High Resolution Timer

Overview

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.

An interrupt level of the handler depends on the CONFIG_ESP_TIMER_INTERRUPT_LEVEL option. It allows to set this: 1, 2 or 3 level (by default 1). Raising the level, the interrupt handler can reduce the timer processing delay.

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, where the implemention depends on CONFIG_ESP_TIMER_IMPL. Available options are:

  • SYSTIMER

Timer callbacks can dispatched by two methods:

ESP_TIMER_TASK. 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.

ESP_TIMER_ISR. Timer callbacks are dispatched directly from the timer interrupt handler. This method is useful for some simple callbacks which aim for lower latency.

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 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.

Using esp_timer APIs

Single timer is represented by 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.

The timer can be started in one-shot mode or in periodic mode.

  • To start the timer in one-shot mode, call 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 esp_timer_start_periodic(), passing the period with which the callback should be called. The timer keeps running until esp_timer_stop() is called.

Note that the timer must not be running when esp_timer_start_once() or esp_timer_start_periodic() is called. To restart a running timer, call esp_timer_stop() first, then call one of the start functions.

Callback functions

Timer callbacks which are processed by ESP_TIMER_ISR method should not call the context switch call - portYIELD_FROM_ISR(), instead of this you should use the esp_timer_isr_dispatch_need_yield() function. The context switch will be done after all ISR dispatch timers have been processed, if required by the system.

esp_timer during the light sleep

During light sleep, the esp_timer counter stops and no callback functions are called. Instead, the time is counted by the RTC counter. Upon waking up, the system gets the difference between the counters and calls a function that advances the esp_timer counter. Since the counter has been advanced, the system starts calling callbacks that were not called during sleep. The number of callbacks depends on the duration of the sleep and the period of the timers. It can lead to overflow of some queues. This only applies to periodic timers, one-shot timers will be called once.

This behavior can be changed by calling esp_timer_stop() before sleeping. In some cases, this can be inconvenient, and instead of the stop function, you can use the skip_unhandled_events option during esp_timer_create(). When the skip_unhandled_events is true, if a periodic timer expires one or more times during light sleep then only one callback is called on wake.

Using the skip_unhandled_events option with automatic light sleep (see Power Management APIs) helps to reduce the consumption of the system when it is in light sleep. The duration of light sleep is also determined by esp_timers. Timers with skip_unhandled_events option will not wake up the system.

Handling callbacks

esp_timer is designed to achieve a high-resolution low latency timer and the ability to handle delayed events. If the timer is late then the callback will be called as soon as possible, it will not be lost. In the worst case, when the timer has not been processed for more than one period (for periodic timers), in this case the callbacks will be called one after the other without waiting for the set period. This can be bad for some applications, and the skip_unhandled_events option was introduced to eliminate this behavior. If skip_unhandled_events is set then a periodic timer that has expired multiple times without being able to call the callback will still result in only one callback event once processing is possible.

Obtaining Current Time

esp_timer also provides a convenience function to obtain the time passed since start-up, with microsecond precision: 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 esp_timer_get_time():

  • Start from zero after the chip wakes up from deep sleep

  • Do not have timezone or DST adjustments applied

Application Example

The following example illustrates usage of esp_timer APIs: system/esp_timer.

API Reference

Functions

esp_err_t esp_timer_early_init(void)

Minimal initialization of esp_timer.

This function can be called very early in startup process, after this call only esp_timer_get_time function can be used.

Note

This function is called from startup code. Applications do not need to call this function before using other esp_timer APIs.

Return

  • ESP_OK on success

esp_err_t esp_timer_init(void)

Initialize esp_timer library.

Note

This function is called from startup code. Applications do not need to call this function before using other esp_timer APIs. Before calling this function, esp_timer_early_init must be called by the startup code.

Return

  • ESP_OK on success

  • ESP_ERR_NO_MEM if allocation has failed

  • ESP_ERR_INVALID_STATE if already initialized

  • other errors from interrupt allocator

esp_err_t esp_timer_deinit(void)

De-initialize esp_timer library.

Note

Normally this function should not be called from applications

Return

  • ESP_OK on success

  • ESP_ERR_INVALID_STATE if not yet initialized

esp_err_t esp_timer_create(const esp_timer_create_args_t *create_args, esp_timer_handle_t *out_handle)

Create an esp_timer instance.

Note

When done using the timer, delete it with esp_timer_delete function.

Return

  • ESP_OK on success

  • ESP_ERR_INVALID_ARG if some of the create_args are not valid

  • ESP_ERR_INVALID_STATE if esp_timer library is not initialized yet

  • ESP_ERR_NO_MEM if memory allocation fails

Parameters

  • create_args: Pointer to a structure with timer creation arguments. Not saved by the library, can be allocated on the stack.

  • [out] out_handle: Output, pointer to esp_timer_handle_t variable which will hold the created timer handle.

esp_err_t esp_timer_start_once(esp_timer_handle_t timer, uint64_t timeout_us)

Start one-shot timer.

Timer should not be running when this function is called.

Return

  • ESP_OK on success

  • ESP_ERR_INVALID_ARG if the handle is invalid

  • ESP_ERR_INVALID_STATE if the timer is already running

Parameters

  • timer: timer handle created using esp_timer_create

  • timeout_us: timer timeout, in microseconds relative to the current moment

esp_err_t esp_timer_start_periodic(esp_timer_handle_t timer, uint64_t period)

Start a periodic timer.

Timer should not be running when this function is called. This function will start the timer which will trigger every ‘period’ microseconds.

Return

  • ESP_OK on success

  • ESP_ERR_INVALID_ARG if the handle is invalid

  • ESP_ERR_INVALID_STATE if the timer is already running

Parameters

  • timer: timer handle created using esp_timer_create

  • period: timer period, in microseconds

esp_err_t esp_timer_stop(esp_timer_handle_t timer)

Stop the timer.

This function stops the timer previously started using esp_timer_start_once or esp_timer_start_periodic.

Return

  • ESP_OK on success

  • ESP_ERR_INVALID_STATE if the timer is not running

Parameters

  • timer: timer handle created using esp_timer_create

esp_err_t esp_timer_delete(esp_timer_handle_t timer)

Delete an esp_timer instance.

The timer must be stopped before deleting. A one-shot timer which has expired does not need to be stopped.

Return

  • ESP_OK on success

  • ESP_ERR_INVALID_STATE if the timer is running

Parameters

  • timer: timer handle allocated using esp_timer_create

int64_t esp_timer_get_time(void)

Get time in microseconds since boot.

Return

number of microseconds since underlying timer has been started

int64_t esp_timer_get_next_alarm(void)

Get the timestamp when the next timeout is expected to occur.

Return

Timestamp of the nearest timer event, in microseconds. The timebase is the same as for the values returned by esp_timer_get_time.

int64_t esp_timer_get_next_alarm_for_wake_up(void)

Get the timestamp when the next timeout is expected to occur skipping those which have skip_unhandled_events flag.

Return

Timestamp of the nearest timer event, in microseconds. The timebase is the same as for the values returned by esp_timer_get_time.

esp_err_t esp_timer_dump(FILE *stream)

Dump the list of timers to a stream.

If CONFIG_ESP_TIMER_PROFILING option is enabled, this prints the list of all the existing timers. Otherwise, only the list active timers is printed.

The format is:

name period alarm times_armed times_triggered total_callback_run_time

where:

name — timer name (if CONFIG_ESP_TIMER_PROFILING is defined), or timer pointer period — period of timer, in microseconds, or 0 for one-shot timer alarm - time of the next alarm, in microseconds since boot, or 0 if the timer is not started

The following fields are printed if CONFIG_ESP_TIMER_PROFILING is defined:

times_armed — number of times the timer was armed via esp_timer_start_X times_triggered - number of times the callback was called total_callback_run_time - total time taken by callback to execute, across all calls

Return

  • ESP_OK on success

  • ESP_ERR_NO_MEM if can not allocate temporary buffer for the output

Parameters

  • stream: stream (such as stdout) to dump the information to

bool esp_timer_is_active(esp_timer_handle_t timer)

Returns status of a timer, active or not.

This function is used to identify if the timer is still active or not.

Return

  • 1 if timer is still active

  • 0 if timer is not active.

Parameters

  • timer: timer handle created using esp_timer_create

Structures

struct esp_timer_create_args_t

Timer configuration passed to esp_timer_create.

Public Members

esp_timer_cb_t callback

Function to call when timer expires.

void *arg

Argument to pass to the callback.

esp_timer_dispatch_t dispatch_method

Call the callback from task or from ISR.

const char *name

Timer name, used in esp_timer_dump function.

bool skip_unhandled_events

Skip unhandled events for periodic timers.

Type Definitions

typedef struct esp_timer *esp_timer_handle_t

Opaque type representing a single esp_timer.

typedef void (*esp_timer_cb_t)(void *arg)

Timer callback function type.

Parameters

  • arg: pointer to opaque user-specific data

Enumerations

enum esp_timer_dispatch_t

Method for dispatching timer callback.

Values:

ESP_TIMER_TASK

Callback is called from timer task.

ESP_TIMER_MAX

Count of the methods for dispatching timer callback.

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