ASCII to hex conversion

Sometimes one need to convert from a ASCII  to hex of a particular character. Here is a small function that will take care of that need. It will convert only 0 .. 9 , a .. f , A .. F , rest of the character will be returned as it is.

char atoh(char ch){

    if(ch >= 0x30 && ch <= 0x39){
        return (ch - 0x30);
    }else if(ch >= 0x41 && ch <= 0x46){
        return (ch - 0x37);
    }else if(ch >= 0x61 && ch <= 0x66){
        return (ch - 0x57);
    }else{
        return ch;
    }
}