A 0.1uf capacitor between the A/D input and ground might help.

Regards,

Anand.


----- Original Message ----- 
From: "Jason Wambach" <jwambach@gmail.com>
To: <piclist@mit.edu>
Sent: Saturday, October 29, 2005 9:59 PM
Subject: [pic] Newbie help with 2 channel A/D


> Hi,
> 
> I'm relatively new to the world of embedded programming but have been
> making progress lately up until I tried to read 2 different A/D
> channels.  My circuit is going to be a temperature control set to
> display on a Hitachi 44780 LCD along with a 10k pot for reading the
> temperature set-point.  I'm using a pic16f684 configured to use the
> internal oscillator running at 8Mhz.  I have the LCD connected to
> portc using a 4-bit operation for display.  A 10K thermistor is
> connected to RA0 (pin 13) and a 10K pot connected to RA2 (pin 11).
> 
> I've taken the approach of getting the individual pieces to work by
> themselves and am now trying to get them to work together.  The LCD
> works fine, reading a temperature works as well as reading the pot
> (using the 8 most significant bits).  My problem comes when I try to
> read both the thermistor and the pot in subsequent loops.  If I turn
> off the switch (sw) and only read either the pot or the thermistor, it
> works.  If I oscillate between the two, neither reads an analog value
> properly.
> 
> I tried to add a delay after I set the ADCON0 bits for each read but
> that doesn't seem to have helped.  At this point I am at a bit of a
> loss as to what needs to be done.  I'm hoping its something simple
> that I have overlooked.  I have attached the code below, any insight
> you can provide as to what I am doing wrong would be very much
> appreciated.
> 
> Thanks,
> Jason
> 
> /**
> Thermo2.c
> Jason Wambach
> 
> PICC program for the pic16f684 to read temperature and display on an LCD.
> 
> **/
> #include <pic.h>
> #include <math.h>
> __CONFIG(FCMDIS & IESODIS & BORDIS & UNPROTECT & MCLRDIS & PWRTEN &
> WDTDIS & INTIO);
> 
> 
> int i,j,k,n,Count,b,sw;
> unsigned int a;
> double Temp;
> //                          1234567890123456
> const char TopMessage[]  = "Curr Temp: ";
> const char BotMessage[]  = "Set  Temp: ";
> 
> #define E RC4
> #define RS RC5
> 
> const int Twentyms = 1250;
> 
> const int Fivems = 300;
> const int TwoHundredus = 10;
> 
> //4bit LCD Write Operation
> LCDWrite(int LCDData, int RSValue)
> {
> PORTC = (LCDData >> 4) & 0x0F;
> RS = RSValue;
> E = 1; E = 0;
> 
> PORTC = LCDData & 0x0F;
> 
> RS = RSValue;
> E = 1;
> E = 0;
> 
> if ((0 == (LCDData & 0xFC)) && (0 == RSValue))
> {
> n =  Fivems;
> }
> else
> {
> n = TwoHundredus;
> }
> for (k = 0; k < n; k++);
> 
> }
> 
> //Write formatted decimal to LCD
> TempDec(int i)
> {
> int t;
> 
> if (i >= 100)
> {
> t = i / 100;
> LCDWrite('0' + t, 1);
> i = i % 100; 
> }
> else
> {
> LCDWrite(' ', 1);
> }
> if (i >= 10)
> {
> t = i / 10;
> LCDWrite('0' + t, 1);
> i = i % 10;
> }
> else
> {
> LCDWrite('0', 1);
> }
> LCDWrite('0' + i, 1);
> }
> 
> main()
> {
> PORTC = 0;
> CMCON0 = 7;
> TRISC = 0;
> TRISA0 = 1;
> TRISA2 = 1;
> 
> ANSEL  = 0b00000101; 
> ADCON1 = 0b00010000; //Clock Fosc/8 or 2uS
> 
> //LCD Initialization
> j = Twentyms;
> for (i = 0; i < j; i++);
> PORTC = 3;
> E = 1; E = 0;  //Reset
> j = Fivems;
> for (i = 0; i < j; i++);
> 
> E = 1; E = 0;  //Reset 2
> j = TwoHundredus;
> for (i = 0; i < j; i++);
> 
> E = 1; E = 0;  //Reset 3
> j = TwoHundredus;
> for (i = 0; i < j; i++);
> 
> PORTC = 2;
> E = 1; E = 0;
> j = TwoHundredus;
> for (i = 0; i < j; i++);
> 
> LCDWrite(0b00101000,0);
> LCDWrite(0b00000001,0);  //clear lcd
> LCDWrite(0b00000110,0);
> LCDWrite(0b00001110,0);
> 
> a = 0;
> Count = 0;
> sw = 0;
> while (1 == 1)
> {
> if (sw)
> {
> ADCON0 = 0b10000001; //Selected RA0, Right justified
> j = Twentyms;  //Delay for setup of A/D converter
> for (i = 0; i < j; i++);
> GODONE = 1;
> while (!GODONE);
> a = (ADRESH << 8) + ADRESL;
> //Temperature Calculation
> Temp = (1023.0 / (double)a) - 1.0;
> Temp = log(Temp);
>                 Temp = 32.5 - (Temp / 0.019129);
> Temp = (9f/5f)*Temp+32;
> a = (int)Temp;
> }
> else
> {
> ADCON0 = 0b00001001;  //select RA2, Left justified result
> j = Twentyms;   //Delay for setup of A/D converter
> for (i = 0; i < j; i++);
> GODONE = 1;
> while (!GODONE);
> b = ADRESH;  //only care about 8 most significant bits
> }
> sw = sw ^ 1;  //switch between channels every loop
> 
> //Only update the LCD every 10 iterations
> if (Count == 0)
> {
> LCDWrite(0b00000001,0);  //clear lcd
> for (i = 0; i< TopMessage[i] != 0; i++)
> {
> LCDWrite(TopMessage[i],1);
> //LCDWrite('0' + i, 1);
> }
> 
> TempDec(a);  //Format and display
> LCDWrite(0xDF, 1);  //Degree symbol
> LCDWrite('F', 1);
> 
> LCDWrite(0b11000000,0); //Move to 2nd line
> //2nd Line
> for (i = 0; BotMessage[i] != 0; i++)
> {
> LCDWrite(BotMessage[i],1);
> }
> 
> TempDec(b);
> LCDWrite(0xDF, 1);
> LCDWrite('F', 1);
> Count++;
> }
> else
> {
> Count++;
> if (Count == 10)
> {
> Count = 0;
> }
> } 
> //delay
> for (i = 0; i < 255; i++)
> {
> for (j = 0; j < 129; j++);
> }
> }
> 
> }
> 
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