The 8052 and its various derivatives are digital devices. Its inputs work as two state circuits that will produce a 1 or a 0 in the input port. There is a requirement of course that the designer / user ensure that the inputs be held at the proper levels in order for the input circuit to properly detect the 1 or 0 level. For a 1 to be guarenteed the input must be over the data sheet V_IH voltage level. For a 0 to be guarenteed the input must be under the data sheet V_IL voltage level. If you leave an 8052 type input in an open-circuit state (and the pin register on the output side is set to a 1) the software will read the input as a 1. This is because the onboard weak pullup resistor will be pulling the pin above V_IH. There is NO way to make a completely unconnected input pin read as a 0. But with the addition of a pulldown resistor on the input it is possible for an input pin to go to a 0 as read by software as long as other external circuits are disconnected from the input pin. When you select a pulldown resistor it must be a resistance that is low enough so that pin input voltage is below V_IL as it sinks the current from overcoming the onboard weak pullups.

Michael Karas