PIC16F877 ADC stalled?

[MizuRyuu]

currently, my code seem to do the endless loop and the while(GO) loop correctly.... however, the output seem to stay constant regardless of what the input is....

#include <system.h>

//Target PIC16F877 configuration word
#pragma DATA _CONFIG, _PWRTE_OFF & _BODEN_OFF & _WDT_OFF & _LVP_ON & _CPD_OFF & _DEBUG_OFF & _HS_OSC & _CP_OFF

//Set clock frequency
#pragma CLOCK_FREQ 20000000

void main( void )
{
//Configure port A - E
trisa = 0x2F;
trisb = 0x00;
trisc = 0x00;
trisd = 0x00;

//Initialize port A - E
portd = 0x00;
porta = 0x00;
portc = 0x00;
portb = 0x00;

//Configure A/D pins
adcon0 = 10011001b; //Set clock Fosc/32, ch1(AN0), GO = 0, ADC Operating
adcon1 = 0x80; //Right Justified, All ADC pins on

delay_ms(100);
// intcon = 0x00;

// int check;
volatile unsigned int ADC_VALUE;

// ADIE = 0; /* Masking the interrupt */
// ADIF = 0; /* Resetting the ADC interupt bit */
ADC_VALUE = 0000000000b;

while( 1 ) //Endless loop
{

adcon0 = 10001101b; //turn on ADC
while ((adcon0 & 0x04) == 0x04) //waiting until ADC conversion is finished
{
portd = 11111111b;
}
portd = 00000000b; //test that ADC exits since Go bit suppose to change to 0 when ADC finishes
delay_ms(1);
ADC_VALUE = ADRESL;
ADC_VALUE += (ADRESH << 8);
if (ADC_VALUE.9 == 1) //if equal or bigger than 2.5 Volts
{
portb = 11111111b;
}
else
{
portb = 00000000b;
}
portc = 11111111b;
// portb = ADRESH;
portc = 00000000b;
// delay_ms(1);
}
}

 

[kchriste]

Is your PIC really running at 20Mhz? Because the diagram of the dev board shows a 4Mhz crystal. If it's at 4Mhz, you should change your #pragma CLOCK_FREQ and adcon0 setups to reflect this. Shouldn't stop the ADC from working though it'll be slower.

I am supplying the input through a power supply unit where i can adjust the voltage...

It's hard to see on the diagram exactly where AN0 (Pin2) of the PIC goes to on the DEV board. I assume the input to the pin never went above 5V. If your power supply surged or supplied a voltage higher than 5V, then the AN0 input is fried if you didn't use a series resistor (4K7 is a good value). If so, try a different input pin.

 

[MizuRyuu]

Thank you for your help... I finally got it working... I copied a piece of coding from an ADC sample.... now my ADC is working correctly... don't know why it is working though.... I had my TA's check it out and they have no idea why it is working either as the sample code that i copied should do what i had written anyway..... so while it is now working, i am still puzzled by why it is working...

 

[kchriste]

Could be some weird compiler bug. I guess you could look at the disassembly listing and try to figure it out. It's good you got it going; happy programming!