[AVR] Arduino ADC driven by Timer interrupt Problem.

#include <SdFat.h>

#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))     // simplifying syntax of AVR methods.
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))

SdFat sd;
SdFile rec;

const int chipSelect = 53;          // CS pin of microsd module. 
unsigned long recByteCount = 0L;    // total count of bytes saved to buffer
unsigned long recByteSaved = 0L;    //total count of bytes saved to sdcard
const int btnStart = 6;             // start button pin
const int btnStop = 5;              // stop button pin
const int ledStart = 3;             // start button LED (green)
const int ledStop = 2;             // stop button LED (red)
int recPressed = 0;                 // flag to check start is pressed or not
int stopPressed = 0;                // flag to check stop is pressed or not
byte buf00[510];                    // buffer array 1 for time
byte buf01[510];                    // buffer array 2 for time
//byte buf02[1020];                    // buffer array 2 to 15 for samples
//byte buf03[1020];
unsigned int bufByteCount=1;          // array index variable for buffers mentioned above
byte bufWrite,diff;                      // flag to state which buffer to write (will have only two values 0 or 1 since we are having only two buffers)
long t1,t0;
char name1[]= "audio_1.xls";


void setup() { // THIS RUNS ONCE

  Setup_timer2();       // timer config
  Setup_ADC();      // skipped to check timer function

  pinMode(10, OUTPUT);
  pinMode(ledStart, OUTPUT);
  pinMode(ledStop, OUTPUT);
  pinMode(btnStop, INPUT_PULLUP);
  pinMode(btnStart, INPUT_PULLUP);
  
  if (sd.begin(chipSelect, SPI_FULL_SPEED)) { // initialise card on SPI to 8MHz SPI bus speed
    for (int dloop = 0; dloop < 4; dloop++) {
      digitalWrite(ledStart,!digitalRead(ledStart));
      delay(50);
    }
  } else { // if error, flash LED twice per second, until reset
    while(1) {
      digitalWrite(ledStart,!digitalRead(ledStart));
      delay(50);
    }
  }
  for(int i=2;i<=510;i+=2)
  {
   if(i%10==0){buf00[i]='\n';buf01[i]='\n';}
   else{buf00[i]='\t'; buf01[i]='\t';}

  }
  
}
void loop() { // THIS RUNS CONTI.

  if (digitalRead(btnStart) == LOW && recPressed == 0) 
  {
    StartRec(); // launch StartRec method
  }
  
  if (digitalRead(btnStop) == LOW) 
  {
  cbi (TIMSK2, OCIE2A); 
  rec.close();
  recPressed = 0;
  digitalWrite(ledStart,LOW); // turn off recording LED
  digitalWrite(ledStop,HIGH); // light stop LED
  }
  
  if (recByteCount % 1020 == 510 && recPressed == 1) 
  {      
      rec.write(buf00,510);  
      recByteSaved+= 510; 
  } // save buf00 to card
  
  if (recByteCount % 1020 == 0 && recPressed == 1) 
  {   
       rec.write(buf01,510); 
       recByteSaved+= 510; 
  } // save buf01 to card


}

void StartRec() { // begin recording process

  digitalWrite(ledStart,HIGH);
  digitalWrite(ledStop,LOW);
  recByteCount = 0;
  recByteSaved = 0;
  recPressed = 1; // recording button has been pressed
  stopPressed = 0;
  
  rec.open(name1, O_CREAT | O_TRUNC | O_RDWR);

  sbi (TIMSK2, OCIE2A); // enable timer interrupt, start grabbing samples (here, time )

}

void Setup_timer2() {

  TCCR2B = _BV(CS21);  // Timer2 Clock Prescaler to : 8
  TCCR2A = _BV(WGM21); // Interupt frequency  = 16MHz / (8 x 90 + 1) = 22191Hz
  OCR2A = 221; // Compare Match register set to 221. to match 111uS requirement (i.e. 9.009kHz)

}

void Setup_ADC() {                

   // set ADC to read pin A5, ADLAR to 1 (left adjust)
  cbi(ADCSRA,ADPS2); // set prescaler to 8 / ADC clock = 2MHz
  sbi(ADCSRA,ADPS1);
  cbi(ADCSRA,ADPS0);
}

ISR(TIMER2_COMPA_vect) {            // timer interrupt function will be called automatically.
  t1=micros();      //catch the time when sample taken
 
  ADMUX = 0x65;     // set ADMUX to read analog channel 5.
  sbi(ADCSRA, ADSC); // start ADC sample
  while(bit_is_set(ADCSRA, ADSC));  // wait until ADSC bit goes low = new sample ready
  
  diff=t1-t0;
  t0=t1;

 if (bufByteCount == 510 && bufWrite == 0) { 
    bufByteCount = 0;
    bufWrite = 1;
  } else if (bufByteCount == 510 && bufWrite == 1) {
    bufByteCount = 0;
    bufWrite = 0;
  }
  
  if (bufWrite == 0) { buf00[bufByteCount] = diff;  buf00[bufByteCount+2] = ADCH;}    // we will read analog sensors here.
  if (bufWrite == 1) { buf01[bufByteCount] = diff;  buf01[bufByteCount+2] = ADCH;}    //here, difference between two sampled time is taken for timing purpose. 
  delayMicroseconds(10);
  
ADMUX = 0x66;       // set ADMUX to read analog channel 6.
  sbi(ADCSRA, ADSC); // start ADC sample
  while(bit_is_set(ADCSRA, ADSC));  // wait until ADSC bit goes low = new sample ready
  
  if (bufWrite == 0) {  buf00[bufByteCount+4] = ADCH;}
  if (bufWrite == 1) {  buf01[bufByteCount+4] = ADCH;}
  delayMicroseconds(10);

  ADMUX = 0x67;       // set ADMUX to read analog channel 7.
  sbi(ADCSRA, ADSC); // start ADC sample
  while(bit_is_set(ADCSRA, ADSC));  // wait until ADSC bit goes low = new sample ready
  
  if (bufWrite == 0) {  buf00[bufByteCount+6] = ADCH;}  
  if (bufWrite == 1) {  buf01[bufByteCount+6] = ADCH;}
  delayMicroseconds(10);

  ADMUX = 0x68;       // set ADMUX to read analog channel 7.
  sbi(ADCSRA, ADSC); // start ADC sample
  while(bit_is_set(ADCSRA, ADSC));

  if (bufWrite == 0) {  buf00[bufByteCount+8] = ADCH;}  
  if (bufWrite == 1) {  buf01[bufByteCount+8] = ADCH;}

  recByteCount+=10; // increment sample counter 
  bufByteCount+=10;
  delayMicroseconds(10);

}

 if (bufByteCount == 510 && bufWrite == 0) { 
    bufByteCount = 0;
    bufWrite = 1;
  } else if (bufByteCount == 510 && bufWrite == 1) {
    bufByteCount = 0;
    bufWrite = 0;
  }

 if (bufByteCount >= 510 && bufWrite == 0) { 
    bufByteCount = 0;
    bufWrite = 1;
  } else if (bufByteCount >= 510 && bufWrite == 1) {
    bufByteCount = 0;
    bufWrite = 0;
  }