DS1820 datasheet(5/27 Pages) DALLAS

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DS1820

030598 5/27

TEMPERATURE MEASURING CIRCUITRY Figure 4

SLOPE ACCUMULATOR

PRESET

COUNTER

STOP

INC

COMPARE

TEMPERATURE REGISTER

LOW TEMPERATURE

COEFFICIENT OSCILLATOR

HIGH TEMPERATURE

COEFFICIENT OSCILLATOR

SET/CLEAR

LSB

TEMPERATURE/DATA RELATIONSHIPS Table 1

TEMPERATURE

DIGITAL OUTPUT

(Binary)

DIGITAL OUTPUT

(Hex)

+125

°C

00000000 11111010

00FA

+25

°C

00000000 00110010

0032h

+1/2°C

00000000 00000001

0001h

°C

00000000 00000000

0000h

–1/2°C

11111111 11111111

FFFFh

–25

°C

11111111 11001110

FFCEh

–55

°C

11111111 10010010

FF92h

OPERATION – ALARM SIGNALING

After the DS1820 has performed a temperature conver-

sion, the temperature value is compared to the trigger

values stored in TH and TL. Since these registers are

8–bit only, the 0.5

°C bit is 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 measure-

ment. As long as the alarm flag is set, the DS1820 will

respond to the alarm search command. This allows

many DS1820s to be connected in parallel doing simul-

taneous 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.

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