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DS1920
STRONG PULLUP FOR SUPPLYING DS1920 DURING TEMPERATURE
CONVERSION Figure 2
OPERATION—MEASURING TEMPERATURE
The DS1920 measures temperatures through the use of an on-board proprietary temperature measurement
technique. A block diagram of the temperature measurement circuitry is shown in Figure 3.
The DS1920 measures temperature by counting the number of clock cycles that an oscillator with a low
temperature coefficient goes through during a gate period determined by a high temperature coefficient
oscillator. The counter is preset with a base count that corresponds to -55°C. If the counter reaches 0
before the gate period is over, the temperature register, which is also preset to the -55°C value, is
incremented, indicating 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.
The counter is then clocked again until it reaches 0. If the gate period is still not finished, then this
process repeats.
The slope accumulator compensates for the nonlinear behavior of the oscillators over temperature,
yielding a high-resolution temperature measurement. This is done by changing the number of counts
necessary 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 accumulator) at a given temperature must be known.
Internally, this calculation is done inside the DS1920 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 output data to measured temperature. The data is transmitted serially over the 1-Wire
interface. The DS1920 can measure temperature over the range of -55°C to +100°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 DS1920 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
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