Although FreeRTOS provides software timers, these timers have a few limitations:
- Maximum resolution is equal to RTOS tick period
- Timer callbacks are dispatched from a low-priority task
Hardware timers are free from both of the limitations, but often they are less convenient to use. For example, application components may need timer events to fire at certain times in the future, but the hardware timer only contains one "compare" value used for interrupt generation. This means that some facility needs to be built on top of the hardware timer to manage the list of pending events can dispatch the callbacks for these events as corresponding hardware interrupts happen.
esp_timer set of APIs provides one-shot and periodic timers, microsecond time resolution, and 64-bit range.
Internally, esp_timer uses a 64-bit hardware timer :ref:`CONFIG_ESP_TIMER_IMPL`:
- LAC timer (ESP32)
- (legacy) FRC2 timer (ESP32)
- SYSTIMER for (ESP32-S2)
Timer callbacks are dispatched from a high-priority esp_timer task. Because all the callbacks are dispatched from the same task, it is recommended to only do the minimal possible amount of work from the callback itself, posting an event to a lower priority task using a queue instead.
If other tasks with priority higher than esp_timer are running, callback dispatching will be delayed until esp_timer task has a chance to run. For example, this will happen if a SPI Flash operation is in progress.
Creating and starting a timer, and dispatching the callback takes some time. Therefore there is a lower limit to the timeout value of one-shot esp_timer. If :cpp:func:`esp_timer_start_once` is called with a timeout value less than 20us, the callback will be dispatched only after approximately 20us.
Periodic esp_timer also imposes a 50us restriction on the minimal timer period. Periodic software timers with period of less than 50us are not practical since they would consume most of the CPU time. Consider using dedicated hardware peripherals or DMA features if you find that a timer with small period is required.
Single timer is represented by :cpp:type:`esp_timer_handle_t` type. Timer has a callback function associated with it. This callback function is called from the esp_timer task each time the timer elapses.
- To create a timer, call :cpp:func:`esp_timer_create`.
- To delete the timer when it is no longer needed, call :cpp:func:`esp_timer_delete`.
The timer can be started in one-shot mode or in periodic mode.
- To start the timer in one-shot mode, call :cpp:func:`esp_timer_start_once`, passing the time interval after which the callback should be called. When the callback gets called, the timer is considered to be stopped.
- To start the timer in periodic mode, call :cpp:func:`esp_timer_start_periodic`, passing the period with which the callback should be called. The timer keeps running until :cpp:func:`esp_timer_stop` is called.
Note that the timer must not be running when :cpp:func:`esp_timer_start_once` or :cpp:func:`esp_timer_start_periodic` is called. To restart a running timer, call :cpp:func:`esp_timer_stop` first, then call one of the start functions.
esp_timer also provides a convenience function to obtain the time passed since start-up, with microsecond precision: :cpp:func:`esp_timer_get_time`. This function returns the number of microseconds since esp_timer was initialized, which usually happens shortly before app_main function is called.
Unlike gettimeofday function, values returned by :cpp:func:`esp_timer_get_time`:
- Start from zero after the chip wakes up from deep sleep
- Do not have timezone or DST adjustments applied
The following example illustrates usage of esp_timer APIs: :example:`system/esp_timer`.
.. include-build-file:: inc/esp_timer.inc