ST AN1755 Application note

AN1755

APPLICATION NOTE

A HIGH RESOLUTION / PRECISION THERMOMETER USING ST7 AND NE555

INTRODUCTION

The goal of this application note is to present a realistic example of a thermometer using an ST7 and an NE555.

The NE555 is operating in the a-stable mode. Its frequency is controlled by the resistance changes of a NTC-thermistor. The frequency, as well as the duty cycle, are measured by the ST7 timer. The NE555 output is connected to the timer input capture pin.

Rev. 1.0

AN1755/0304

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A HIGH RESOLUTION / PRECISION THERMOMETER USING ST7 AND NE555

1 DESCRIPTION OF NE555

The NE555 monolithic timing circuit is a highly stable controller capable of producing accurate time delays or oscillation. In the time delay mode of operation, the time is precisely controlled by one external resistor and capacitor. For a stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one capacitor. For more details see NE555 datasheet.

2 ASTABLE OPERATION

The circuit is shown in Figure 1. (pin 2 and 6 connected). It triggers itself and operates as a free running multi vibrator. The external capacitor charges through R1 and R2 and discharges through R2. Thus the duty cycle is precisely set by the ratio of these two resistors. In the a- stable mode of operation, C1 charges and discharges between 1/3 VCC and 2/3 VCC. Due to the self-triggered mode, the charge and discharge times and therefore frequency are independent of the supply voltage.

Figure 1. Circuit Diagram of NE555 in a-stable mode

VCC+ = 5V

																							R1
Output								4				8
				3					NE555							7							R2
		Control
		Voltage
0.01µF				5												6
							1						2											C1

The charge time (output HIGH) is given by

(1)t1 = 0.693 (R1 + R2) C1

and the discharge time (output LOW) by

(2)t2 = 0.693 (R2) C1

Thus the total period T is given by

(3)T = t1 + t2 = 0.693 (R1 + 2R2) C1

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A HIGH RESOLUTION / PRECISION THERMOMETER USING ST7 AND NE555

Figure 2. NE555 Timing Diagram

output
t1	t2
voltage on C1

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.

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