CTC ISR runs without setting OCR1A

I'm trying to understand timers but it's very difficult. I set up a simple program to toggle an LED when the compare match value is matched however the ISR runs without me even setting the OCR1A value. Even if I leave it out the ISR runs and the LED blinks. How is this possible?

I set up a simple program

. . . which despite 424 posts, I didn't post here in the programming section.

How is this possible?

  1. You failed to set the timer mode to CTC.

  2. What does the datasheet say about an initial value for OCR1A? Nothing. Which means it has whatever value the transistors fall into when first powered.

  3. The core code set an initial value for OCR1A.