LAPIS Semiconductor ML610471, User Manual, Page: 218 / 281

ML610471/472/473/Q471/Q472/Q473 User's Manual
Chapter 17 RC Oscillation Type A/D Converter
17-13
17.3.3 Example of Use of RC Oscillation Type A/D Converter
This section describes the method of performing A/D conversion for sensor values in Counter A and B reference modes
by taking temperature measurement by a thermistor as an example.
Figure 17-5 shows the circuit configuration of 1-thermistor RC oscillator circuit using RCOSC1.
Figure 17-5 Configuration of 1-Thermistor RC Oscillator Circuit Using RCOSC1
Figure 17-6 shows the temperature characteristics of the thermistor resistance RT1.
Th
er
m
is
to
r r
es
is
tanc
e R
T
1
Temperature T
RT1 = f(T)
D
ig
ital
v
al
ue n
T1
RT1
nT0 = K•RT1
= K•f(T)
Figure 17-6 Temperature Characteristics of
Thermistor Characteristics
Figure 17-7 A/D Conversion
(Ideal characteristics when nT0 is proportional to
RT1)
RT1 is expressed as a function of temperature T by the following equation:
RT1 = f (T)
Figure 17-7 shows the ideal characteristics of A/D conversion with the assumption that RT1 is an analog quantity. In the
ideal characteristics, the A/D conversion value nT0 will purely depend on RT1 only. Assuming that nT0 is proportional
to RT1, let proportional constant be K, then nT0 has the following relationship with temperature T:
nT0 = K
•
RT1 = K
•
f (T) ... Expression A
Therefore, temperature T can be expressed as a digital value by performing the conversion processing that accords with
the characteristics shown in Figure 17-6 for nT0 by software.
To convert from an RT1 value to a digital value, the ratio is used between a) the oscillation frequency by the thermistor
connected to the RT1 pin and the capacitor connected to the CS1 pin and b) the oscillation frequency by the reference
resistor (which ideally should have no temperature characteristics) connected to the RS1 pin and the capacitor connected
to the CS1 pin. This is for making the conditions other than resistance equal to eliminate the error factor in oscillation
characteristics.
As shown in Figures 17-6 and 17-8, the RT1 value depends on temperature T and the RS1 value is assumed to be
constant regardless of temperature T. It is ideal if the characteristics of the oscillation frequency f
OSC
to temperature T
using these resistances will be like the solid lines in Figures 17-9 and 17-10; however, in reality, it would appear that
they will be like the dotted lines due to error factors such as IC temperature characteristics.
Since the condition of f
RCCLK
(RT1) and that of f
RCCLK
(RS1) are the same except for the resistances, the error ratios are
almost the same; therefore, errors can almost be eliminated by using the ratio between f
RCCLK
(RT1) and f
RCCLK
(RS1).
CS1
RS1
RT1
IN1
CS1
Reference resistor
Thermistor RT1