Example sampling code for FSK

void ADC_Init(void){
    //Configure Inputs
    TRISA |= 0x01;
    ANSEL |= 0x01;

    ADCON0 = (1<<0) | (0<<2); //Enable ADC and select channel 0
    ADCON1 = (0<<4) | (0<<5); //Set both references to PIC supply
    ADCON2 = (0<<0) | (7<<3) | (0<<7); //Use 20 acquisition cycles and FOsc/2, left justified
}

void ADC_StartConv(void){
    ADCON0 |= (1<<1);
}

unsigned int ADC_CheckConv(void){
    if(ADCON0 & 2) return 1;
    else return 0;
}

void ADC_WaitForConv(void){
    while(ADC_CheckConv());
}

unsigned char ADC_GetConv(void){
    return ADRESH;
}

volatile unsigned int ISample = 0, AmntSamples = 0;
volatile int LPF = 0, HSample = 0;
volatile int SZCrossP = 0;
volatile unsigned char IZC = 0;

//Long is 32bit?
volatile unsigned long Frequency = 0;

void InterruptServiceLow(void) {
        //This interrupt takes 25 instructions (approx 25 cycles = 1.6us @ 64MHz=16MIPS)
        //Happens every 16us (1/(16MHz/256)), uses about 1.6us/16us = 10% of CPU time
        //Interrupt frequency: 62.5kHz

     if  (INTCON & 0b00000100) {     //TIMER0 Interrupt
        INTCON &= 0b11111011;   //CLEAR Timer0 interrupt flag
        tick++;
        timer0intflag = 1;

        ISample = ADC_GetConv(); //Get previously converted value
        ADC_StartConv(); //Start conversion in the background

        //High pass the sample to get rid of DC offset, cutoff determined through Matlab
        //as 40Hz
        LPF += (ISample - LPF)>>7;

        AmntSamples++;

        //Set the last sample
        LSample = HSample;
        HSample = ISample - LPF;

        //Detect zero crossing point
        //If current sample is positive and last sample was negative
        //Therefore, this finds the rising edge of a signal
        if(HSample>0 && LSample <=0){
            if(!IZC){
                AmntSamples = 0;
                IZC = 1;
            }
            //Second rising edge
            else{
                IZC = 0;
                SZCrossP = AmntSamples;

                //The whole period took 'SZCrossP' samples. Knowing one tick is 16us
                //The frequency will be equal to 1/(SZCrossP*16us) e.g. if there were 100
                //samples for the whole period, the frequency would be 625Hz (1/(100*16e-6).
                //The equation could also be 1e6/(SZCrossP*16).

                //Multiplication by 16 is the same as upshifting by 4.
                Frequency = 1000000ul/((unsigned long)SZCrossP<<4);
            }
        }
    }
}