DS18S20

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DETAILED PIN DESCRIPTION

PIN

8-PIN 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

power mode.

DS18S20Z (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 DS18S20. The DS18S20 has three

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

triggers TH and TL. 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 DS18S20 may also be powered from an external 3 volt – 5 volt

supply.

Communication to the DS18S20 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 DS18S20 to perform a temperature measurement. The result

of this measurement will be placed in the DS18S20’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

DS18S20, these registers may be used as general purpose user memory. Writing TH and TL 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.