Analog-to-Digital Converter - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 622

Example 12.10
▼

Construct a humidity measurement system that consists of an HCS12, an IH-3605 humid-
ity sensor, and an LCD. The E-clock frequency of the HCS12 is 24 MHz.
Solution: Since the output of the IH-3605 is between 0.8 and 3.9 V with a 5-V power supply, it
would be beneficial to use a circuit to translate and then scale it to between 0 and 5 V so that
the best accuracy can be achieved. The circuit connection is shown in Figure 12.20. A set of
resistor values and V 1 voltage are given in Figure 12.20. A low-pass filter that consists of a 1-k

Ω

resistor and a 0.16- F capacitor is added to meet the requirement.

To translate from the conversion result to the relative humidity, divide the conversion re-
sult by 10.23. Since the HCS12 MCU does not support floating-point operation directly, we can
multiply the conversion result by 100 (or 10) and then divide the product by 1023 (or 102). The
C program that configures the A/D module, starts the A/D conversion, translates the conver-
sion result to the relative humidity, converts the humidity into an ASCII string, and displays
the string on the LCD is as follows:

μ

V S (= 5 V)

R 0

R f

R 0 +12 V

1 k Ω

V OUT

+12 V

- R 1

-

0.16 μ F 74 1

IH-3605

74 1

+ V OUT

+

GND

R 2

- 12 V

- 12 V

V 1

R 0 = R 1 = 10 k Ω

HCS12

R 2 = 62 k Ω

R f = 16 k Ω

V 1 = 5 V AN7

Figure 12.20 ■ Relative humidity measurement circuit

#include “c:\cwHCS12\include\hcs12.h”
#include “c:\cwHCS12\include\delay.c”
#include “c:\cwHCS12\include\convert.c”
#include “c:\cwHCS12\include\lcd_util_dragon12.c”
void openAD0(void);
char buf[10];
char *msg1 5 “humidity 5 ”;
char *blanks 5 “ ”;
void main (void)
{