void delayMicrosecondsInterruptible(unsigned int us)
{
  // for a one-microsecond delay, simply return.  the overhead
  // of the function call yields a delay of approximately 1 1/8 us.
  if (--us == 0)
    return;

  // the following loop takes a quarter of a microsecond (4 cycles)
  // per iteration, so execute it four times for each microsecond of
  // delay requested.
  us <<= 2;

  // account for the time taken in the preceeding commands.
  us -= 2;

  // busy wait
  __asm__ __volatile__ ("1: sbiw %0,1" "\n\t" // 2 cycles
"brne 1b" : 
  "=w" (us) : 
  "0" (us) // 2 cycles
    );
}