System Time
Overview
ESP32-S2 uses two hardware timers for the purpose of keeping system time. System time can be kept by using either one or both of the hardware timers depending on the application’s purpose and accuracy requirements for system time. The two hardware timers are:
RTC timer: This timer allows time keeping in various sleep modes, and can also persist time keeping across any resets (with the exception of power-on resets which reset the RTC timer). The frequency deviation depends on the RTC Timer Clock Sources and affects the accuracy only in sleep modes, in which case the time will be measured at 6.6667 μs resolution.
High-resolution timer: This timer is not available in sleep modes and will not persist over a reset, but has greater accuracy. The timer uses the APB_CLK clock source (typically 80 MHz), which has a frequency deviation of less than ±10 ppm. Time will be measured at 1 μs resolution.
The possible combinations of hardware timers used to keep system time are listed below:
RTC and high-resolution timer (default)
RTC
High-resolution timer
None
It is recommended that users stick to the default option as it provides the highest accuracy. However, users can also select a different setting via the CONFIG_NEWLIB_TIME_SYSCALL configuration option.
RTC Timer Clock Sources
The RTC timer has the following clock sources:
Internal 90 kHz RC oscillator
(default): Features the lowest Deep-sleep current consumption and no dependence on any external components. However, the frequency stability of this clock source is affected by temperature fluctuations, so time may drift in both Deep-sleep and Light-sleep modes.External 32 kHz crystal
: Requires a 32 kHz crystal to be connected to the XTAL_32K_P and XTAL_32K_N pins. This source provides a better frequency stability at the expense of a slightly higher (by 1 μA) Deep-sleep current consumption.External 32 kHz oscillator at XTAL_32K_P pin
: Allows using 32 kHz clock generated by an external circuit. The external clock signal must be connected to the XTAL_32K_P pin. The amplitude should be less than 1.2 V for sine wave signal and less than 1 V for square wave signal. Common mode voltage should be in the range of 0.1 < Vcm < 0.5xVamp, where Vamp stands for signal amplitude. In this case, the XTAL_32K_P pin cannot be used as a GPIO pin.Internal 8.5 MHz oscillator, divided by 256 (~33 kHz)
: Provides better frequency stability than theInternal 90 kHz RC oscillator
at the expense of a higher (by 5 μA) Deep-sleep current consumption. It also does not require external components.
The choice depends on your requirements for system time accuracy and power consumption in sleep modes. To modify the RTC clock source, set CONFIG_RTC_CLK_SRC in project configuration.
More details about the wiring requirements for the external crystal or external oscillator, please refer to ESP32-S2 Hardware Design Guidelines.
Get Current Time
To get the current time, use the POSIX function gettimeofday()
. Additionally, you can use the following standard C library functions to obtain time and manipulate it:
gettimeofday
time
asctime
clock
ctime
difftime
gmtime
localtime
mktime
strftime
adjtime*
To stop smooth time adjustment and update the current time immediately, use the POSIX function settimeofday()
.
If you need to obtain time with one second resolution, use the following code snippet:
time_t now;
char strftime_buf[64];
struct tm timeinfo;
time(&now);
// Set timezone to China Standard Time
setenv("TZ", "CST-8", 1);
tzset();
localtime_r(&now, &timeinfo);
strftime(strftime_buf, sizeof(strftime_buf), "%c", &timeinfo);
ESP_LOGI(TAG, "The current date/time in Shanghai is: %s", strftime_buf);
If you need to obtain time with one microsecond resolution, use the code snippet below:
struct timeval tv_now;
gettimeofday(&tv_now, NULL);
int64_t time_us = (int64_t)tv_now.tv_sec * 1000000L + (int64_t)tv_now.tv_usec;
SNTP Time Synchronization
To set the current time, you can use the POSIX functions settimeofday()
and adjtime()
. They are used internally in the lwIP SNTP library to set current time when a response from the NTP server is received. These functions can also be used separately from the lwIP SNTP library.
The function to use inside the lwIP SNTP library depends on the sync mode for system time. Use the function sntp_set_sync_mode()
to set one of the following sync modes:
SNTP_SYNC_MODE_IMMED
(default): Updates system time immediately upon receiving a response from the SNTP server after usingsettimeofday()
.SNTP_SYNC_MODE_SMOOTH
: Updates time smoothly by gradually reducing time error using the functionadjtime()
. If the difference between the SNTP response time and system time is more than 35 minutes, update system time immediately by usingsettimeofday()
.
The lwIP SNTP library has API functions for setting a callback function for a certain event. You might need the following functions:
sntp_set_time_sync_notification_cb()
: Can be used to set a callback function that will notify of the time synchronization process.sntp_get_sync_status()
andsntp_set_sync_status()
: Can be used to get/set time synchronization status.
To start synchronization via SNTP, just call the following three functions:
esp_sntp_setoperatingmode(ESP_SNTP_OPMODE_POLL);
esp_sntp_setservername(0, "pool.ntp.org");
esp_sntp_init();
An application with this initialization code will periodically synchronize the time. The time synchronization period is determined by CONFIG_LWIP_SNTP_UPDATE_DELAY (the default value is one hour). To modify the variable, set CONFIG_LWIP_SNTP_UPDATE_DELAY in project configuration.
A code example that demonstrates the implementation of time synchronization based on the lwIP SNTP library is provided in the protocols/sntp directory.
Timezones
To set the local timezone, use the following POSIX functions:
Call
setenv()
to set theTZ
environment variable to the correct value based on the device location. The format of the time string is the same as described in the GNU libc documentation (although the implementation is different).Call
tzset()
to update C library runtime data for the new timezone.
Once these steps are completed, call the standard C library function localtime()
, and it will return the correct local time taking into account the timezone offset and daylight saving time.
Year 2036 and 2038 Overflow Issues
SNTP/NTP 2036 Overflow
SNTP/NTP timestamps are represented as 64-bit unsigned fixed point numbers, where the first 32 bits represent the integer part, and the last 32 bits represent the fractional part. The 64-bit unsigned fixed point number represents the number of seconds since 00:00 on 1st of January 1900, thus SNTP/NTP times will overflow in the year 2036.
To address this issue, lifetime of the SNTP/NTP timestamps has been extended by convention by using the MSB (bit 0 by convention) of the integer part to indicate time ranges between years 1968 to 2104 (see RFC2030 for more details). This convention is implemented in lwIP library SNTP module. Therefore SNTP-related functions in ESP-IDF are future-proof until year 2104.
Unix Time 2038 Overflow
Unix time (type time_t
) was previously represented as a 32-bit signed integer, leading to an overflow in year 2038 (i.e., Y2K38 issue). To address the Y2K38 issue, ESP-IDF uses a 64-bit signed integer to represent time_t
starting from release v5.0, thus deferring time_t
overflow for another 292 billion years.
API Reference
Header File
Functions
-
void sntp_sync_time(struct timeval *tv)
This function updates the system time.
This is a weak-linked function. It is possible to replace all SNTP update functionality by placing a sntp_sync_time() function in the app firmware source. If the default implementation is used, calling sntp_set_sync_mode() allows the time synchronization mode to be changed to instant or smooth. If a callback function is registered via sntp_set_time_sync_notification_cb(), it will be called following time synchronization.
- Parameters
tv – Time received from SNTP server.
-
void sntp_set_sync_mode(sntp_sync_mode_t sync_mode)
Set the sync mode.
Modes allowed: SNTP_SYNC_MODE_IMMED and SNTP_SYNC_MODE_SMOOTH.
- Parameters
sync_mode – Sync mode.
-
sntp_sync_mode_t sntp_get_sync_mode(void)
Get set sync mode.
- Returns
SNTP_SYNC_MODE_IMMED: Update time immediately. SNTP_SYNC_MODE_SMOOTH: Smooth time updating.
-
sntp_sync_status_t sntp_get_sync_status(void)
Get status of time sync.
After the update is completed, the status will be returned as SNTP_SYNC_STATUS_COMPLETED. After that, the status will be reset to SNTP_SYNC_STATUS_RESET. If the update operation is not completed yet, the status will be SNTP_SYNC_STATUS_RESET. If a smooth mode was chosen and the synchronization is still continuing (adjtime works), then it will be SNTP_SYNC_STATUS_IN_PROGRESS.
- Returns
SNTP_SYNC_STATUS_RESET: Reset status. SNTP_SYNC_STATUS_COMPLETED: Time is synchronized. SNTP_SYNC_STATUS_IN_PROGRESS: Smooth time sync in progress.
-
void sntp_set_sync_status(sntp_sync_status_t sync_status)
Set status of time sync.
- Parameters
sync_status – status of time sync (see sntp_sync_status_t)
-
void sntp_set_time_sync_notification_cb(sntp_sync_time_cb_t callback)
Set a callback function for time synchronization notification.
- Parameters
callback – a callback function
-
void sntp_set_sync_interval(uint32_t interval_ms)
Set the sync interval of SNTP operation.
Note: SNTPv4 RFC 4330 enforces a minimum sync interval of 15 seconds. This sync interval will be used in the next attempt update time throught SNTP. To apply the new sync interval call the sntp_restart() function, otherwise, it will be applied after the last interval expired.
- Parameters
interval_ms – The sync interval in ms. It cannot be lower than 15 seconds, otherwise 15 seconds will be set.
-
uint32_t sntp_get_sync_interval(void)
Get the sync interval of SNTP operation.
- Returns
the sync interval
-
bool sntp_restart(void)
Restart SNTP.
- Returns
True - Restart False - SNTP was not initialized yet
-
void esp_sntp_setoperatingmode(esp_sntp_operatingmode_t operating_mode)
Sets SNTP operating mode. The mode has to be set before init.
- Parameters
operating_mode – Desired operating mode
-
void esp_sntp_init(void)
Init and start SNTP service.
-
void esp_sntp_stop(void)
Stops SNTP service.
-
void esp_sntp_setserver(u8_t idx, const ip_addr_t *addr)
Sets SNTP server address.
- Parameters
idx – Index of the server
addr – IP address of the server
-
void esp_sntp_setservername(u8_t idx, const char *server)
Sets SNTP hostname.
- Parameters
idx – Index of the server
server – Name of the server
-
const char *esp_sntp_getservername(u8_t idx)
Gets SNTP server name.
- Parameters
idx – Index of the server
- Returns
Name of the server
-
const ip_addr_t *esp_sntp_getserver(u8_t idx)
Get SNTP server IP.
- Parameters
idx – Index of the server
- Returns
IP address of the server
-
bool esp_sntp_enabled(void)
Checks if sntp is enabled.
- Returns
true if sntp module is enabled
Macros
-
esp_sntp_sync_time
Aliases for esp_sntp prefixed API (inherently thread safe)
-
esp_sntp_set_sync_mode
-
esp_sntp_get_sync_mode
-
esp_sntp_get_sync_status
-
esp_sntp_set_sync_status
-
esp_sntp_set_time_sync_notification_cb
-
esp_sntp_set_sync_interval
-
esp_sntp_get_sync_interval
-
esp_sntp_restart
Type Definitions
-
typedef void (*sntp_sync_time_cb_t)(struct timeval *tv)
SNTP callback function for notifying about time sync event.
- Param tv
Time received from SNTP server.
Enumerations
-
enum sntp_sync_mode_t
SNTP time update mode.
Values:
-
enumerator SNTP_SYNC_MODE_IMMED
Update system time immediately when receiving a response from the SNTP server.
-
enumerator SNTP_SYNC_MODE_SMOOTH
Smooth time updating. Time error is gradually reduced using adjtime function. If the difference between SNTP response time and system time is large (more than 35 minutes) then update immediately.
-
enumerator SNTP_SYNC_MODE_IMMED