ADC on arduino in C

Question

I'm trying to use the Arduio Uno in pure C because I can't use the Arduino IDE for my senior design project. I've succeeded in getting the serial communication to work, the digital outputs/inputs, and the analog inputs, to an extent. I'm getting a reading off the analog input, but most of them are 20,000+, which is way to high. This is supposedly a 10 bit ADC, and I'm only trying to use 8 bits. Why is my resulting read 100X what the highest supposedly is?

void init_aio(){
      DIDR0 = 0x00;           //Digital input disabled on all ADC ports
      PRR &= ~(1<<PRADC);     //ADC turned on
      ADMUX = 0x60;           //AVcc, right adjusted, ADC0 pin
      ADCSRA = 0xcF;          //ADC Enabled, no auto trigger, Iterrupt enabled, 128 prescaller
}

int read_analog(){
    reading = APin0;
    ADCSRA |= 1<<ADSC; //conversion start
    reading = abs(reading);
    return reading;
}

The only thing I can think of is that I'm using "int reading_str = itoa(reading, buffer, 10);" to make it a printable value. When I print reading directly, it prints garbage to the terminal. (char buffer[100]; is what buffer is)

The Uno uses an Atmega328P: www.atmel.com/Images/doc8161.pdf‎

Thanks for any help.

Answer 1

The top bits of the ADC aren't zero-initialised, so you need to manually mask them away:

reading &= 0x3FF; // binary 0000001111111111, i.e. ten bits for ADC

You can then verify that this is correct by tying your analog pin to ground and Vref respectively, and checking that the resulting values match expectations.

In case anyone else runs into the same problem, using a different implementation of reading the ADC, ensure that your endianness and bit-ordering are correct.