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High ADC values on 16F819

 
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fasteddye



Joined: 13 Apr 2005
Posts: 6

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High ADC values on 16F819
PostPosted: Wed Apr 13, 2005 9:38 pm     Reply with quote
Hi Everybody,
I am a newbie to CCS and microchip so bare with me if i say/do anything stupid. I am working on some code for a school project and the value that the adc is reading are not what i see on a voltmeter attached to the same pin. I am using 4 of the ADCs with the 5th pin as Vref+. The Vref is set to 2.5V from a voltage divider and the inputs to each of the pins is an opamp output which is a voltage between 0 to 2.5 (mainly .4 to 2.3V). The trouble is that i have a meter attached to a adc pin and its reading .4V and the chip is turning on flickering ledbank1 ( (0) means 0 leds on, (1) means 1 led on, etc) that should not come on unless there is a voltage greater than .74V (thus the threshold of 300 in the code below). I dont know whether i am not putting something in the code or if my math operations are wrong. My compiler version is 3.212. Please let me know if i have forgotten anything. Thanks for the help!!

Threshold Math:
2.5/1024 = .00244V = 2.44mV

300 * 2.44mV = .7324V

btw: this code is checking to see how close the voltages from the ADCs are and turn on 3 leds when the voltages are close enough....figured it might help


Code:
#include <16F819.h>
#device *=16 adc=10
#fuses INTRC_IO,NOWDT,NOPROTECT,NOLVP, NODEBUG
#use delay(clock=8000000)

void main() {
   int intensitySwitch = 0;
   int startup = 0;
   long adc0, adc1, adc2, adc3;
   long adcdata0, adcdata1, adcdata2, adcdata3;
   long baseline0, baseline1, baseline2, baseline3;
   long threshold;
   long highvalue;
   int variance = 100;
   float temp;
   float minthreeledvalue;



   // setup the adc converters
   SETUP_ADC_PORTS(AN0_AN1_AN2_AN4_VSS_VREF);
   SETUP_ADC(ADC_CLOCK_INTERNAL);
   do {
         // check adc0
         SET_ADC_CHANNEL(0);
         delay_us(10);
         //delay_ms(1);
         adc0 = READ_ADC();

         // check adc1
         SET_ADC_CHANNEL(1);
         delay_us(10);
         //delay_ms(1);
         adc1 = READ_ADC();

         // check adc2
         SET_ADC_CHANNEL(2);
         delay_us(10);
         //delay_ms(1);
         adc2 = READ_ADC();

         // check adc3
         SET_ADC_CHANNEL(4);
         delay_us(10);
         //delay_ms(1);
         adc3 = READ_ADC();


         threshold = 300;

         adcdata0 = adc0;// - baseline0;
         adcdata1 = adc1;// - baseline1;
         adcdata2 = adc2;// - baseline2;
         adcdata3 = adc3;// - baseline3;

         // compares adc outputs
         highvalue = adcdata0;

         if(adcdata1 > highvalue) {
            highvalue = adcdata1;
         }

         if(adcdata2 > highvalue) {
            highvalue = adcdata2;
         }

         if(adcdata3 > highvalue) {
            highvalue = adcdata3;
         }

         minthreeledvalue = 0.95 * (float)highvalue;

         if(adcdata0 < threshold && adcdata1 < threshold && adcdata2 < threshold && adcdata3 < threshold) {
            ledbank0(0);
            ledbank1(0);
            ledbank2(0);
            ledbank3(0);
         }
         else if(adcdata0 >= minthreeledvalue && adcdata1 >= minthreeledvalue && adcdata2 >= minthreeledvalue && adcdata3 >= minthreeledvalue) {
            ledbank0(3);
            ledbank1(3);
            ledbank2(3);
            ledbank3(3);
         }
            else {
            if(adcdata0 >= minthreeledvalue) {
               ledbank0(2);
            }
            else if(adcdata0 < threshold) {
               ledbank0(0);
            }
            else {
               ledbank0(1);
            }

            if(adcdata1 >= minthreeledvalue) {
               ledbank1(2);
            }
            else if(adcdata1 < threshold) {
               ledbank1(0);
            }
            else {
               ledbank1(1);
            }

            if(adcdata2 >= minthreeledvalue) {
               ledbank2(2);
            }
            else if(adcdata2 < threshold) {
               ledbank2(0);
            }
            else {
               ledbank2(1);
            }

            if(adcdata3 >= minthreeledvalue) {
               ledbank3(2);
            }
            else if(adcdata3 < threshold) {
               ledbank3(0);
            }
            else {
               ledbank3(1);
            }
         }
   } while (TRUE);
}
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Wed Apr 13, 2005 10:19 pm     Reply with quote
My suggestion is that if you have doubts about the output of the
read_adc(), then you should put in printf("%lx \n\r", value)" statements
and look at what you're getting.

Currently, you have a complicated group of if-else statements,
and the situation could be clarified instantly if you were to see
the actual output from the read A/D functions.
fasteddye



Joined: 13 Apr 2005
Posts: 6

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PostPosted: Thu Apr 14, 2005 7:16 am     Reply with quote
where would the printf() statement output the value. I am programing the chip on a demo board using the icd 2 then moving it to my PCB.
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Thu Apr 14, 2005 12:50 pm     Reply with quote
I assume you don't have a LCD or a serial port circuit on your board.

In that case, you can temporarily add a transmit-only serial port to
your board. The 16F819 doesn't have a hardware UART module, so
you can use any free i/o pin for a software UART transmit pin.

Let's say that Pin B1 is a free pin on your PIC. Connect a wire between
pin B1 and pin 2 of the DB-9 connector for COM1, on the back of your PC.
Also connect another wire between Ground on your board and pin 5
of the COM1 DB-9 connector on your PC. (Make sure that you don't
accidently connect the PIC to pin 3 on the DB-9. That pin is the RS232
output from the PC and it's at a negative voltage, such as -10v. It might
damage the PIC's i/o pin if you connect it to the PIC).

You don't need a MAX232 type chip for this temporary UART.
Because of this, we will select the INVERT option for the software UART.

Here is a sample program that shows you how to do it. My demo board
has a 16F877 on it, so that's why this program uses that PIC. This
program reads the A/D value every 1/2 second and sends it to the PC.
I can turn the trimpot with a screwdriver and watch the output change
on my terminal window on the PC. Example of output:

0186
0186
0187
018e
0196
01a8
01b0
01ba
01c8

The range of values goes from 0 to 03FF. There are gaps in the
values displayed above because I only have a 3/4 turn trimpot and
I didn't try to make a fine adjustment with the screwdriver.

Code:

#include <16F877.H>
#device adc=10
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_B1, INVERT)

void main()
{
int16 result;

setup_adc_ports(AN0);

setup_adc(ADC_CLOCK_DIV_8);

set_adc_channel(0);

while(1)
  {
   result = read_adc();
   printf("%lx\n\r", result);
   delay_ms(500);
  }

}
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