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Hi,
Sourcing current of a pin of a device specifies how much current it can supply to a load connected to it. Sinking on the other hand specifies how much current can be driven into the pin.
Regards, Laktronics
When a pin is HIGH, it is called a "source" of current into a ground referenced load.
When a pin is LOW it is called a current "sink" because it draws current into ground from a Vcc referenced load.
So if your load is tied to ground, the pin can supply up to it's "source" current into the load; a load tied to the supply voltage can have up to the "sink" current pulled by the pin.
Due to the fact that each state has a different path inside the chip, each state is inherently different. That is why both source and sink are given. (In fact, if source and sink are the same, the chip designers most likely had to use different sized transistors because of the inherent difference between 'N' and 'P' type transistors).
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