DS1820 Ver la hoja de datos (PDF) - Dallas Semiconductor -> Maxim Integrated

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DS1820

1–Wire™ Digital Thermometer

Dallas Semiconductor -> Maxim Integrated 

DS1820

USING VDD TO SUPPLY TEMPERATURE CONVERSION CURRENT Figure 3

DS1820

TO OTHER 1–WIRE

DEVICES

+5V

4.7K

µP

VDD

I/O

EXTERNAL +5V SUPPLY

OPERATION – MEASURING TEMPERATURE

The DS1820 measures temperature through the use of

an on–board proprietary temperature measurement

technique. A block diagram of the temperature mea-

surement circuitry is shown in Figure 4.

The DS1820 measures temperature by counting the

number of clock cycles that an oscillator with a low tem-

perature coefficient goes through during a gate period

determined by a high temperature coefficient oscillator.

The counter is preset with a base count that corre-

sponds to –55°C. If the counter reaches zero before the

gate period is over, the temperature register, which is

also preset to the –55°C value, is incremented, indicat-

ing that the temperature is higher than –55°C.

At the same time, the counter is then preset with a value

determined by the slope accumulator circuitry. This cir-

cuitry is needed to compensate for the parabolic behav-

ior of the oscillators over temperature. The counter is

then clocked again until it reaches zero. If the gate

period is still not finished, then this process repeats.

The slope accumulator is used to compensate for the

non–linear behavior of the oscillators over temperature,

yielding a high resolution temperature measurement.

This is done by changing the number of counts neces-

sary for the counter to go through for each incremental

degree in temperature. To obtain the desired resolution,

therefore, both the value of the counter and the number

of counts per degree C (the value of the slope accumu-

lator) at a given temperature must be known.

Internally, this calculation is done inside the DS1820 to

provide 0.5°C resolution. The temperature reading is

provided in a 16–bit, sign–extended two’s complement

reading. Table 1 describes the exact relationship of out-

put data to measured temperature. The data is trans-

mitted serially over the 1–Wire interface. The DS1820

can measure temperature over the range of –55°C to

+125°C in 0.5°C increments. For Fahrenheit usage, a

lookup table or conversion factor must be used.

Note that temperature is represented in the DS1820 in

terms of a 1/2°C LSB, yielding the following 9–bit format:

MSB

LSB

1

1

1

0

0

1

1

1

0

= –25°C

The most significant (sign) bit is duplicated into all of the

bits in the upper MSB of the two–byte temperature reg-

ister in memory. This “sign–extension” yields the 16–bit

temperature readings as shown in Table 1.

Higher resolutions may be obtained by the following

procedure. First, read the temperature, and truncate

the 0.5°C bit (the LSB) from the read value. This value is

TEMP_READ. The value left in the counter may then be

read. This value is the count remaining

(COUNT_REMAIN) after the gate period has ceased.

The last value needed is the number of counts per

degree C (COUNT_PER_C) at that temperature. The

actual temperature may be then be calculated by the

user using the following:

(COUNT_PER_C – COUNT_REMAIN)

TEMPERATURE = TEMP_READ – 0.25 )

COUNT_PER_C

030598 4/27

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