hi,

Jez is right, you have to undergo to the port internals first. when you write one to a LATCH the out put FET turns off and currecnts from the pullups is available to source some sink. if no sink is at input this pulls the input of tristate buffer high so that it reads one( that means if you write a one to a port exept port0, and leave it open MCU will read one). when some output which is at low state it sinks all the current from pullups so that voltage at the input buffer drops and it read low.

where the short circuit come from?

it will happen if you connect the pin directly to 5 v or to some output which can source current with magnitude more than the max. permissible limits. now if you write a '0' to the port, you blow the output transister of the port.

conclusion if you have written one to a port pin, you can connect it it to ground with no harm , but a pull down resistor is prefered.

abhishek

abhishek