Raspberry Pi ADC Interrupt

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>

#include <wiringPi.h>
#include <wiringPiSPI.h>

#define CS_MCP3208  8       // BCM_GPIO8

#define BUTTON_PIN 3
#define DG_OUT 7
#define CHANNEL_BIT_MASK 0x07
#define BYTE_I_MASK 0x06
#define BYTE_II_MASK 0x00
#define BYTE_II_RCV_MASK 0x0F
#define SPI_CHANNEL 0
#define SPI_SPEED   2000000 // !! Start low here and if all works try to increase if needed on a breadboard I could go upto about 750000 
int toggle = 1;
int start, end;  int read_mcp3208_adc(unsigned char adcChannel);


void myInterrupt(void) {  int adc1Channel = 0;  int adc1Value   = 0;    start = micros();  toggle ^= 1;  digitalWrite(DG_OUT, toggle);   adc1Value = read_mcp3208_adc(adc1Channel);  adc1Value = (3.3/4096) * adc1Value;  end = micros();
  //printf("\n int: 0 %d \n", end - start);
}


int main (void)
{  //int adc1Channel = 0;  //int adc1Value   = 0;
     if(wiringPiSetup() == -1)  {    fprintf (stdout, "Unable to start wiringPi: %s\n", strerror(errno));    return 1 ;  }
  wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED);      if ( wiringPiISR (BUTTON_PIN, INT_EDGE_RISING, &myInterrupt) < 0 ) {      fprintf (stderr, "Unable to setup ISR: %s\n", strerror (errno));      return 1;  }
  pinMode(CS_MCP3208, OUTPUT);  pinMode(DG_OUT, OUTPUT);
  while(1)  {    //system("clear");    //printf("\n\nMCP3208 channel output.\n\n");    //adc1Value = read_mcp3208_adc(adc1Channel);    //printf("adc0 Value = %04u", adc1Value);    //printf("\tVoltage = %.6f\n", ((3.3/4096) * adc1Value));
    usleep(1000000);    //printf("\n int: 0 %d \n", end - start);    //waitForInterrupt(BUTTON_PIN,1);
    }  return 0;
}



int read_mcp3208_adc(unsigned char adcChannel)
{  unsigned char buff[3];  int adcValue = 0;  
  buff[0] = BYTE_I_MASK | ((adcChannel & CHANNEL_BIT_MASK) >> 2);  buff[1] = ((adcChannel & CHANNEL_BIT_MASK) << 6);  buff[2] = BYTE_II_MASK;
  digitalWrite(CS_MCP3208, 0);  // Low : CS Active
  wiringPiSPIDataRW(SPI_CHANNEL, buff, 3);
  buff[1] = BYTE_II_RCV_MASK & buff[1];  adcValue = ( buff[1] << 8) | buff[2];
  digitalWrite(CS_MCP3208, 1);  // High : CS Inactive  
  return adcValue;
}