why PORT0.27 of LPC1768 is open drain?

Hi all
Does any one know why PORT0.27 at LPC1768 is open drain?when i add external 10k pull up resistor then this pin work properly but when i remove external resistor it doesnt work and for example if i apply high level via LPC_GPIO0->FIOSET|=(1<<27); then this pin goes to 0.3volt instead of 3.3volt and when apply low level via LPC_GPIO0->FIOCLR|=(1<<27); it goes to 0 volt.
I enable chip pull up resistor on this pin but it seems doesnt work.What should i do?

More questions in this forum