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[Reply #15 on:]

The best way to sample a logic signal is to send the signal down a matched transmission line, such as 70 ohm coax - however to do that you might need something like a 70 ohm resistor in series with 1k, the logic signal drives this voltage divider, the coax samples the small voltage from the 70 ohm resistor (with very little distortion up to 10GHz or more), and at the other end of the signal a 70 ohm input-impedance amplifier can reconstruct the signal.  The whole voltage divider setup ought to be in a coaxial cavity with carefully tuned impedance
to ground to match the signal impedance.

Its all very complex and inconvenient and is an acknowledgement that logic signals switch at microwave frequencies to microwave
circuit techniques are required to catch the fastest details.

Standard analog scope probes are a resistive divider and capacitive divider in parallel, which ought to have low distortion in theory, but the
length of coax acts as a complex impedance dependent on frequency at higher frequencies.

When you have a significant area of loop between the probe tip and the ground croc-clip this is going cause distortion
by adding significant stray inductance, and pick up noise by electromagnetic induction as well.

[Reply #16 on:]

70 ohm resistor in series with 1k

What about 140:140 ohm divider, or etc, for more bang?

[Reply #17 on:]

You need to match the transmission line impedance - in my example 70 ohm coax - fail to match impedances and you
lose the distortion-free properties of a transmission line.  In my example I should have said a 75 ohm and 1k divider - the
two resistors in parallel are the effective impedance seen by the transmission line.

Having said that you can just have the 1k at the board end and purely use the 70ohm termination at the scope end,
probably better.

[Reply #18 on:]

140 || 140 = 70.

[Reply #19 on:]

If you really want to know your Due's pin flipping capability, google "arm bit banding".

I wouldn't expect bit-banding to be any faster than the parallel port "set" and "reset" registers...  Either way you can modify a single bit with a single instruction.  (now, whether the compiler produces equally good code for both cases is a separate question.)

Any discrepancies you see in the "square wave" shape that are due to the scope probe are also going to be present in any other "wire" that you connect to a pin toggling at that frequency.  Putting a ~20MHz signal down a wire is not as trivial as it sounds (consider the old "thick ethernet", which had a 20MHz signal rate, sort of.)