delayMicroseconds

Code ideas and analysis for delayMicroseconds()
I choosed a license, but I don't know it good for here.

Under construction...

How to accomplish delayMicroseconds() function

- Calculating the cycle counter

• Multiplication (Not multiplying more than 4 for the same delaying capability than the old 16 MHz Uno)
• Subtraction (Decrease the cycle counter for compensate partially the wasted clock cycles before the waiting cycle) (or get a calculated counter from a table to avoid multiplication when frequency is not a round value in MHz, or MCU has no instruction for mulptilication)

- Tuning with some nops

• This is the rest of partially compensated wasted cycles (for the clock accurate timings. I hope this is comprehensible.)

- Doing the waiting cycle

• variable nops (for higher frequencies than 16 MHz)
• variable decrement (for lower frequencies than 4 MHz)

It must be an assember source, because of LTO, compiler may generate shorter code than desired if it written in C.
This function must be inline to avoid call and return timings.
It does not check the zero delay, because if user calls it with a zero parameter, it is a user program error anyway. Impossible to delay zero time.
When parameter not a constant, the function optimized for register variables. So when variable isn't a register optimized variable (like a volatile variable) the delay will be longer with 3 clock cycles.
This function does not care about if the clock frequency has changed in runtime by user program.