Sync reset with push-button not reliable

This is an example of misnomers and misleading wording of available literature.

synchronous (for any signal) is wrongly perceived as the case of applying it at the input to registers but we should make distinction between "applying it" and "synchronising it at origin".

in your case I am not surprised that asynchronous application was safer because it bypassed the clk edge.

Thus asynchronous reset is far more superior to the so called synchronous provided it is pre-synchronised which should be done in any case. There is no extra cost on resource when you use the dedicated port.

Another unrelated example of bad wording that put me in trouble was this:

-- combinatorial assignment

a <= b; -- means a is wired to b

It was written in a MentorGraphics paper(1998). I will keep you guessing what is wrong with it...