DS18B20

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OPERATION - MEASURING TEMPERATURE

The core functionality of the DS18B20 is its direct-to-digital temperature sensor. The resolution of the

DS18B20 is configurable (9, 10, 11, or 12 bits), with 12-bit readings the factory default state. This

equates to a temperature resolution of 0.5

°C, 0.25°C, 0.125°C, or 0.0625°C. Following the issuance of

the Convert T [44h] command, a temperature conversion is performed and the thermal data is stored in

the scratchpad memory in a 16-bit, sign-extended two’s complement format.

The temperature

information can be retrieved over the 1-Wire interface by issuing a Read Scratchpad [BEh] command

once the conversion has been performed. The data is transferred over the 1-Wire bus, LSB first. The

MSB of the temperature register contains the “sign” (S) bit, denoting whether the temperature is positive

or negative.

Table 2 describes the exact relationship of output data to measured temperature. The table assumes 12-bit

resolution. If the DS18B20 is configured for a lower resolution, insignificant bits will contain zeros. For

Fahrenheit usage, a lookup table or conversion routine must be used.

Temperature/Data Relationships Table 2

2

3

2

2

2

1

2

0

2

-1

2

-2

2

-3

2

-4

LSB

MSb

(unit =

°C)

LSb

SS

S

S

S

2

6

2

5

2

4

MSB

TEMPERATURE

DIGITAL OUTPUT

(Binary)

DIGITAL

OUTPUT

(Hex)

+125°C

0000 0111 1101 0000

07D0h

+85°C

0000 0101 0101 0000

0550h*

+25.0625°C

0000 0001 1001 0001

0191h

+10.125°C

0000 0000 1010 0010

00A2h

+0.5°C

0000 0000 0000 1000

0008h

0°C

0000 0000 0000 0000

0000h

-0.5°C

1111 1111 1111 1000

FFF8h

-10.125°C

1111 1111 0101 1110

FF5Eh

-25.0625°C

1111 1110 0110 1111

FF6Fh

-55°C

1111 1100 1001 0000

FC90h

*The power on reset register value is +85°C.

OPERATION - ALARM SIGNALING

After the DS18B20 has performed a temperature conversion, the temperature value is compared to the

trigger values stored in TH and TL.

Since these registers are 8-bit only, bits 9-12 are ignored for

comparison. The most significant bit of TH or TL directly corresponds to the sign bit of the 16-bit

temperature register. If the result of a temperature measurement is higher than TH or lower than TL, an

alarm flag inside the device is set. This flag is updated with every temperature measurement. As long as

the alarm flag is set, the DS18B20 will respond to the alarm search command.

This allows many

DS18B20s to be connected in parallel doing simultaneous temperature measurements. If somewhere the

temperature exceeds the limits, the alarming device(s) can be identified and read immediately without

having to read non-alarming devices.