History

Ivan Grokhotkov 4813ab2d28 examples: check return value of nvs_flash_init nvs_flash_init may return an error code in some cases, and applications should check this error code (or at least assert on it being ESP_OK, to make potential issues more immediately obvious). This change modifies all the examples which use NVS to check the error code. Most examples get a simple ESP_ERROR_CHECK assert, while NVS examples, OTA example, and NVS unit tests get a more verbose check which may be used in real applications.		2017-03-15 10:44:54 +08:00
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main	examples: check return value of nvs_flash_init	2017-03-15 10:44:54 +08:00
Makefile	Moved examples to new folders / categories. Removed example numbers from example names	2017-01-16 23:08:35 +01:00
README.md	Moved examples to new folders / categories. Removed example numbers from example names	2017-01-16 23:08:35 +01:00

README.md

Example: using LwIP SNTP module and time functions

This example demonstrates the use of LwIP SNTP module to obtain time from Internet servers. See the README.md file in the upper level 'examples' directory for more information about examples.

Obtaining time using LwIP SNTP module

When this example boots first time after ESP32 is reset, it connects to WiFi and obtains time using SNTP. See initialize_sntp function for details.

Timekeeping

Once time is synchronized, ESP32 will perform timekeeping using built-in timers.

RTC clock is used to maintain accurate time when chip is in deep sleep mode
FRC1 timer is used to provide time at microsecond accuracy when ESP32 is running.

Timekeeping using RTC timer is demonstrated in this example by going into deep sleep mode. After wake up, ESP32 will print current time without connecting to WiFi.

To use this functionality, make sure "Timers used for gettimeofday function" option in "ESP32-specific config" menu of menuconfig is set to "RTC and FRC1" or "RTC".

Working with time

To get current time, gettimeofday function may be used. Additionally the following standard C library functions can be used to obtain time and manipulate it:

gettimeofday
time
asctime
clock
ctime
difftime
gmtime
localtime
mktime
strftime

To set time, settimeofday POSIX function can be used. It is used internally in LwIP SNTP library to set current time when response from NTP server is received.

Timezones

To set local timezone, use setenv and tzset POSIX functions. First, call setenv to set TZ environment variable to the correct value depending on device location. Format of the time string is described in libc documentation. Next, call tzset to update C library runtime data for the new time zone. Once these steps are done, localtime function will return correct local time, taking time zone offset and daylight saving time into account.