Hi Arun Kumar V,

So many members have suggested you, NOT to connect the IO pin directly to MCU Pin. The main reason is that, it is too dangerous and will kill you, or your technician, or the end-user. It is plain and in simple language.

Even then, if you are still stuck to doing this, then it entirely your fault.

So either you are NOT listening to what the senior members are saying, or you deliberately ignoring them.

You are relying more on the application notes. Even that is pretty dangerous.

There are many application notes that tell you that this is how it can be done. But they DO NOT give the finer details.

I have seen some application notes, that do NOT work at all. In fine print, it is written: "the schematic is just to explain the concept." Read the fine print in those application notes.
These days many web-sites just copy the applicaion note, and never publish the fine-print of these application notes, that are written by the original author. So beware !!!

Just to tell you, even Phillips has an app.note about 89LPC76x series that talks about connecting the Mains wire, for detecting the zero-cross. But that does not mean that you can build a safe product by connecting the Live wire to the MCU pin. So this should answer your question whether MCS51 can be connected to Live wire. There are certain devices that give this protection, such as P89LPC76x. This protection is only to save the microcontroller, and not the end-user. There is a chance that the end-user/operator will get electricuted.

Let me give you an example:
Will you drink Baygon(used to kill insects), to kill the germs inside your body? Baygon will kill you rather than the germs. And so will a Live wire kill you, if you connect it directly to MCU pin. But your MCU will still be working !!! What is the use, if your product works, but you are not in this world to see it ???!!!