As mentioned in Step 6 once an interrupt is enabled it will be triggered even when the corresponding pin is configured as output.
Task: Enable 16-bit Timer 3 in CTC mode with OC3B pin (PE4 pin - configured as output) (also INT4 pin) toggling on compare match. Enable the interrupt INT4 with the ISR containing a routine that turns on and off the LED0 connected to PORTA.
Without using STK600, connect two LED circuits as shown in Figure 1; one at PA0 and another at PE4.
When running the example code, the LED1 (connected to OC3B pin) toggles because of timer action. Whenever the LED1 switches OFF (means a transition from low to high – a rising edge), INT4 is triggered and so LED0 blinks once.