I am intending to write a code where a user will send lines through serial to affect timer interrupts. I am noticing a problem with serial where as soon as I enable the timer interrupts, the serial communication becomes garbled. More specifically, as soon as I enable the timer interrupts and I open the serial monitor, the monitor gets flooded with garbage. I haven't written too much code and this is already happening. Am I making a silly mistake? I am using an Arduino Mega 2560. My code is below:

void setup() {
  Serial.println("Hello World!");

void loop() {
  // put your main code here, to run repeatedly:

void timer_setup() {
  // waveform generation = 0100 = CTC
  TCCR1B &= ~(1 << WGM13);
  TCCR1B |=  (1 << WGM12);
  TCCR1A &= ~(1 << WGM11);
  TCCR1A &= ~(1 << WGM10);
  // output mode = 00 (disconnected)
  TCCR1A &= ~(3 << COM1A0);
  TCCR1A &= ~(3 << COM1B0);
  // Set the timer pre-scaler
  TCCR1B = (TCCR1B & ~(0x07 << CS10)) | (2 << CS10);
  OCR1A = 0x4000;
  TCNT1 = 0;
  // Enable timer interrupt mask 1
  TIMSK1 |= (1<<OCIE1A);

If I comment out the TIMSK1 |= (1<<OCIE1A) line, serial doesn't mess up.


You've enabled interrupts for Timer 1 Compare A interrupt. Where is the interrupt handler?

For example:

  // whatever
}  // end of TIMER1_COMPA_vect

Without that, the processor is just restarting. No wonder serial output is messed up.

  • Thanks, Nick. I hadn't added it yet because I wasn't testing interrupts yet, just setting it up. I didn't know it would do that if I didn't include the handler. Mar 28 '16 at 11:44
  • 1
    You can't "set up" interrupts without a handler and expect things to work. Once you enable the flag, you are promising something will handle them. The compiler generates a default handler that jumps to the start of the program (same place Reset goes).
    – Nick Gammon
    Mar 28 '16 at 20:32

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