A debug pin can be used to probe timer interrupts with an oscilloscope or
other time measurement equipment. Thus, determine when an interrupt occurs
and how long the timer ISR takes.
The pin should be defined in the makefile as follows:
CFLAGS += -DDEBUG_TIMER_PORT=PORTF -DDEBUG_TIMER_DDR=DDRF \
-DDEBUG_TIMER_PIN=PORTF4
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