Atmel ASF - Problem mit dem ADC

/*

 In this use case, the ADC will be configured with the following settings:

 1V from internal bandgap reference

 Div 4 clock prescaler

 12 bit resolution

 Window monitor disabled

 No gain

 Positive input on ADC PIN 0

 Negative input on ADC PIN 1

 Averaging disabled

 Oversampling disabled

 Right adjust data

 Single-ended mode

 Free running disabled

 All events (input and generation) disabled

 Sleep operation disabled

 No reference compensation

 No gain/offset correction

 No added sampling time

 Pin scan mode disabled

 */

#include <asf.h>

struct adc_module adc_instance;

void configure_adc(void);

void configure_adc(void)

{

  struct adc_config config_adc;        // Create a ADC module configuration struct, which can be filled out to adjust the configuration of a physical ADC peripheral.

  adc_get_config_defaults(&config_adc);    // Initialize the ADC configuration struct with the module's default values.

                        // This should always be performed before using the configuration struct to ensure that all values are initialized to known default settings.

  adc_init(&adc_instance, ADC, &config_adc);  // Set ADC configurations

  adc_enable(&adc_instance);          // Enable the ADC module so that conversions can be made.

}

uint16_t adc_result;              // global variable

int main (void)

{

  system_init();    

  configure_adc();

  uint16_t result;

  while (1) 

  {

    cpu_delay_ms(20);  // delay in sys ticks

    adc_start_conversion(&adc_instance);

    do

    {

      /* Wait for conversion to be done and read out result */

    } while (adc_read(&adc_instance, &result) == STATUS_BUSY);

    adc_result = result;

    asm ("NOP");

  }

}