Because any 8051 derivate goes high during reset, any load should be wired in this manner, that it was switched off, if the port pin was high and switched on with low level.

And some derivates with more sink current (e.g. 20mA on the AT89C2051) can drive also standard LEDs directly against VCC and only the current limiting resistor in series.


Furthermore to drive bi-color LEDS with common cathode, you can use the open drain characteristic of the 8051. Then connect the common cathode to GND and every anode over a resistor on VCC directly and a port pin in paralel. If the port goes low, it shorten the LED and thus the LED was switched off.

If the port pin was high, a small additional current was drawn trough the LED from the internal pull up.


Peter