Direct port manipulation really is fast, so you may have to
add calls to delayMicroseconds () or asm nops to control the timing
if it is too fast. Here you are driving a stepper controller I think, most
of which specify a 5--10us minimum step pulse length, so digitalWrite()
is actually slow enough to work (usually, since its about 4us), whereas the 125ns pulses
direct port manipulation gives are way too fast.