how to display -ve temp reading on LCD??

[hhhsssmmm]

Hello

Im using PIC18F4220, 20MHz ceramic, C18 compiler.

I have a running project where im taking temperature readings after every 300ms intervals and display on 16x2 LCD.

Im using thermistor MCP9701A. I have 8bit ADC enabled on my PIC and im only reading the high byte ADRESH.

My V+ref == Vdd and V-ref == Vss.

All seems to work fine and im getting good temp read outs going up to room temperature. This i compared with a
precision digital thermometer. The only problem is when the temperature drops below zero, i get only two fixed
numbers showing on LCD.... -55C and -35C.

Please can anyone explain why im getting some crazing -ve reading where in fact my -ve temperature reading
should not be dropping more than -2C....and this should show up on LCD as " -02C ".

Below is my code...plz see it and kindly let me know where the mistake might be.

Thanks
Haseeb

#include <p18f4220.h>    
#include <delays.h>
        
#pragma config OSC = HS
#pragma config WDT = OFF  
#pragma config LVP = OFF 
 
void SendLCD(unsigned char Byte, unsigned char type); //LCD display function
 
#define LCD LATD //LCD Latch PORT
#define LCD_RS LATEbits.LATE0 //Register Select (0: Instruction/ 1: Data)
#define LCD_E LATEbits.LATE2 //Enable (Starts data read/write)
#define LED LATCbits.LATC3  //power up LED

unsigned char minus = 0, //-ve flag...for -ve sign activation             

              bin = 0, //temp buffer variable for ADC value for BCD conversion process                     

             //BCD coversion variables
             d0 = 0,       
             d1 = 0,
              d2 = 0;   
      
void main(void)
{     

  //Configure ADC
  ADCON1 = 0b00001110 ; //VREF- = VSS,
                         //VREF+ = VDD,
                        //Enable AN0 only...rest all Digital
   
  ADCON0 = 0b00000000;  //Select channel 0 (AN0),
                         //ADC Idle,
                        //ADC Dissabled
              
  ADCON2 = 0b00001100;    //Left Justified for 8bit ADC scaling
                         //ADC Acquisition Time 2TAD,
                        //ADC Conversion Clock FOSC/4         

  PORTA = 0; //initiallize PORTA
  TRISAbits.TRISA0 = 1; // Analog Channel 0 (AN0 input) 
 
  PORTB = 0; //intialize PORTB 
  TRISBbits.TRISB3 = 1;    //swtich 1 input
  TRISBbits.TRISB4 = 1;    //switch 2 input
  TRISBbits.TRISB5 = 1;    //switch 3 input   

  PORTC = 0; //intialize PORTC
  TRISCbits.TRISC3 = 0;  //LED      
 
  PORTD = 0; //initiallize PORTD 
  TRISD = 0x00; //LCD output   

  PORTE = 0; //initiallize PORTE   
  TRISEbits.TRISE0 = 0;  //LCD Register Select (to LCD Pin 4)
  TRISEbits.TRISE2 = 0;  //LCD Enable (to LCD Pin 6)
 

    INTCON = 0; //dissable all interrupts


    Delay10KTCYx(10); //20ms wait for LCD power up


    //Initialize the LCD
    LCD = 0x00; //clear LCD PORTD pins
    Delay1KTCYx(25); //delay 5mS
    SendLCD(0x03,0); //Initialization command
    Delay1KTCYx(25); //delay 5mS
    SendLCD(0x03,0); //Initialization command
    Delay100TCYx(8); //delay 160uS
    SendLCD(0x03,0); //Initialization command
    Delay100TCYx(8); //delay 160uS
    SendLCD(0x3C,0); //Interface lenght is 8 bits long, 2-lines, 5x10 dots
    Delay100TCYx(8); //delay 160uS
    SendLCD(0x10,0); //Turn off LCD
    Delay100TCYx(8); //delay 160uS
    SendLCD(0x01,0); //Clear LCD   
    Delay1KTCYx(25); //delay 5mS
    SendLCD(0x06,0); //Increment the cursor after each byte written
    Delay100TCYx(8); //delay 160uS
    SendLCD(0x0C,0); //Turn on LCD, cursor off, cursor blinking off
    Delay100TCYx(8); //delay 160uS


    //Power up flashing LED
    LED = 1;   
    Delay10KTCYx(125); //250ms delay            
    LED = 0;
    Delay10KTCYx(125); //250ms delay
    LED = 1;
    Delay10KTCYx(125); //250ms delay
    LED = 0;


    ADCON0bits.ADON = 1; //Enable ADC


    while(1) //loop forever
    {         
   
        ADCON0bits.GO_DONE = 1; //Start ADC Conversion
                                           
        while (ADCON0bits.GO_DONE == 1);  //Wait until Conversion finishes
 
        bin = ADRESH;  // reading ADC High byte only for 8bit ADC scale

        if(bin < 20) //below 0 deg C ... (0 deg C @ 400mV)                   

            minus = 1;    //activate -ve sign   

        bin = bin - 20; //offset adjustment ... (0 deg C @ 400mV)               

        //convert ADC value to BCD
        d1 = bin % 10;                   
        d0 = (bin / 10) % 10;                  
       
        //convert BCD digit numbers to ASCII text characters for LCD           
        d0 += '0';
        d1 += '0';           

        SendLCD(0x80,0); //activate LCD line 1
           
        //if temperature is in -ve ... then print -ve sign on LCD
        if(minus == 1)
        {

            SendLCD('-',1);    

            minus = 0; //reset minus sign flag

        }   

        else //temperature is in +ve so erase minus sign (blank space print)
       
            SendLCD(' ',1);

        //print the temperature on LCD
        SendLCD(d0,1);    
        SendLCD(d1,1);    
        SendLCD('C',1);                            
       
        Delay10KTCYx(150); //300ms delay            

    }// end of main while loop




} //end of main()



void SendLCD(unsigned char Byte, unsigned char type)
{
   
    LCD_RS = type; //Set whether it is a Command (0) or Data/Char (1)   

    Delay100TCYx(8); //delay 160uS

    LCD = Byte; //assign the new data to the LCD PORTD
   
    Delay100TCYx(8); //delay 160uS

    LCD_E = 1; //Set Enable High   
   
    Delay100TCYx(8); //delay 160uS
   
    LCD_E = 0; //Set Enable Low

}

 

[dougy83]

Well the problem I would think is here:

        if(bin < 20) //below 0 deg C ... (0 deg C @ 400mV)                   

            minus = 1;    //activate -ve sign   

        bin = bin - 20; //offset adjustment ... (0 deg C @ 400mV)

if the temperature is < 0C, bin <20, but bin is an unsigned char so bin - 20 is 255 for -1C, which would explain why you get -55 displayed.

I would suggest declaring bin as a signed char, and replacing the above code with:

bin = bin - 20; //offset adjustment ... (0 deg C @ 400mV)
if(bin < 0) //below 0 deg C
{
    minus = 1;    //activate -ve sign   
    bin = -bin;    // make bin printable
}

Or even more simply, just replace those above lines with (keep the unsigned char decl):

if(bin < 20) //below 0 deg C @ 400mV offset
{
    minus = 1;    //activate -ve sign   
    bin = 20-bin;    // make bin printable
}
else
    bin = bin - 20;

 

[hhhsssmmm]

Thank you so much for your help...your suggestion #2 worked perfectly...

thanks so much..

best regards

Haseeb

 

[dougy83]

You're of course most welcome. Glad to be of assistance.