[mikejmoffitt]

> Do not transition on any negative (falling) edges. > Falling edge clocks should be considered a violation of the one clock principle, as they act like separate clocks.

I think this is a good thing to point out if you are working in a primarily rising-edge system, which is common, but if your _entire design_ uses falling edges for one reason or another, I don't see the problem.

I have had to transition a design to falling edge when I needed interop with existing external hardware that operated on the falling edge. Rather than invert my clock, using the falling edge was fine.