问在atmega16中输出错误

EN

#include<avr/io.h>
int adc(void);
void pump(void);
int adc_value;
int main(void)
 {
  DDRC=0x01;                          //Defining PC0 as output
  ADCSRA=0x87;                    //Setting the mode of operation
  ADMUX=0x00;                     //Selection of channel and bit alignment
  while(1)
   {
     adc_value=adc();                //reading moisture level
     pump();                               //Pump activator routine
   }
   return 0;
 }

int adc(void)
{
   int lower_bits,higher_bits,result; 
   ADCSRA |= (1 << ADSC)|(1 << ADIF);  //Turn on conversion and clear flag
   while(ADCSRA & (1 << ADIF) == 0);  //wait for flag
   lower_bits=ADCL;
   higher_bits=ADCH;
   result=lower_bits|(higher_bits<<8);         //Accessing converted value by shifting
   return result;
 }

void pump(void)
 {
  if(adc_value>=700)                                //Pump ON trigger point
   {
     PORTC|=(1<<0);
   }
  else if(adc_value<=600)                        //Pump Off trigger point
   {
     PORTC&=~(1<<0);
   }
 }