ADC with 4.096 reference voltage

[lloydi12345]

I am building a temperature reading circuit and I am using a thermistor for temperature reading with the help of PIC18F4620. How can I use 4.096V as reference voltage? What are the guidelines? The reading pin is RA0/Analog0. I assigned TRISA to 0x0F to have inputs from RA0 to RA3. I also set ADCON1 to 0x3B.

Should TRISA's Vref+ and Vref- be set as input?
Should TRISA's Vref+ and Vref- be set as analog?

RA2 was connected to GND while RA3 was connected to 4.096 voltage. The precision voltage was generated through MCP1541.

Here is a snippet of my code:

    LATA = 0x00;
    LATB = 0x00;
    LATC = 0x00;
    LATC = 0x00;
    LATE = 0x00;

    PORTA = 0x00;
    PORTB = 0x00;
    PORTC = 0x00;
    PORTD = 0x00;
    PORTE = 0x00;

    TRISA = 0x0F;    //default-0x01 make port0 as input for analog reading, make pin ra2 and ra3 inputs also
    TRISB = 0x0F;    //make port0-3 as input for button pressing
    TRISC = 0x00;
    TRISD = 0x00;
    TRISE = 0x00;

    //Analog settings
    ADCON0 = 0x00; //choose analog0
    ADCON1 = 0x3B;
    //Details:
    //Connected Vref+(AN2/Pin5) to 4.096V and Vref-(AN3/Pin4) to GND
    //RA2 & RA3 is analog and rest are digital, changed vref- and vref+
    ADCON2 = 0xA9; //12 TAD Fosc/8

    // configure Timer1
    T1CONbits.RD16 = 1;     // 16-bit read/write
    T1CONbits.T1CKPS0 = 1;  // 1:8 prescale
    T1CONbits.T1CKPS1 = 1;
    T1CONbits.T1OSCEN = 0;  // not using external osc, so disable driver
    T1CONbits.TMR1CS = 0;   // internal clock

    TMR1H = 0xDD;
    TMR1L = 0xAA;

    // enable Timer1
    T1CONbits.TMR1ON = 1;   // turn on timer1

    // other Timer1 settings
    RCONbits.IPEN = 1;            // IPEN = 1
    IPR1bits.TMR1IP = 1;        // Timer1 high priority int
    INTCON2bits.TMR0IP = 1;        // Timer0 high priority int
   
    PIE1bits.TMR1IE = 1;        // Enable timer1

    INTCONbits.GIE = 1;
    INTCONbits.PEIE = 1;

    // configure A/D convertor
    OpenADC(ADC_FOSC_64 & ADC_RIGHT_JUST
            & ADC_12_TAD, ADC_CH0 & ADC_INT_OFF
            & ADC_REF_VDD_VSS, ADC_1ANA);

void get_temperature(void){
    temp_holder = 0;//variable declaration for temperature

    for (convert_var=0;convert_var<3;convert_var++){
        ConvertADC(); // Start conversion
        while(BusyADC()); // Wait for completion
        adc_result = ReadADC(); // Read result
   
        //this is the formula based on Steinharts-harts equation
        res = 3300.0 / (1024.0 / adc_result - 1.0);
        resistance = log(res);
        tempk = (1.0 / (Ra + (Rb * resistance) + (Rc * (pow(resistance,3.0)))));
        tempf = (tempk * 1.8) - 459.67;  //1.8 = 9.0/5.0
        temp_holder = temp_holder + tempf;
    }
    temp1 = temp_holder / 3.0; // original = 3.0

    temp3 = (temp1 + temp3)/2.0;

    if (temp3 < 75.0){
        temp3 = 75.0;
    }

//in case of intense temperature fluctuations
//include this line of code below

    tempB = (tempC + tempB)/2.0;
    tempC = (tempC + tempB + temp3) / 3.0;

    temp_holder = tempB;
}

R27 is currently connected to 4.096V. With these current settings, the temperature reading is inaccurate. When I connect RA3 to 5V the reading doesn't change but when I connect R27 to 5V the reading goes right. It looks like Vref+ is still using the internal Vdd as its reference. What else did I miss?

lloyd

 

[nickelflippr]

TRISA as analog for Vref+, no need to fool with Vref- as internal GND is default. The MCP1541 output can only source a couple of ma's, so the R27 connection to a switch (to what?) and AN0 is a bit of a mystery.

 

[lloydi12345]

Found the problem. It was OpenADC configuration. Thanks anyway nickelflippr.