using the A/D converter in the PIC16f874

[Akhila]

The value I'm getting for the read_adc() is not what it is supposed to be. The range of Voltage inputing into PIN 2(A0) is 0v--5v.
input expected output actual output
0 0 0
0.5 26 103
1 51 209
1.5 77 52
2 102 161
2.5 128 5
3 154 113
3.5 179 210
4 205 64
4.5 230 154
5 256 255

My code also includes an 8bit counter 0--255, at 1 sec delay. It is supposed to print the correct value from read_adc(). Here is my code.
/*********************************************************/
#include <16F874.h>
#use delay(CLOCK = 20000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)


int P7 = 0; // output most significant bit
int P6 = 0;
int P5 = 0;
int P4 = 0;
int P3 = 0;
int P2 = 0;
int P1 = 0;
int P0 = 0; // output least significant bit
byte value;
int ScanNumber =0; // declaration for number of scan times
int counter = 0; // counter for ten-minute time delay


void nextloop(void); // declaration for the 4 least significant bits
void delay(void); // declaration for ten-minute time delay
void Scan(void); // declaration for the 4 most significant bits
void dcvol(void); // declaration for printing the dc voltage

main ()
{
setup_adc(ADC_CLOCK_INTERNAL); // set clock for PIC
set_adc_channel(0); // set analog-to-digital input to pin2
setup_adc_ports(ALL_ANALOG);

for(ScanNumber=0; ScanNumber<9; ScanNumber++)
{
Scan();
delay();

}
}
void Scan(void)
{

for (P7=0; P7<2; P7++)
{
output_low(PIN_B7);
for (P6=0; P6<2; P6++)
{
output_low(PIN_B6);
for (P5=0; P5<2; P5++)
{
output_low(PIN_B5);
for(P4=0; P4<2; P4++)
{

output_low(PIN_B4);
if (P4 == 1) { output_high(PIN_B4); }
if (P5 == 1) { output_high(PIN_B5); }
if (P6 == 1) { output_high(PIN_B6); }
if (P7 == 1) { output_high(PIN_B7); }
nextloop();
}
}
}
}
output_low(PIN_B4);
output_low(PIN_B5);
output_low(PIN_B6);
output_low(PIN_B7);
}


void nextloop(void)
{
for (P3=0; P3<2; P3++)
{
output_low(PIN_B3);
for (P2=0; P2<2; P2++)
{
output_low(PIN_B2);
for (P1=0; P1<2; P1++)
{
output_low(PIN_B1);
for (P0=0; P0<2; P0++)
{

output_low(PIN_B0);
if (P0 == 1) { output_high(PIN_B0); }
if (P1 == 1) { output_high(PIN_B1); }
if (P2 == 1) { output_high(PIN_B2); }
if (P3 == 1) { output_high(PIN_B3); }
dcvol();
delay_ms(1000);

}
}
}
}
output_low(PIN_B0);
output_low(PIN_B1);
output_low(PIN_B2);
output_low(PIN_B3);
}

void delay(void)
{
for (counter = 0; counter < 120; counter++)
{
delay_ms(5000);
}
}
void dcvol(void)
{
value = 0;
value = Read_ADC();
printf("\%u\n\r", value);
}
___________________________
This message was ported from CCS's old forum
Original Post ID: 13841

[PCM programmer]

:=The value I'm getting for the read_adc() is not what it is supposed to be. The range of Voltage inputing into PIN 2(A0) is 0v--5v.
<PRE>
input expected output actual output
0 0 0
0.5 26 103
1 51 209
1.5 77 52
</PRE>
------------------------------------------------------------

Based on the number series above, you're getting 4x your
expected value, and it's being truncated to the lower 8 bits.
For example, 4 x 77 = 308.
and 308 - 256 = 52.

So you need to tell the compiler to return an 8-bit value from
read_adc(), instead of a 10-bit value.
So modify your pre-processor statements like this:

#include <16F874.h>
#device adc=8 // Add this line

If that doesn't fix it, then post the version of your compiler.
___________________________
This message was ported from CCS's old forum
Original Post ID: 13846