ST AN1755 Application note
AN1755
APPLICATION NOTE
A HIGH RESOLUTION / PRECISION THERMOMET ER 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 tim e r input capture pi n.
Rev. 1.0
AN1755/0304 1/7
1
A HIGH RESOLUTION / PRECISION THERMOMETER USING ST7 AND NE555
1 DESCRIPTION OF NE 55 5
The NE555 monolithic timing circui t is a hi ghly stable controll er 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. F or a stable operation as an oscil lator , the free runni ng
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 sho wn in Figure 1. (pin 2 and 6 connect ed). I t triggers itself and operates as a
free running multi vibrator. The external capacitor charges through R
through R
stable mode of operation, C
. Thus the duty cycle is precisely set by the ratio of the se two resistor s. In the a-
2
charges and di scharges betw ee n 1/3 VCC and 2/3 VCC. Due to
1
the self-triggered mode, the charge and discharge times and therefore frequency are independent of the supply voltage.
Figure 1. Circuit Diagram of NE 555 in a-stable mode
and R2 and discharges
1
+
V
= 5V
CC
Output
Control
Voltage
0.01µF
3
5
1 2
The charge time (output HIGH) is given by
(1) t
= 0.693 (R1 + R2) C
1
1
and the discharge time (output LOW) by
4
NE555
R
1
8
7
R
2
6
C
1
(2) t
= 0.693 (R2) C
2
1
Thus the total period T is given by
(3) T = t
2/7
+ t2 = 0.693 (R1 + 2R2) C
1
2
1
A HIGH RESOLUTION / PRECISION THERMOMETER USING ST7 AND NE555
Figure 2. NE555 Timing Diagram
output
t
1
t
2
voltage on
C
1
3 THEORY OF OPERATION - NE555
In general we can use the R
of R
. The times needed to charge (1) and to discharge (2) the capacitor will be
1
(4) t
= 0.693 (R
1
(5) t2 = 0.693 (R2) C
The result of the measurement should be dependent on the value of R
C
should not v ary with tem pera ture or age. That is why the capac itor C1 is to be eliminated
1
from the equations.The peri ods t
and (5) we can calculate two variables.
Expressing C
(6) R
from (5) and putting it in (4) we obtain
1
= R2 (t1-t2) / t
ntc
The result depends on the precision of the time measuring (t1, t2) and tolerance of R2 only. It
depends neither on C
nor the supply voltage.
1
in place of R1 as well as in place of R2. We assume R
ntc
+ R2) C
ntc
1
1
ntc
and t2 are measured with the ST7 ti mer. F rom form ula (4)
1
2
in place
ntc
only. For that R2 and
4 THEORY OF OP ERATION - 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 c alc ulate equation (6) we can use multiples of 125ns for simplicity.
3/7
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