If your micro's port pins can sink the current that you wish to pass through LED you dont need a transistor for driving. But there are some issues that needs to be consider.

1. Be sure to read the maximum sink limit for the chip if it specified. For example for some of the LPC932A1 has LED drive capability on all port pins. With a max. limit of 80mA per package.

2. As the sink current increases the Vol [ Low level logic pin voltage ] also increases and with LPC932 @ Sink 3.2mA Vol max = 0.3V and @ Sink 20 mA Vol max = 1V. Thus you may not get the same brightness in LED that you would get with same current limit resistor + transistor. It may require to use smaller current limit resistor when sinking directly to port pin due to Vol rise.

Regards,
Prahlad Purohit