As my first PIC project I tried to make a simple robot. It has two motors, 
driving two wheels. It also has two bumper switches. This has been done many 
times, but not by me.
I used the normally open contacts of the switches. The program would have 
been slightly simpler if I had used the normally closed contacts.

 I typed a C program, using the MCC18 compiler.
Here is the program:

#include <p18f4520.h>

#pragma config WDT = OFF
#pragma config OSC = HSPLL

#define bumper_left 1  // RD0
#define bumper_right 2 // RD1
#define motor_left_rev 1 // RA0
#define motor_left_fwd 2 // RA1
#define motor_right_fwd 4 // RA2
#define motor_right_rev 8 // RA3
#define LED 16   // RA4

void wait(int i){
long count;
for(count = 100000*i; count>0; count--)Nop();
}

void main (void)
{
unsigned char bump;
TRISD = bumper_left + bumper_right; // two bits input
TRISA = 0xE0;    // bits 0-4: output
while(1){
   PORTA = motor_left_fwd + motor_right_fwd;
   bump = PORTD & (bumper_left + bumper_right);
   if(bump<(bumper_left + bumper_right)){
      PORTA = motor_left_rev + motor_right_rev + LED;
      wait(2);
     // turn
     if(bump == bumper_right) PORTA = motor_left_fwd + LED;
     else  PORTA = motor_right_fwd + LED;
     wait(1);
    }
 }
}

When the program is started the contraption moves forward until it hits 
something. Then it just switches off the motors, does not go back, does not 
light the led. I tried every change I could think of, nothing worked. Until 
finally I moved just one line:

void main (void)
{
unsigned char bump;
TRISD = bumper_left + bumper_right; // two bits input
TRISA = 0xE0;    // bits 0-4: output
while(1){
// the PORTA line is moved to the end.............................
   bump = PORTD & (bumper_left + bumper_right);
   if(bump<(bumper_left + bumper_right)){
      PORTA = motor_left_rev + motor_right_rev + LED;
      wait(2);
     // turn
      if(bump == bumper_right) PORTA = motor_left_fwd + LED;
      else  PORTA = motor_right_fwd + LED;
      wait(1);
    }
   PORTA = motor_left_fwd + motor_right_fwd;
 }
}

...and then it worked. Once the while(1) loop is entered it should not make 
any difference where this line is.
I tried to read the .lst file as produced by MCC18. I am not too familiar 
with the Assembler code yet, but I could not find anything unexpected there.
Do you people have any ideas?

Frank Abbing



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