File content as of revision 2:b30e3a46ae2b:

#include "mbed.h"
#include "HEPTA_EPS.h"
#include "HEPTA_CDH.h"
HEPTA_CDH cdh(PB_5, PB_4, PB_3, PA_8, "sd");
HEPTA_EPS eps(PA_0,PA_4);
DigitalOut condition(PB_1);
Serial gs(USBTX,USBRX,9600);
Timer sattime;
int rcmd = 0, cmdflag = 0; //command variable

//getting command and command flag
void commandget()
{
    rcmd = gs.getc();
    cmdflag = 1;
}
//interrupting process by command receive
void receive(int rcmd, int cmdflag)
{
    gs.attach(commandget,Serial::RxIrq);
}
//initializing
void initialize()
{
    rcmd = 0;
    cmdflag = 0;
    condition = 0;
}

int main()
{
    gs.printf("From Sat : Nominal Operation\r\n");
    int flag = 0; //condition flag
    float batvol, temp; //voltage, temperature 
    sattime.start();
    receive(rcmd,cmdflag); //interrupting
    eps.turn_on_regulator();//turn on 3.3V conveter
    for(int i=0;i<50;i++){
        //satellite condition led
        condition = !condition;
        
        //senssing HK data(dummy data)
        eps.vol(&batvol);
        temp   = 28.5;
        
        //Transmitting HK data to Ground Station(GS)
        gs.printf("HEPTASAT::Condition = %d, Time = %f [s], batvol = %2f [V], temp = %2f [deg C]\r\n",flag,sattime.read(),batvol,temp);
        wait_ms(1000);
        
        //Power Saving Mode 
        if(batvol <= 3.5){
            eps.shut_down_regulator();
            gs.printf("Power saving mode ON\r\n"); 
            flag = 1;
        } else if((flag == 1) & (batvol > 3.7)) {
            eps.turn_on_regulator();
            gs.printf("Power saving mode OFF\r\n");
            flag = 0;
        }
        
        if(cmdflag == 1){
            if(rcmd == 'a'){
                for(int j=0;j<5;j++){
                    gs.printf("Hello World!\r\n");
                    condition = 1;
                    wait_ms(1000);
                }
            }else if(rcmd == 'b') {
                char str[100];
                mkdir("/sd/mydir", 0777);
                FILE *fp = fopen("/sd/mydir/satdata.txt","w");
                if(fp == NULL) {
                    error("Could not open file for write\r\n");
                }
                for(int i = 0; i < 10; i++) {
                    eps.vol(&batvol);
                    fprintf(fp,"%f\r\n",batvol);
                    condition = 1;
                    wait_ms(1000);
                }
                fclose(fp);
                fp = fopen("/sd/mydir/satdata.txt","r");
                for(int i = 0; i < 10; i++) {
                    fgets(str,100,fp);
                    gs.puts(str);
                }
                fclose(fp);
            }
            initialize(); //initializing
        }
    }
    sattime.stop();
    gs.printf("From Sat : End of operation\r\n");
}