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

volatile uint16_t time, wait=50, avg_cnt=20;
volatile uint32_t avg_sum=0, avg;


ISR(TIMER1_CAPT_vect)
{
static int16_t icr1;
static int16_t delta;

time = ICR1-icr1;
icr1 = ICR1;

if(wait)  // power on wait
   wait--;
else
  if(avg_cnt>0) // measure LC frequency on startup
   {
   avg_cnt--;
   avg_sum += time;
   } 

avg = avg_sum / 20;
delta = time - avg;

// no magnetic metal
if( delta < -10)   PORTC = (1<<2);
if( delta < -20)   PORTC = (1<<1);
if( delta < -60)   PORTC = (1<<0);

// magnetic metal
if( delta > 10)    PORTC = (1<<4);
if( delta > 60)   PORTC = (1<<5);

// nothing
if (( delta <= 10) && (delta >= -10)) PORTC = (1<<3);


// autotune
if(time > avg)
     avg_sum += 5;
if(time < avg)
    avg_sum -= 5;
}


int main(void)
{

// Timer0 prescaler div 0xff
TCCR0B = (1<<CS01) | (1<<CS02);
OCR0B = 128;
TCCR0A = (1<<COM0B0);
DDRD = (1<<5);  // output from prescaler


// Timer1 as capture
TCCR1B = (1<< CS10) | (1<< WGM13);  // clock 20MHz
TCCR1B = (1<< CS10) | (1<< ICNC1);
TIMSK1 = (1<< ICIE1);
PORTD = (1<<4); // pull up for input

DDRC = (1<<0) | (1<<1) |(1<<2) |(1<<3) |(1<<4) |(1<<5);    // LEDs

sei();

while(1);  //
}