Posted on September 21, 2012 at 16:04It relates to the slew control on the pin, and defines how sharp the rising/falling edges are, and how hard/rapidly the pin is driven.
Define it too slow, and your output waveform will be less square, will depend on the capacitive load being driven. Drive it too hard, you tend to get under/over shoot, which will also add to the EMI generated.
Generally you'd want to set it appropriately for the signal being output.
