Even though the signal is a sine wave, the pin has an activation voltage. Before this activation it will be a 0, and after it will read a 1. This is usually a consequence of diodes which pass almost no current until they pass a threshold voltage. Everything below threshold will be blocked from doing anything.

After threshold, the pin will register a 1. The pin has a range of voltages which it can function regularly in, so even if the voltage of a '1' changes, say from 3.31 to 3.35 volts, during the peak of the sine wave, it will operate in the desired way.

So, the pin goes from not functioning at all, a 0, to functioning, a 1. Of course, too much voltage and it will begin operating in unexpected ways, usually damaging the chip.

Even though the signal is a sine wave, the pin has an threshold voltage. Below this threshold it will be a 0, and above it will read a 1. This is usually a consequence of internal circuitry.

Above threshold, the pin will register a 1. The pin has a range of voltages which it can function regularly in, so even if the voltage of a '1' changes, say from 3.31 to 3.35 volts, during the peak of the sine wave, it will operate in the desired way.

So, the pin goes from functioning as a 0 to functioning as a 1, even though the actual voltage varies slightly. Of course, too much voltage and it will begin operating in unexpected ways, usually damaging the chip.