General Purpose Timer (GPTimer)

Introduction

GPTimer (General Purpose Timer) is the driver of ESP32-C2 Timer Group peripheral. The hardware timer features high resolution and flexible alarm action. The behavior when the internal counter of a timer reaches a specific target value is called a timer alarm. When a timer alarms, a user registered per-timer callback would be called.

Typically, a general purpose timer can be used in scenarios like:

Free running as a wall clock, fetching a high-resolution timestamp at any time and any places
Generate period alarms, trigger events periodically
Generate one-shot alarm, respond in target time

Functional Overview

The following sections of this document cover the typical steps to install and operate a timer:

Resource Allocation - covers which parameters should be set up to get a timer handle and how to recycle the resources when GPTimer finishes working.
Set and Get Count Value - covers how to force the timer counting from a start point and how to get the count value at anytime.
Set up Alarm Action - covers the parameters that should be set up to enable the alarm event.
Register Event Callbacks - covers how to hook user specific code to the alarm event callback function.
Enable and Disable Timer - covers how to enable and disable the timer.
Start and Stop Timer - shows some typical use cases that start the timer with different alarm behavior.
Power Management - describes how different source clock selections can affect power consumption.
IRAM Safe - describes tips on how to make the timer interrupt and IO control functions work better along with a disabled cache.
Thread Safety - lists which APIs are guaranteed to be thread safe by the driver.
Kconfig Options - lists the supported Kconfig options that can be used to make a different effect on driver behavior.

Resource Allocation

Different ESP chips might have different numbers of independent timer groups, and within each group, there could also be several independent timers. 1

A GPTimer instance is represented by gptimer_handle_t. The driver behind will manage all available hardware resources in a pool, so that you do not need to care about which timer and which group it belongs to.

To install a timer instance, there is a configuration structure that needs to be given in advance: gptimer_config_t:

gptimer_config_t::clk_src selects the source clock for the timer. The available clocks are listed in gptimer_clock_source_t, you can only pick one of them. For the effect on power consumption of different clock source, please refer to Section Power Management.
gptimer_config_t::direction sets the counting direction of the timer, supported directions are listed in gptimer_count_direction_t, you can only pick one of them.
gptimer_config_t::resolution_hz sets the resolution of the internal counter. Each count step is equivalent to 1 / resolution_hz seconds.
gptimer_config::intr_priority sets the priority of the timer interrupt. If it is set to 0, the driver will allocate an interrupt with a default priority. Otherwise, the driver will use the given priority.
Optional gptimer_config_t::intr_shared sets whether or not mark the timer interrupt source as a shared one. For the pros/cons of a shared interrupt, you can refer to Interrupt Handling.

With all the above configurations set in the structure, the structure can be passed to gptimer_new_timer() which will instantiate the timer instance and return a handle of the timer.

The function can fail due to various errors such as insufficient memory, invalid arguments, etc. Specifically, when there are no more free timers (i.e. all hardware resources have been used up), then ESP_ERR_NOT_FOUND will be returned. The total number of available timers is represented by the SOC_TIMER_GROUP_TOTAL_TIMERS and its value will depend on the ESP chip.

If a previously created GPTimer instance is no longer required, you should recycle the timer by calling gptimer_del_timer(). This will allow the underlying HW timer to be used for other purposes. Before deleting a GPTimer handle, please disable it by gptimer_disable() in advance or make sure it has not enabled yet by gptimer_enable().

Creating a GPTimer Handle with Resolution of 1 MHz

gptimer_handle_t gptimer = NULL;
gptimer_config_t timer_config = {
    .clk_src = GPTIMER_CLK_SRC_DEFAULT,
    .direction = GPTIMER_COUNT_UP,
    .resolution_hz = 1 * 1000 * 1000, // 1MHz, 1 tick = 1us
};
ESP_ERROR_CHECK(gptimer_new_timer(&timer_config, &gptimer));

Set and Get Count Value

When the GPTimer is created, the internal counter will be reset to zero by default. The counter value can be updated asynchronously by gptimer_set_raw_count(). The maximum count value is dependent on the bit width of the hardware timer, which is also reflected by the SOC macro SOC_TIMER_GROUP_COUNTER_BIT_WIDTH. When updating the raw count of an active timer, the timer will immediately start counting from the new value.

Count value can be retrieved by gptimer_get_raw_count(), at any time.

Set up Alarm Action

For most of the use cases of GPTimer, you should set up the alarm action before starting the timer, except for the simple wall-clock scenario, where a free running timer is enough. To set up the alarm action, you should configure several members of gptimer_alarm_config_t based on how you make use of the alarm event:

gptimer_alarm_config_t::alarm_count sets the target count value that will trigger the alarm event. You should also take the counting direction into consideration when setting the alarm value. Specially, gptimer_alarm_config_t::alarm_count and gptimer_alarm_config_t::reload_count cannot be set to the same value when gptimer_alarm_config_t::auto_reload_on_alarm is true, as keeping reload with a target alarm count is meaningless.
gptimer_alarm_config_t::reload_count sets the count value to be reloaded when the alarm event happens. This configuration only takes effect when gptimer_alarm_config_t::auto_reload_on_alarm is set to true.
gptimer_alarm_config_t::auto_reload_on_alarm flag sets whether to enable the auto-reload feature. If enabled, the hardware timer will reload the value of gptimer_alarm_config_t::reload_count into counter immediately when an alarm event happens.

To make the alarm configurations take effect, you should call gptimer_set_alarm_action(). Especially, if gptimer_alarm_config_t is set to NULL, the alarm function will be disabled.

Note

If an alarm value is set and the timer has already exceeded this value, the alarm will be triggered immediately.

Register Event Callbacks

After the timer starts up, it can generate a specific event (e.g. the “Alarm Event”) dynamically. If you have some functions that should be called when the event happens, please hook your function to the interrupt service routine by calling gptimer_register_event_callbacks(). All supported event callbacks are listed in gptimer_event_callbacks_t:

gptimer_event_callbacks_t::on_alarm sets a callback function for alarm events. As this function is called within the ISR context, you must ensure that the function does not attempt to block (e.g., by making sure that only FreeRTOS APIs with ISR suffix are called from within the function). The function prototype is declared in gptimer_alarm_cb_t.

You can save your own context to gptimer_register_event_callbacks() as well, via the parameter user_data. The user data will be directly passed to the callback function.

This function will lazy install the interrupt service for the timer but not enable it. So please call this function before gptimer_enable(), otherwise the ESP_ERR_INVALID_STATE error will be returned. See Section Enable and Disable Timer for more information.

Enable and Disable Timer

Before doing IO control to the timer, you needs to enable the timer first, by calling gptimer_enable(). This function will:

Switch the timer driver state from init to enable.
Enable the interrupt service if it has been lazy installed by gptimer_register_event_callbacks().
Acquire a proper power management lock if a specific clock source (e.g. APB clock) is selected. See Section Power Management for more information.

Calling gptimer_disable() will do the opposite, that is, put the timer driver back to the init state, disable the interrupts service and release the power management lock.

Start and Stop Timer

The basic IO operation of a timer is to start and stop. Calling gptimer_start() can make the internal counter work, while calling gptimer_stop() can make the counter stop working. The following illustrates how to start a timer with or without an alarm event. Calling gptimer_start() will transit the driver state from enable to run, and vice versa. You need to make sure the start and stop functions are used in pairs, otherwise, the functions may return ESP_ERR_INVALID_STATE. Most of the time, this error means that the timer is already stopped or in the “start protection” state (i.e. gptimer_start() is called but not finished).

Start Timer as a Wall Clock

ESP_ERROR_CHECK(gptimer_enable(gptimer));
ESP_ERROR_CHECK(gptimer_start(gptimer));
// Retrieve the timestamp at any time
uint64_t count;
ESP_ERROR_CHECK(gptimer_get_raw_count(gptimer, &count));

Trigger Period Events

typedef struct {
    uint64_t event_count;
} example_queue_element_t;

static bool example_timer_on_alarm_cb(gptimer_handle_t timer, const gptimer_alarm_event_data_t *edata, void *user_ctx)
{
    BaseType_t high_task_awoken = pdFALSE;
    QueueHandle_t queue = (QueueHandle_t)user_ctx;
    // Retrieve the count value from event data
    example_queue_element_t ele = {
        .event_count = edata->count_value
    };
    // Optional: send the event data to other task by OS queue
    // Do not introduce complex logics in callbacks
    // Suggest dealing with event data in the main loop, instead of in this callback
    xQueueSendFromISR(queue, &ele, &high_task_awoken);
    // return whether we need to yield at the end of ISR
    return high_task_awoken == pdTRUE;
}

gptimer_alarm_config_t alarm_config = {
    .reload_count = 0, // counter will reload with 0 on alarm event
    .alarm_count = 1000000, // period = 1s @resolution 1MHz
    .flags.auto_reload_on_alarm = true, // enable auto-reload
};
ESP_ERROR_CHECK(gptimer_set_alarm_action(gptimer, &alarm_config));

gptimer_event_callbacks_t cbs = {
    .on_alarm = example_timer_on_alarm_cb, // register user callback
};
ESP_ERROR_CHECK(gptimer_register_event_callbacks(gptimer, &cbs, queue));
ESP_ERROR_CHECK(gptimer_enable(gptimer));
ESP_ERROR_CHECK(gptimer_start(gptimer));

Trigger One-Shot Event

typedef struct {
    uint64_t event_count;
} example_queue_element_t;

static bool example_timer_on_alarm_cb(gptimer_handle_t timer, const gptimer_alarm_event_data_t *edata, void *user_ctx)
{
    BaseType_t high_task_awoken = pdFALSE;
    QueueHandle_t queue = (QueueHandle_t)user_ctx;
    // Stop timer the sooner the better
    gptimer_stop(timer);
    // Retrieve the count value from event data
    example_queue_element_t ele = {
        .event_count = edata->count_value
    };
    // Optional: send the event data to other task by OS queue
    xQueueSendFromISR(queue, &ele, &high_task_awoken);
    // return whether we need to yield at the end of ISR
    return high_task_awoken == pdTRUE;
}

gptimer_alarm_config_t alarm_config = {
    .alarm_count = 1 * 1000 * 1000, // alarm target = 1s @resolution 1MHz
};
ESP_ERROR_CHECK(gptimer_set_alarm_action(gptimer, &alarm_config));

gptimer_event_callbacks_t cbs = {
    .on_alarm = example_timer_on_alarm_cb, // register user callback
};
ESP_ERROR_CHECK(gptimer_register_event_callbacks(gptimer, &cbs, queue));
ESP_ERROR_CHECK(gptimer_enable(gptimer));
ESP_ERROR_CHECK(gptimer_start(gptimer));

Dynamic Alarm Update

Alarm value can be updated dynamically inside the ISR handler callback, by changing gptimer_alarm_event_data_t::alarm_value. Then the alarm value will be updated after the callback function returns.

typedef struct {
    uint64_t event_count;
} example_queue_element_t;

static bool example_timer_on_alarm_cb(gptimer_handle_t timer, const gptimer_alarm_event_data_t *edata, void *user_ctx)
{
    BaseType_t high_task_awoken = pdFALSE;
    QueueHandle_t queue = (QueueHandle_t)user_data;
    // Retrieve the count value from event data
    example_queue_element_t ele = {
        .event_count = edata->count_value
    };
    // Optional: send the event data to other task by OS queue
    xQueueSendFromISR(queue, &ele, &high_task_awoken);
    // reconfigure alarm value
    gptimer_alarm_config_t alarm_config = {
        .alarm_count = edata->alarm_value + 1000000, // alarm in next 1s
    };
    gptimer_set_alarm_action(timer, &alarm_config);
    // return whether we need to yield at the end of ISR
    return high_task_awoken == pdTRUE;
}

gptimer_alarm_config_t alarm_config = {
    .alarm_count = 1000000, // initial alarm target = 1s @resolution 1MHz
};
ESP_ERROR_CHECK(gptimer_set_alarm_action(gptimer, &alarm_config));

gptimer_event_callbacks_t cbs = {
    .on_alarm = example_timer_on_alarm_cb, // register user callback
};
ESP_ERROR_CHECK(gptimer_register_event_callbacks(gptimer, &cbs, queue));
ESP_ERROR_CHECK(gptimer_enable(gptimer));
ESP_ERROR_CHECK(gptimer_start(gptimer));

Power Management

There’re some power management strategies, which might turn off or change the frequency of GPTimer’s source clock to save power consumption. For example, during DFS, APB clock will be scaled down. If light-sleep is also enabled, PLL and XTAL clocks will be powered off. Both of them can result in an inaccurate time keeping.

The driver can prevent the above situation from happening by creating different power management lock according to different clock source. The driver will increase the reference count of that power management lock in the gptimer_enable() and decrease it in the gptimer_disable(). So we can ensure the clock source is stable between gptimer_enable() and gptimer_disable().

IRAM Safe

By default, the GPTimer interrupt will be deferred when the cache is disabled because of writing or erasing the flash. Thus the alarm interrupt will not get executed in time, which is not expected in a real-time application.

There is a Kconfig option CONFIG_GPTIMER_ISR_IRAM_SAFE that will:

Enable the interrupt being serviced even when the cache is disabled
Place all functions that used by the ISR into IRAM 2
Place driver object into DRAM (in case it is mapped to PSRAM by accident)

This will allow the interrupt to run while the cache is disabled, but will come at the cost of increased IRAM consumption.

There is another Kconfig option CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM that can put commonly used IO control functions into IRAM as well. So, these functions can also be executable when the cache is disabled. These IO control functions are as follows:

gptimer_start()
gptimer_stop()
gptimer_get_raw_count()
gptimer_set_raw_count()
gptimer_set_alarm_action()

Thread Safety

All the APIs provided by the driver are guaranteed to be thread safe, which means you can call them from different RTOS tasks without protection by extra locks. The following functions are allowed to run under ISR context.

gptimer_start()
gptimer_stop()
gptimer_get_raw_count()
gptimer_set_raw_count()
gptimer_set_alarm_action()

Kconfig Options

CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM controls where to place the GPTimer control functions (IRAM or flash), see Section IRAM Safe for more information.
CONFIG_GPTIMER_ISR_IRAM_SAFE controls whether the default ISR handler can work when the cache is disabled, see Section IRAM Safe for more information.
CONFIG_GPTIMER_ENABLE_DEBUG_LOG is used to enabled the debug log output. Enable this option will increase the firmware binary size.

Application Examples

Typical use cases of GPTimer are listed in the example peripherals/timer_group/gptimer.

API Reference

Header File

components/driver/include/driver/gptimer.h

Functions

esp_err_t gptimer_new_timer(const gptimer_config_t *config, gptimer_handle_t *ret_timer)

Create a new General Purpose Timer, and return the handle.

Note

The newly created timer is put in the “init” state.

Parameters

config – [in] GPTimer configuration
ret_timer – [out] Returned timer handle

Returns

ESP_OK: Create GPTimer successfully
ESP_ERR_INVALID_ARG: Create GPTimer failed because of invalid argument
ESP_ERR_NO_MEM: Create GPTimer failed because out of memory
ESP_ERR_NOT_FOUND: Create GPTimer failed because all hardware timers are used up and no more free one
ESP_FAIL: Create GPTimer failed because of other error

esp_err_t gptimer_del_timer(gptimer_handle_t timer)

Delete the GPTimer handle.

Note

A timer must be in the “init” state before it can be deleted.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()

Returns

ESP_OK: Delete GPTimer successfully
ESP_ERR_INVALID_ARG: Delete GPTimer failed because of invalid argument
ESP_ERR_INVALID_STATE: Delete GPTimer failed because the timer is not in init state
ESP_FAIL: Delete GPTimer failed because of other error

esp_err_t gptimer_set_raw_count(gptimer_handle_t timer, uint64_t value)

Set GPTimer raw count value.

Note

When updating the raw count of an active timer, the timer will immediately start counting from the new value.

Note

This function is allowed to run within ISR context

Note

If CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM is enabled, this function will be placed in the IRAM by linker, makes it possible to execute even when the Flash Cache is disabled.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()
value – [in] Count value to be set

Returns

ESP_OK: Set GPTimer raw count value successfully
ESP_ERR_INVALID_ARG: Set GPTimer raw count value failed because of invalid argument
ESP_FAIL: Set GPTimer raw count value failed because of other error

esp_err_t gptimer_get_raw_count(gptimer_handle_t timer, uint64_t *value)

Get GPTimer raw count value.

Note

With the raw count value and the resolution set in the gptimer_config_t, you can convert the count value into seconds.

Note

This function is allowed to run within ISR context

Note

If CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM is enabled, this function will be placed in the IRAM by linker, makes it possible to execute even when the Flash Cache is disabled.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()
value – [out] Returned GPTimer count value

Returns

ESP_OK: Get GPTimer raw count value successfully
ESP_ERR_INVALID_ARG: Get GPTimer raw count value failed because of invalid argument
ESP_FAIL: Get GPTimer raw count value failed because of other error

esp_err_t gptimer_register_event_callbacks(gptimer_handle_t timer, const gptimer_event_callbacks_t *cbs, void *user_data)

Set callbacks for GPTimer.

Note

User registered callbacks are expected to be runnable within ISR context

Note

The first call to this function needs to be before the call to gptimer_enable

Note

User can deregister a previously registered callback by calling this function and setting the callback member in the cbs structure to NULL.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()
cbs – [in] Group of callback functions
user_data – [in] User data, which will be passed to callback functions directly

Returns

ESP_OK: Set event callbacks successfully
ESP_ERR_INVALID_ARG: Set event callbacks failed because of invalid argument
ESP_ERR_INVALID_STATE: Set event callbacks failed because the timer is not in init state
ESP_FAIL: Set event callbacks failed because of other error

esp_err_t gptimer_set_alarm_action(gptimer_handle_t timer, const gptimer_alarm_config_t *config)

Set alarm event actions for GPTimer.

Note

This function is allowed to run within ISR context, so that user can set new alarm action immediately in the ISR callback.

Note

If CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM is enabled, this function will be placed in the IRAM by linker, makes it possible to execute even when the Flash Cache is disabled.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()
config – [in] Alarm configuration, especially, set config to NULL means disabling the alarm function

Returns

ESP_OK: Set alarm action for GPTimer successfully
ESP_ERR_INVALID_ARG: Set alarm action for GPTimer failed because of invalid argument
ESP_FAIL: Set alarm action for GPTimer failed because of other error

esp_err_t gptimer_enable(gptimer_handle_t timer)

Enable GPTimer.

Note

This function will transit the timer state from “init” to “enable”.

Note

This function will enable the interrupt service, if it’s lazy installed in gptimer_register_event_callbacks.

Note

This function will acquire a PM lock, if a specific source clock (e.g. APB) is selected in the gptimer_config_t, while CONFIG_PM_ENABLE is enabled.

Note

Enable a timer doesn’t mean to start it. See also gptimer_start() for how to make the timer start counting.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()

Returns

ESP_OK: Enable GPTimer successfully
ESP_ERR_INVALID_ARG: Enable GPTimer failed because of invalid argument
ESP_ERR_INVALID_STATE: Enable GPTimer failed because the timer is already enabled
ESP_FAIL: Enable GPTimer failed because of other error

esp_err_t gptimer_disable(gptimer_handle_t timer)

Disable GPTimer.

Note

This function will transit the timer state from “enable” to “init”.

Note

This function will disable the interrupt service if it’s installed.

Note

This function will release the PM lock if it’s acquired in the gptimer_enable.

Note

Disable a timer doesn’t mean to stop it. See also gptimer_stop for how to make the timer stop counting.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()

Returns

ESP_OK: Disable GPTimer successfully
ESP_ERR_INVALID_ARG: Disable GPTimer failed because of invalid argument
ESP_ERR_INVALID_STATE: Disable GPTimer failed because the timer is not enabled yet
ESP_FAIL: Disable GPTimer failed because of other error

esp_err_t gptimer_start(gptimer_handle_t timer)

Start GPTimer (internal counter starts counting)

Note

This function will transit the timer state from “enable” to “run”.

Note

This function is allowed to run within ISR context

Note

If CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM is enabled, this function will be placed in the IRAM by linker, makes it possible to execute even when the Flash Cache is disabled.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()

Returns

ESP_OK: Start GPTimer successfully
ESP_ERR_INVALID_ARG: Start GPTimer failed because of invalid argument
ESP_ERR_INVALID_STATE: Start GPTimer failed because the timer is not enabled or is already in running
ESP_FAIL: Start GPTimer failed because of other error

esp_err_t gptimer_stop(gptimer_handle_t timer)

Stop GPTimer (internal counter stops counting)

Note

This function will transit the timer state from “run” to “enable”.

Note

This function is allowed to run within ISR context

Note

If CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM is enabled, this function will be placed in the IRAM by linker, makes it possible to execute even when the Flash Cache is disabled.

Parameters

timer – [in] Timer handle created by gptimer_new_timer()

Returns

ESP_OK: Stop GPTimer successfully
ESP_ERR_INVALID_ARG: Stop GPTimer failed because of invalid argument
ESP_ERR_INVALID_STATE: Stop GPTimer failed because the timer is not in running.
ESP_FAIL: Stop GPTimer failed because of other error

Structures

struct gptimer_alarm_event_data_t

GPTimer alarm event data.

Public Members

uint64_t count_value: Current count value

uint64_t alarm_value: Current alarm value

struct gptimer_event_callbacks_t

Group of supported GPTimer callbacks.

Note

The callbacks are all running under ISR environment

Note

When CONFIG_GPTIMER_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM.

Public Members

gptimer_alarm_cb_t on_alarm: Timer alarm callback

struct gptimer_config_t

General Purpose Timer configuration.

Public Members

gptimer_clock_source_t clk_src: GPTimer clock source

gptimer_count_direction_t direction: Count direction

uint32_t resolution_hz: Counter resolution (working frequency) in Hz, hence, the step size of each count tick equals to (1 / resolution_hz) seconds

int intr_priority: GPTimer interrupt priority, if set to 0, the driver will try to allocate an interrupt with a relative low priority (1,2,3)

uint32_t intr_shared: Set true, the timer interrupt number can be shared with other peripherals

struct gptimer_config_t::[anonymous] flags: GPTimer config flags

struct gptimer_alarm_config_t

General Purpose Timer alarm configuration.

Public Members

uint64_t alarm_count: Alarm target count value

uint64_t reload_count: Alarm reload count value, effect only when auto_reload_on_alarm is set to true

uint32_t auto_reload_on_alarm: Reload the count value by hardware, immediately at the alarm event

struct gptimer_alarm_config_t::[anonymous] flags: Alarm config flags

Type Definitions

typedef struct gptimer_t *gptimer_handle_t: Type of General Purpose Timer handle.

typedef bool (*gptimer_alarm_cb_t)(gptimer_handle_t timer, const gptimer_alarm_event_data_t *edata, void *user_ctx)

Timer alarm callback prototype.

Param timer: [in] Timer handle created by gptimer_new_timer()
Param edata: [in] Alarm event data, fed by driver
Param user_ctx: [in] User data, passed from gptimer_register_event_callbacks()
Return: Whether a high priority task has been waken up by this function

Header File

components/hal/include/hal/timer_types.h

Type Definitions

typedef soc_periph_gptimer_clk_src_t gptimer_clock_source_t: GPTimer clock source.

Note

User should select the clock source based on the power and resolution requirement

Enumerations

enum gptimer_count_direction_t

GPTimer count direction.

Values:

enumerator GPTIMER_COUNT_DOWN: Decrease count value

enumerator GPTIMER_COUNT_UP: Increase count value

1: Different ESP chip series might have different numbers of GPTimer instances. For more details, please refer to ESP32-C2 Technical Reference Manual > Chapter Timer Group (TIMG) [PDF]. The driver will not forbid you from applying for more timers, but it will return error when all available hardware resources are used up. Please always check the return value when doing resource allocation (e.g. gptimer_new_timer()).
2: gptimer_event_callbacks_t::on_alarm callback and the functions invoked by the callback should also be placed in IRAM, please take care of them by yourself.

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