Here is what the people making those devices are trying to tell you.1) the If vs. Vfwd curve comes the led datasheet.2) The thick green curve comes from those nice folks at Atmel. They are the I-V curve of a mcu's pin.The two curves together determines the output current on the led, without any resistors. It is about 15ma If and 3v Vfwd. That's due to the pin's internal resistance (0.3v / 15ma = ohm).If the pin had no output resistance, its I-V curve would be like that red line at 3.3v. The output current would then be at 30ma.That is essentially the basis why some designs have no serial resistors.
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