[avr-chat] ADCW keeps returning 1023

[Mikey Sklar]

I recently built a circuit board to measure the temperature of the
grease flowing through my car. The car is a old Mercedes running on
Waste Vegetable Oil from restaurant dumpsters. I was able to upload my
code and single step with gdb then I found a issue. The ADCW register
reports back 1023 despite my sensors running a wide range of values
between 1.9V - 5V.

Setup:
- atmega169 (TQFP) 
- avarice (v2.5) 
- jtag clone (MK-I only)
- avr-gcc (4.0.2)
- avr-gdb (6.5)
- Mac OS/X 10.4.11

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

#include <avr/io.h>
#include <avr/interrupt.h>


#define SENSORS 2       // three thermistors measuring temperature (0-2)


SIGNAL(SIG_ADC);
int main (void);


unsigned int adc_data;          //variable for ADC results
unsigned int illuminate;        //led display pattern


SIGNAL(SIG_ADC)                 //ADC ISR
{
        
        adc_data = ADCW;        //read all 10 bits into variable


        /* ADC data from thermistors translated into five different hex values.
           Hex values are stored for display to appropriate output LED port.
           Shows driver grease temperature by illuminating LEDs.
           The more LEDs that are on, the hotter the fuel is. */

        if ( adc_data <= 432 )
                illuminate = 0x00;      // less than 65F - all lights off
        else if ( adc_data > 432 && adc_data <= 530)
                illuminate = 0x01;      // 65F - 85F   bin: 0001  dec: 1  hex: 
0x01
        else if (adc_data > 530 && adc_data <= 770 )
                illuminate = 0x03;      // 85F - 105F  bin: 0011  dec: 3  hex: 
0x03
        else if (adc_data > 770 && adc_data <= 827)
                illuminate = 0x07;      // 105F - 125F bin: 0111  dec: 7  hex: 
0x07
        else if (adc_data > 827 && adc_data <= 1023)
                illuminate = 0x0F;      // 135F+       bin: 1111  dec: 0  hex: 
0x0F
        else 
                illuminate = 0x00;      //error all lights off


        if ( ADMUX == 0 ) {
                PORTA = illuminate;     //store sensor results for display
        }
        else if ( ADMUX == 1 )  {
                PORTC = illuminate;     //store sensor results for display
        }
        else if ( ADMUX == 2 ) {
                PORTD = illuminate;     //store sensor results for display
        }
        else {
                PORTA = 0x0A;           //MUX Error: 0101 (led pattern)
                PORTC = 0x0A;           //MUX Error: 0101 (led pattern)
                PORTD = 0x0A;           //MUX Error: 0101 (led pattern)
        }


        // rotate through sensors
        if ( ADMUX < SENSORS  ) 
                ADMUX += 1;             //change mux to next sensor
        else 
                ADMUX = 0;              //reset mux to first sensor
                

        ADCSRA = ADCSRA | 0x40; //start the next conversion

}


int main(void)
{
        DDRA=0x0F;              //LED output (first four pins)
        DDRC=0x0F;              //LED output (first four pins)
        DDRD=0x0F;              //LED output (first four pins)
        ADCSRA=0xCE;            //ADC on, /64, interrupt enable, and started
        ADMUX=0x00;             //ADC registers PF0, PF1, PF2 one at a time

        PORTA = 0x0F;           //power on: 1111 (led pattern)
        PORTC = 0x0F;           //power on: 1111 (led pattern)
        PORTD = 0x0F;           //power on: 1111 (led pattern)

        sei();                  //global interrupt enable bit

        while(1) {
        }
}