Measurement not acquired by ADC @8MHz.

Using Arduino Programming Questions
1

I'm trying to get a measurement from my ADC on an Arduino Uno rev3 using different prescaling factors. I have connected a pot at A0 to be able to modify the voltage. When the measurement is higher than 300, a pulse is output at pin 6, otherwise a constant value is output. Here is my source code:

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

#define _FREE_RUNNING_MODE 100
#define _SIGNLE_CONVERSION_MODE 101

#define mode _FREE_RUNNING_MODE

#define prescalingFactor 2

int main(void)
{

  DDRC &= ~(1<<DDC0); // Pin A0 as Input
  DDRD |=  (1<<PIND7);//Pin 6 on arduino as output
  if (prescalingFactor==16){
    ADCSRA |= 1<<ADPS2;
  }
  else if (prescalingFactor==2){
    ADCSRA |= 1<<ADPS0;
  }
  else if (prescalingFactor==4){
    ADCSRA |= 1<<ADPS1;
  }
  else {
    //default to 128
    ADCSRA |= 1<<ADPS2 | 1<<ADPS1 | 1<<ADPS0;
  }
  ADMUX |= (1<<REFS0) | (1<<REFS1); // Internal 1.1V reference
  if (mode==_FREE_RUNNING_MODE){
    ADCSRB &= ~((1<<ADTS2)|(1<<ADTS1)|(1<<ADTS0));//ADC in free-running mode
    ADCSRA |= (1<<ADATE);//Signal source, in this case is the free-running
  }
  ADCSRA |= 1<<ADIE; // Enable the interrupt
  ADCSRA |= 1<<ADEN;// Enable the ADR
  sei();// Enable Interrupts (Global)

  ADCSRA |= 1<<ADSC;//Start first conversion (necessary always)  
  while(1){     
  }
}

ISR(ADC_vect)
{
  uint8_t lowPart = ADCL;
  uint16_t myMeasurement = ADCH<<8 | lowPart;
  if (mode==_SIGNLE_CONVERSION_MODE) { 
    ADCSRA |= 1<<ADSC; 
  }//trigger new conversion
  if (myMeasurement>=300){
    PIND ^= 1<<PIND6;//flip pin 6 on arduino
  }
}

For debugging I use Serial.println(myMeasurement); inside the while(1){} loop. Everything works as expected for every prescaling factor greater than 2 (4,8,...,128). But, when I set the prescaler's division factor to 2 (i.e. 8MHz):

  • I constantly get a pulse at the output, meaning that it seems that myMeasurement>300 holds true regardless of the position of the potentiometer.
  • If I use Serial.println for debugging, I get the measurement 1023 always.

Any ideas? Why isn't my measurement updated on runtime?

2

Read the atmega328p datasheet. Here is an excerpt:

By default, the successive approximation circuitry requires an input clock frequency between 50kHz and 200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency to the ADC can be higher than 200kHz to get a higher sample rate.

See also section 29.8 to find the maximum ADC clock frequency.

3

chung:
Any ideas? Why isn't my measurement updated on runtime?

The part of the datasheet that explains how to set the prescalar also discusses the limits of the ADC...