Supply)

DS18S20

GND

4.7K

To Other

1-Wire Devices

1-Wire Bus

Micro-

processor

4.7K

1-Wire Bus

Micro-

processor

DS18S20

GND

To Other

1-Wire Devices