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   1 #define BTN_PIN   2
   2
   3 volatile uint8_t timer_en = 0;
   4
   5 void button_pressed_ISR(void);
   6
   7 void setup()
   8 {
   9   // Stop reception of interrupts
  10   noInterrupts(); //cli();
  11
  12   pinMode(BTN_PIN, INPUT_PULLUP);
  13   attachInterrupt(digitalPinToInterrupt(BTN_PIN), button_pressed_ISR, FALLING);
  14
  15   // Set PB1 to be an output (Pin9 Arduino UNO)
  16   DDRB |= (1 << PB1);
  17
  18   // Clear Timer/Counter Control Registers
  19   TCCR1A = 0;
  20   TCCR1B = 0;
  21   TIMSK1 = 0;
  22
  23   // Set non-inverting mode - Table 15-3 (page 108)
  24   TCCR1A |= (1 << COM1A1);
  25
  26   // Set Fast-PWM Mode (Mode 14) - Table 15-5 (page 109)
  27   TCCR1A |= (1 << WGM11);
  28   TCCR1B |= (1 << WGM12);
  29   TCCR1B |= (1 << WGM13);
  30
  31   // Clear Timer 1 Counter
  32   TCNT1 = 0;
  33
  34   // Set PWM frequency/top value - Output PWM 1Hz
  35   ICR1 = 15625;
  36   OCR1A = 10000;
  37
  38   // Enable interrupts
  39   interrupts();
  40 }
  41
  42 void loop() {
  43   // some useless code here
  44   }
  45
  46 void button_pressed_ISR(void)
  47 {
  48   if (timer_en)
  49   {
  50     timer_en = 0;
  51     // Disable the timer by selecting "none" as the clock source
  52     TCCR1B &= ~((1 << CS11) | (1 << CS12) | (1 << CS10));
  53   }
  54   else
  55   {
  56     timer_en = 1;
  57     // Enable the timer
  58     TCCR1B |= (1 << CS12) | (1 << CS10);
  59     TCCR1B &= ~(1 << CS11);
  60   }
  61 }