16F877A problem: "coupled" digital values

[Lupo83]

Hi everybody here's my problem with PIC 16F877A on PICDEM Mechatronics board.

I get the 2 analog inputs produced by temperature sensor and light sensor, then I convert them into digital values and send them to a PC via RS-232 (serial port), and somehow I light some LEDs.
I actually use HyperTerminal to view the produced data.

I noticed that temp and light values are "coupled": if I cover the light sensor with a sheet, temp values go crazy... fluctuating from 26°C to -5°C (!!!)
So used a multimeter to measure the voltage levels on the output pins of the sensors, BEFORE being converted by the ADC of the PIC, and everything worked fine: no voltage coupling or wrong fluctuations.
So the fault is of the ADC (and mine, because I don't program it well ) What could it be?
Microcontroller's internal interferences?
What do you think?

My code:
[code]
// ANALOG SIGNALS ACQUISITION (TEMP, LIGHT)
// FROM PICdem Mechatronics board --- TARGET PIC = 16F877A

/******************************
* TEMP --> RA0 *
* LIGHT --> RA1 *
* RB0… RB7 --> LED1… LED8 *
* *
******************************/


#define XTAL_FREQ 20MHZ

#include <pic.h>
#include "delay.h"
#include "macro.h"
#include "ascii.h"
#include "uart.h"

// CONFIGURATION BITS
__CONFIG(HS & WDTDIS & PWRTEN & BORDIS & LVPDIS & DUNPROT & WRTEN & UNPROTECT);


main(void)
{
// VARIABLES DECLARATIONS, SERIAL INITIALIZATION
int temp, light, i;
unsigned char clock = 20;
unsigned long baudrate = 9600;
char c,tempstr[4], lightstr[4];

UartInit(clock, baudrate, UART_CFG_BITSTOP_1);


//PORTS AND REGISTERS INITIALIZATION
PORTA = 0b00000000;
PORTB = 0b00000000;

TRISA = 0b00000011; // Port A WITH INPUTS AN0, AN1 (temp, light)
TRISB = 0b00000000; // Port B AS OUTPUT

//ADC CONFIG
ADCON1 = 0b10000000; // RESULT IN LITTLE ENDIAN (ADFM BIT = 1)
// f_clock of ADC like in ADCON0
// AN0... AN7: all analog inputs



while(1)
{
ADCON0 = 0b01000001; //f_clock of ADC = 1/8 f_clock of the system
//Now I set AN0 (to reset to AN1!)
//ADC enabled

ADRESH = ADRESL = 0b00000000;

/*TEMP SIGNAL CONVERSION*/
ADGO = 1;
while(ADGO) // WAIT FOR ADC...
continue;

temp = ADRESL + (ADRESH<<8);
/****************************************/

ADRESH = ADRESL = 0b00000000;

/*LIGHT SIGNAL CONVERSION*/
ADCON0 = 0b01001001; //Switch input from AN0 to AN1

ADGO = 1;
while(ADGO)
continue;

light = ADRESL + (ADRESH<<8);
/****************************************/

// TEMP in RANGE -40 -- 125 ° C
// ==> 0.1 -- 1.75 V (v. datasheet sensor TC1047)
// ==> 21 -- 358 in 10 bit ADC values (Volt * 206)

// IRRADIANCE IN RANGE 0 -- 233 uW/cm^2
// ==> 0.005 -- 3.73 V (v. datasheet sensor TSL251RD)
// ==> 1 -- 768 in 10 bit ADC values (Volt * 206)


temp = (int)(temp*0.4896 - 50.282); // Map: 21 -- 358
// in -40 -- 125 °C

if (temp<21) //limiting absurd fluctuations...
temp=21;
if (temp>100)
temp=100;


light = (int)( 0.304*(light - 1) ); // Map: 1 -- 768
// in 0 -- 233 uW/cm^2

// LEDs
if (temp==26)
PORTB=0b00000001; // Light RB0
if (temp==27)
PORTB=0b00000010; // Light RB1
if (temp==28)
PORTB=0b00000100; // Light RB2
if (temp==29)
PORTB=0b00001000; // Light RB3
if (temp==30)
PORTB=0b00010000; // Light RB4
if (temp==31)
PORTB=0b00100000; // Light RB5
if (temp==32)
PORTB=0b01000000; // Light RB6
if (temp==33)
PORTB=0b10000000; // Light RB7

// VIEW DATA
for(i = 0; i < 14; i++)
DelayMs(100);

UartPutch('-');
UartPuts("Temperature level: ");
int2dec(temp,tempstr);
UartPuts(tempstr);
UartPuts(" C\r\n\n");

DelayMs(250);

UartPutch('-');
UartPuts("Light level: ");
int2dec(light,lightstr);
UartPuts(lightstr);
UartPuts(" uW/cm^2\r\n\n\n\n");
}

}

[mackrackit]

Something looks veerry funny with your code.

Are you sure you are using PIC BASIC? That is the language we use here.

[BobK]

Ah-hem, Lupo83,

You probably didn't notice it but we are a PicBasic, forum not a "C" forum.

BobK

[Lupo83]

Yes guys, you are both perfectly right but I DID notice it I was just posting wherever there could be someone familiar with PICs and crosstalk troubles (electronical aspects do not depend on the language you use).
Besides, I was thinking that some basic expert could have also been involved in C matters.
I've already tried to write in C forums but nobody's been helpful yet, so it was despair that drove me here

Of course, I apologize if my post here was unappropriate: in that case, delete it and sorry again

ciao!

[Darrel Taylor]

Hi Lupo,

Good answer. Not a problem.

I think you need some delay between selecting the A/D channel (with ADCON0) and starting the conversion (ADGO = 1;) for sample Aquisition time.

Starting it too soon won't allow the internal Sample and Hold capacitor to fully charge. Which leaves part of the charge from the last channel in it.

HTH,

[Lupo83]

Ciao Darrel,

your answer is the most useful I've read so far, so I thank you very much for that!

I'll test your solution tomorrow morning, since the board/PIC I'm working on are not mine, but have to stay in a laboratory at my university.

Then, I'll let you know.

Best regards.

[mackrackit]

Of course, I apologize if my post here was unappropriate:

I do not think your post was inappropriate, just thought you posted to the wrong forum.
There are a few here that use "C" and I understand needing help.

Tried PIC programming in "C" a few years ago and like you I could not find help. That was one of the reasons I ended up using PIC BASIC.

Good luck.