You claimed:

"am sampling the input port using a while(1) infinite loop inside main()."

But you don't show how you sample the input. All you show is a function call.

And your code looks a bit strange:

Key_Scan(); 
ch = keypad[R][C]; 

Normal code would probably look like:

ch = Key_Scan();
if (ch != 0) {
    /* a key was detected */
    ...
}

I assume that keypad[][] is a translation from a key position to a character. But what exactly are the values of R and C if no key is pressed? Wouldn't they either point to a valid key - in which case you get a false match - or point outside the lookup table?