DS18B20

2 of 27

DETAILED PIN DESCRIPTION Table 1

PIN

8PIN SOIC

PIN

TO92

SYMBOL

DESCRIPTION

5

1

GND

Ground.

42

DQ

Data Input/Output pin. For 1-Wire operation: Open

drain. (See “Parasite Power” section.)

33

operation in parasite power mode.

DS18B20Z (8-pin SOIC): All pins not specified in this table are not to be connected.

OVERVIEW

The block diagram of Figure 1 shows the major components of the DS18B20. The DS18B20 has four

main data components: 1) 64-bit lasered ROM, 2) temperature sensor, 3) nonvolatile temperature alarm

triggers TH and TL, and 4) a configuration register. The device derives its power from the 1-Wire

communication line by storing energy on an internal capacitor during periods of time when the signal line

is high and continues to operate off this power source during the low times of the 1-Wire line until it

returns high to replenish the parasite (capacitor) supply. As an alternative, the DS18B20 may also be

powered from an external 3 volt - 5.5 volt supply.

Communication to the DS18B20 is via a 1-Wire port. With the 1-Wire port, the memory and control

functions will not be available before the ROM function protocol has been established. The master must

first provide one of five ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, 4)

Skip ROM, or 5) Alarm Search. These commands operate on the 64-bit lasered ROM portion of each

device and can single out a specific device if many are present on the 1-Wire line as well as indicate to

the bus master how many and what types of devices are present. After a ROM function sequence has

been successfully executed, the memory and control functions are accessible and the master may then

provide any one of the six memory and control function commands.

One control function command instructs the DS18B20 to perform a temperature measurement. The result

of this measurement will be placed in the DS18B20’s scratch-pad memory, and may be read by issuing a

memory function command which reads the contents of the scratchpad memory. The temperature alarm

triggers TH and TL consist of 1 byte EEPROM each. If the alarm search command is not applied to the

DS18B20, these registers may be used as general purpose user memory. The scratchpad also contains a

configuration byte to set the desired resolution of the temperature to digital conversion. Writing TH, TL,

and the configuration byte is done using a memory function command. Read access to these registers is

through the scratchpad. All data is read and written least significant bit first.