[SOLVED] Pic 18F2431 Multiple ADC conversion problem.

[Athosza]

Greetings all,

I am attempting to use a PIC 18F2431 to read more than one analogue input voltage. I would like pin AN0 to read 2.5V and pin AN1 to read 5V. I have pin AN0 reading and storing the 2.5V value perfect however when I try to read AN1 it returns the same value of 2.5V that is on pin AN0. I do not understand why. I have debugged the code and it seems to select pin AN1 in the code but no matter what I cannot get the 5V reading even if I only try to read pin AN1 by itself, I still get the 2.5V on pin AN0.

Here is the code I am using to test this:

#include <p18f2431.h>
#include <stdlib.h>
#include <adc.h>

#define ADC_CH0 0x00 //Select Channel 0
#define ADC_CH1 0x01 //Select Channel 0
int var0;
int var1;

void measure_0(void);
void measure_1(void);


void oscillator_setup()
{
OSCCON = 0xf0; // 8MHz internal clock frequency
TRISA = 1;
TRISB = 0x00;
PORTB = 0;
}

void Setup_ADC()
{


OpenADC ( ADC_FOSC_RC &
ADC_RIGHT_JUST &
ADC_12_TAD,
ADC_CH0 &
ADC_INT_OFF &
ADC_VREFPLUS_VDD, 0);

}

void measure_0()
{
SelChanConvADC(ADC_CH0);
Delay10TCYx( 5 ); // Delay for 50TCY
ConvertADC(); // Start conversion
BusyADC(); // Wait for completion
var0 = ReadADC();
}

void measure_1()
{
SelChanConvADC(ADC_CH1);
Delay10TCYx( 5 ); // Delay for 50TCY
ConvertADC(); // Start conversion
BusyADC(); // Wait for completion
var1 = ReadADC();
}

void main()
{

var0 = 0;
var1 = 0;
oscillator_setup();
Setup_ADC();


while(1) {

measure_0();
measure_1();
//variable = variable;
//CloseADC();


}
}


Any help will be greatly appreciated. I hope I am overlooking something really basic.

Thanks,
Athosza

 

[john blue]

SCS<1:0>: System Clock Select bits
1x = Internal oscillator block
01 = Secondary (Timer1) oscillator
00 = Primary oscillator

OSCCON = 0xf0; // 8MHz internal clock frequency. You set it to primary oscillator ( refer to page 36 of the datasheet)

 

[Athosza]

Hey John,

Thanks for the reply. I see what you mean that was a bit of an oversight on my part thanks for pointing that out. I have now corrected it but the code is still not giving me a difference between AN0 and AN1.

Athosza

 

[CronusZA]

Hi Athos,

Try manually setting the ACMOD bits in the ADCON0 register (page 248 of the datasheet), I dont trust those built compiler functions.

Regards,
Cronus

 

[Athosza]

Thanks CronusZA it worked perfectly! I had to set each of the ACMOD bits indivdually:

void measure_0()
{
SelChanConvADC(0);
Delay10TCYx( 5 ); // Delay for 50TCY
ADCON0bits.ACMOD0 = 0;
ConvertADC(); // Start conversion
while( BusyADC() ); // Wait for completion
result_0 = ReadADC(); // Read result
}

void measure_1()
{
SelChanConvADC(1);
Delay10TCYx( 5 ); // Delay for 50TCY
ADCON0bits.ACMOD0 = 1;
ConvertADC(); // Start conversion
while( BusyADC() ); // Wait for completion
result_1 = ReadADC(); // Read result

I hope this helps other people with the same problem.

Case closed South African style