Speeding up the ADC's on an arduino

gistfile1.c

#define FASTADC 1

// defines for setting and clearing register bits
#ifndef cbi
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#endif
#ifndef sbi
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))
#endif

void setup() {
  int start ;
  int i ;
  
#if FASTADC
  // set prescale to 16
  sbi(ADCSRA,ADPS2) ;
  cbi(ADCSRA,ADPS1) ;
  cbi(ADCSRA,ADPS0) ;
#endif

  Serial.begin(9600) ;
  Serial.print("ADCTEST: ") ;
  start = millis() ;
  for (i = 0 ; i < 1000 ; i++)
    analogRead(0) ;
  Serial.print(millis() - start) ;
  Serial.println(" msec (1000 calls)") ;
}