ESP_timeHandling

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 |______|_____/|_|         |_|  |_____|_|  |_|______| handling...
                                                      
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## There are basically two categories of time in the ESP system. They can be classified as :

	1.	Interval Time	->	Time via which intervals can be measured.
					This can be used to time events, or rather gap between events.
					Basic timing delays fall into this category. 
							
					#Data Structure to handle interval time:
					---------------------------------------
					struct timeval tv;	//This struct type stores the amount of time elapsed 
								//since system REBOOT in sec as well as usec form
					gettimeofday(&tv);	//gettimeofday() populates the struct timeval

					#Using FREERTOS TICKS to determine interval:
					--------------------------------------------
					uint32_t tick_counter=xTaskGetTickCount(); //no. of ticks since REBOOT
					/*TICK is a const accessed by portTICK_PERIOD_MS = 1msec*/

					#Even more precise timing ~ usec interval:
					------------------------------------------
					>> Rare scenario 
					>> ESP has a 'ccount' reg --> stores machine cycles since REBOOT
					>> uint32_t value = xthal_get_ccount();
	
	
	2.	Wall Clock	->	> Real world time
					> Can be used to synchonize sensor data across various IOT devices
					> Can be synced to an sntp server 
					> SNTP server generally returns time in seconds elasped since EPOCH
					  i.e. Jan 1 1970.
					> Various data structure are available to distribute this data into
					  readable format.
					
					#Data Structure to handle:
					--------------------------
					time_t time1;	//a typedef of a large integer type
					struct tm tm1;	//a structure which converts time_t ie. secs since Jan 1 1970
							//to a readable format having year, month, day etc...

					### CONNECT TO AN SNTP SERVER ###
					---------------------------------
					//we use the lpip libraries
					void start_sntp()
					{
						ip_addr_t addr;
						sntp_setoperatingmode(SNTP_OPMODE_POLL);
						inet_pton(AF_INET,"/*DNS of sntp server*/",&addr);
						sntp_setserver(0,&addr);
						sntp_init();
					}
										
					#Retrieve data (secs) from 1970
					-------------------------------
					time_t time1; 
					time(&time1);	//time() populates time1 variable with secs since Jan1 1970
						
					#Once data is retrieved we could use various structs to display it
					------------------------------------------------------------------
					struct tm tm1;	//struct to display time_t data in year,month,day etc...
					gmtime_r(&time1,&tm1); //populates the structure with GM time
					printf("%d\t%d\n",tm1.tm_year,tm1.tm_mon);//prints year and month from tm1
					
					***IMPORTANT***
					>> Year and month in tm1 struct will be since Jan 1 1970
					>> Much better way is to dump the data of time1 into a formated string
					   using asctime() api

					#To display local time we have to set the timezone via env variable:
					---------------------------------------------------------------------
					setenv("TZ","UTC-5:30",1); //timezone for India UTC-5:30
					tzset();

					
					#Once SNTP server has started and timezone has been set, we can use the APIs
					---------------------------------------------------------------------------
					/*Display GMT time in as a formated text string*/
					
					char buffer[20]; //to store the formated time string
					printf("Formated string \t%s\n",asctime_r(gmtime(&time1),buffer); 

					/*Display Local time (UTC-5:30)*/

					char buffer[20];
					printf("Formated string \t%s\n",asctime_r(localtime(&time1),buffer));