File content as of revision 0:52cef1cc1434:

#include "mbed.h"
BusOut num(D3,D4,D5,D6);
AnalogIn v(A2) ;
Serial tel(D8, D2);

int main() {
    uint8_t data;
    float vol;
    bool exit = false;
    bool show = true;
    while(1)
    {   
        if (show == true)
        {   
            tel.printf("------------------------------------\n");
            tel.printf("            Hi! Pitiwut             \n");
            tel.printf("------------------------------------\n");
            tel.printf("Menu 1 : Show number\n");
            tel.printf("Menu 2 : Show voltage\n");
            show = false;
        }
        if (tel.readable())
        {
            data = tel.getc();
            show = true;
            switch (data)
            {   
                case '1': 
                    tel.printf("insert 0-9 \n");
                    tel.printf("exit plz input 'x' \n");
                    while (exit == false){
                        if (tel.readable())
                        {
                            data = tel.getc();
                            switch (data)
                            {
                                case '0': 
                                    num = 0;
                                    break;
                                case '1': 
                                    num = 1;
                                    break;
                                case '2': 
                                    num = 2;
                                    break;
                                case '3': 
                                    num = 3;
                                    break;
                                case '4': 
                                    num = 4;
                                    break;
                                case '5': 
                                    num = 5;
                                    break;
                                case '6':
                                    num = 6; 
                                    break;
                                case '7': 
                                    num = 7; 
                                    break;
                                case '8': 
                                    num = 8; 
                                    break;
                                case '9': 
                                    num = 9; 
                                    break;
                                case 'x': 
                                    exit = true; 
                                    break;
                                default:
                                    tel.printf("plz input 0-9 \n");      
                                    break;   
                            }
                        }
                    }
                    if (exit == true)
                    {
                        exit = false;
                        break;    
                    }
                    break;
                        
                case '2':                                                                                                                      
                    tel.printf("exit plz input 'x' \n");
                    while(exit == false)
                    {
                        vol = v.read();
                        vol = vol * 3.30f ;
                        tel.printf("voltage : %.2f \n" , vol);
                        wait(0.7);
                        if (tel.readable())
                        {   
                            data = tel.getc();
                            if (data == 'x')
                            {
                                exit = true;
                            }
                            else
                            {
                                tel.printf("exit plz input 'x' \n");
                            }
                        }
                    }
                    if (exit == true)
                    {
                        exit = false;
                        break;
                    }
            }
            
        }
    }
}