AN1755 datasheet(3/7 Pages) STMICROELECTRONICS

номер детали	AN1755

подробное описание детали	The goal of this application note is to present

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A HIGH RESOLUTION / PRECISION THERMOMETER USING ST7 AND NE555

Figure 2. NE555 Timing Diagram

3 THEORY OF OPERATION - NE555

In general we can use the Rntc in place of R1 as well as in place of R2. We assume Rntc in place

of R1. The times needed to charge (1) and to discharge (2) the capacitor will be

(4)

t1 = 0.693 (Rntc + R2) C1

(5)

t2 = 0.693 (R2) C1

The result of the measurement should be dependent on the value of Rntc only. For that R2 and

C1 should not vary with temperature or age. That is why the capacitor C1 is to be eliminated

from the equations.The periods t1 and t2 are measured with the ST7 timer. From formula (4)

and (5) we can calculate two variables.

Expressing C1 from (5) and putting it in (4) we obtain

(6)

Rntc = R2 (t1-t2) / t2

The result depends on the precision of the time measuring (t1, t2) and tolerance of R2 only. It

depends neither on C1 nor the supply voltage.

4 THEORY OF OPERATION - ST7

The rising and falling edges of the input signal are captured by the micro and periods t1, t2 are

measured with the built-in timer. The timer resolution (125 ns @ 8MHz) is sufficient to capture

these edges.

Note: To calculate equation (6) we can use multiples of 125ns for simplicity.

output

voltage on

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