Why does enabling TIMSK1 mess up serial?

Question

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.begin(9600);
 Serial.println("Hello World!");
 timer_setup();
}
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.

Answer 1

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

For example:

ISR(TIMER1_COMPA_vect)
{
 // whatever
} // end of TIMER1_COMPA_vect

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

• Sebastian Freeman: 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.
• Nick Gammon: 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).