Page 213 - Programming Microcontrollers in C

198 Chapter 4 Small 8-Bit Systems

adc_data[j]=(read_adc(j)+adc_data[j])/2;

Hidden in this code is the fact that both read_adc and
adc_data are unsigned results. When two unsigned numbers are
added together, the most significant bit of each can be 1 so there can
be a carry or overflow when the addition takes place. Problems from
this carry can be avoided in this case by merely using a long or
double precision add routine in adding the two numbers. Then,
when the result is divided by 2, if a bit is carried into the upper byte
of the result it will be shifted back into the lower byte. The result of
this operation is correct and will fit into a single unsigned int.
The compiled version of the above program with read_adc()
merged into it follows:
#include “hc05b6.h”

0050 0008 unsigned adc_data[8];
unsigned read_adc(int);

void main(void)
{
0058 unsigned int j;

0100 3F 09 CLR $09 AD_CTST=0;
0102 1A 09 BSET 5,$09 AD_CTST.ADON=1;

0104 3F 58 CLR $58 for(j=0;j<8;j++)
0106 B6 58 LDA $58
0108 A1 08 CMP #$08
010A 24 36 BCC $0142
{
010C CD 01 43 JSR $0143 adc_data[j]=(read_adc(j)+
adc_data[j])/2;
010F BE 58 LDX $58
0111 EB 50 ADD $50,X
0113 AE 02 LDX #$02
0115 CD 01 57 JSR $0157
0118 9F TXA
0119 BE 58 LDX $58
011B E7 50 STA $50,X