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I want to sense when a port pin goes high. I have connected it to Vcc through a resitor and pushbutton , so when pushbutton is pressed the port pin goes high .But to configure port for input I must first write 0xFF to Port .
V | R1 PB | |_____/\/\/\____.--.____|Port Pin |
"You should get a data book for the particular chip you are using and read it through" Absolutely, however most newer datsheets omit the basic '51 info which can be found in the 3 80C51_Fam... documents found at: http://www.semiconductors.philips.com/products/all_other.html.
and the original Intel datasheet is on your Keil CD!