Event Task Matrix (ETM)
=======================

:link_to_translation:`zh_CN:[中文]`

Introduction
------------

Normally, if a peripheral X needs to notify peripheral Y of a particular event, this could only be done via a CPU interrupt from peripheral X, where the CPU notifies peripheral Y on behalf of peripheral X. However, in time-critical applications, the latency introduced by CPU interrupts is non-negligible.

With the help of the Event Task Matrix (ETM) module, some peripherals can directly notify other peripherals of events through pre-set connections without the intervention of CPU interrupts. This allows precise and low latency synchronization between peripherals, and lessens the CPU's workload as the CPU no longer needs to handle these events.

.. blockdiag:: /../_static/diagrams/etm/etm_channel.diag
    :caption: ETM channels Overview
    :align: center

The ETM module has multiple programmable channels, they are used to connect a particular **Event** to a particular **Task**. When an event is activated, the ETM channel will trigger the corresponding task automatically.

Peripherals that support ETM functionality provide their or unique set of events and tasks to be connected by the ETM. An ETM channel can connect any event to any task, even looping back an event to a task on the same peripheral. However, an ETM channel can only connect one event to one task at a time (i.e., 1 to 1 relation). If you want to use different events to trigger the same task, you can set up more ETM channels.

Typically, with the help of the ETM module, you can implement features like:

-  Toggle the GPIO when a timer alarm event happens
-  Start an ADC conversion when a pulse edge is detected on a GPIO

Functional Overview
-------------------

The following sections of this document cover the typical steps to configure and use the ETM module.

- :ref:`etm-channel-allocation` - describes how to install and uninstall the ETM channel.
- :ref:`etm-event` - describes how to allocate a new ETM event handle or fetch an existing handle from various peripherals.
- :ref:`etm-task` - describes how to allocate a new ETM task handle or fetch an existing handle from various peripherals.
- :ref:`etm-channel-control` - describes common ETM channel control functions.
- :ref:`etm-power-management` - describes the options and strategies provided by the driver in order to save power.
- :ref:`etm-thread-safety` - lists which APIs are guaranteed to be thread-safe by the driver.
- :ref:`etm-kconfig-options` - lists the supported Kconfig options that can be used to make a different effect on driver behavior.

.. _etm-channel-allocation:

ETM Channel Allocation
^^^^^^^^^^^^^^^^^^^^^^

There are many identical ETM channels in {IDF_TARGET_NAME} [1]_, and each channel is represented by :cpp:type:`esp_etm_channel_handle_t` in the software. The ETM core driver manages all available hardware resources in a pool so that you do not need to care about which channel is in use and which is not. The ETM core driver will allocate a channel for you when you call :cpp:func:`esp_etm_new_channel` and delete it when you call :cpp:func:`esp_etm_del_channel`. All requirements needed for allocating a channel are provided in :cpp:type:`esp_etm_channel_config_t`.

Before deleting an ETM channel, please disable it by :cpp:func:`esp_etm_channel_disable` in advance or make sure it has not been enabled yet by :cpp:func:`esp_etm_channel_enable`.

.. _etm-event:

ETM Event
^^^^^^^^^

ETM Event abstracts the event source, masking the details of specific event sources, and is represented by :cpp:type:`esp_etm_event_handle_t` in the software, allowing applications to handle different types of events more easily. ETM events can be generated from a variety of peripherals, thus the way to get the event handle differs from peripherals. When an ETM event is no longer used, you should call :cpp:func:`esp_etm_channel_connect` with a ``NULL`` event handle to disconnect it and then call :cpp:func:`esp_etm_del_event` to free the event resource.

GPIO Events
~~~~~~~~~~~

GPIO **edge** event is the most common event type, it can be generated by any GPIO pin. You can call :cpp:func:`gpio_new_etm_event` to create a GPIO event handle, with the configurations provided in :cpp:type:`gpio_etm_event_config_t`:

- :cpp:member:`gpio_etm_event_config_t::edge` or :cpp:member:`gpio_etm_event_config_t::edges` decides which edge(s) to trigger the event(s), supported edge types are listed in the :cpp:type:`gpio_etm_event_edge_t`.

You need to build a connection between the GPIO ETM event handle and the GPIO number. So you should call :cpp:func:`gpio_etm_event_bind_gpio` afterwards. Please note, only the ETM event handle that created by :cpp:func:`gpio_new_etm_event` can set a GPIO number. Calling this function with other kinds of ETM events returns :c:macro:`ESP_ERR_INVALID_ARG` error. Needless to say, this function does not help with the GPIO initialization, you still need to call :cpp:func:`gpio_config` to set the property like direction, pull up/down mode separately.

Other Peripheral Events
~~~~~~~~~~~~~~~~~~~~~~~

.. list::

    :SOC_SYSTIMER_SUPPORT_ETM: - You can call :cpp:func:`esp_systick_new_etm_alarm_event` to get the ETM event from RTOS Systick, one per CPU core.
    :SOC_SYSTIMER_SUPPORT_ETM: - Refer to :doc:`/api-reference/system/esp_timer` for how to get the ETM event handle from esp_timer.
    :SOC_TIMER_SUPPORT_ETM: - Refer to :ref:`gptimer-etm-event-and-task` for how to get the ETM event handle from GPTimer.
    :SOC_GDMA_SUPPORT_ETM: - Refer to :doc:`/api-reference/peripherals/async_memcpy` for how to get the ETM event handle from async memcpy.
    :SOC_MCPWM_SUPPORT_ETM: - Refer to :doc:`/api-reference/peripherals/mcpwm` for how to get the ETM event handle from MCPWM.
    :SOC_ANA_CMPR_SUPPORT_ETM: - Refer to :doc:`/api-reference/peripherals/ana_cmpr` for how to get the ETM event handle from analog comparator.
    :SOC_TEMPERATURE_SENSOR_SUPPORT_ETM: - Refer to :doc:`/api-reference/peripherals/temp_sensor` for how to get the ETM event handle from temperature sensor.
    :SOC_I2S_SUPPORTS_ETM:  - Refer to :doc:`/api-reference/peripherals/i2s` for how to get the ETM event handle from I2S.
    :SOC_LEDC_SUPPORT_ETM:  - Refer to :doc:`/api-reference/peripherals/ledc` for how to get the ETM event handle from LEDC.

.. _etm-task:

ETM Task
^^^^^^^^

ETM Task abstracts the task action and is represented by :cpp:type:`esp_etm_task_handle_t` in the software, allowing tasks to be managed and represented in the same way. ETM tasks can be assigned to a variety of peripherals, thus the way to get the task handle differs from peripherals. When an ETM task is no longer used, you should call :cpp:func:`esp_etm_channel_connect` with a ``NULL`` task handle to disconnect it and then call :cpp:func:`esp_etm_del_task` to free the task resource.

GPIO Tasks
~~~~~~~~~~

GPIO task is the most common task type. One GPIO can take one or more GPIO ETM task actions, and one GPIO ETM task action can even manage multiple GPIOs. When the task gets activated by the ETM channel, all managed GPIOs can set/clear/toggle at the same time. You can call :cpp:func:`gpio_new_etm_task` to create a GPIO task handle, with the configurations provided in :cpp:type:`gpio_etm_task_config_t`:

- :cpp:member:`gpio_etm_task_config_t::action` or :cpp:member:`gpio_etm_task_config_t::actions` decides what GPIO action(s) would be taken by the ETM task. Supported actions are listed in the :cpp:type:`gpio_etm_task_action_t`. If one GPIO needs to take more than one actions, the action tasks have to be created in one :cpp:func:`gpio_new_etm_task` call with filling the actions into the array of :cpp:member:`gpio_etm_task_config_t::actions`.

To build a connection between the GPIO ETM task and the GPIO number, you should call :cpp:func:`gpio_etm_task_add_gpio`. You can call this function by several times if you want the task handle to manage more GPIOs. Please note, only the ETM task handle that created by :cpp:func:`gpio_new_etm_task` can manage a GPIO. Calling this function with other kinds of ETM tasks returns :c:macro:`ESP_ERR_INVALID_ARG` error. Needless to say, this function does not help with the GPIO initialization, you still need to call :cpp:func:`gpio_config` to set the property like direction, pull up/down mode separately.

Before you call :cpp:func:`esp_etm_del_task` to delete the GPIO ETM task, make sure that all previously added GPIOs are removed by :cpp:func:`gpio_etm_task_rm_gpio` in advance.

Other Peripheral Tasks
~~~~~~~~~~~~~~~~~~~~~~

.. list::

    :SOC_TIMER_SUPPORT_ETM: - Refer to :ref:`gptimer-etm-event-and-task` for how to get the ETM task handle from GPTimer.
    :SOC_TEMPERATURE_SENSOR_SUPPORT_ETM: - Refer to :doc:`/api-reference/peripherals/temp_sensor` for how to get the ETM task handle from temperature sensor.
    :SOC_I2S_SUPPORTS_ETM:  - Refer to :doc:`/api-reference/peripherals/i2s` for how to get the ETM task handle from I2S.
    :SOC_LEDC_SUPPORT_ETM:  - Refer to :doc:`/api-reference/peripherals/ledc` for how to get the ETM task handle from LEDC.

.. _etm-channel-control:

ETM Channel Control
^^^^^^^^^^^^^^^^^^^

Connect Event and Task
~~~~~~~~~~~~~~~~~~~~~~

An ETM event has no association with an ETM task, until they are connected to the same ETM channel by calling :cpp:func:`esp_etm_channel_connect`. Especially, calling the function with a ``NULL`` task/event handle means disconnecting the channel from any task or event. Note that, this function can be called either before or after the channel is enabled. But calling this function at runtime to change the connection can be dangerous, because the channel may be in the middle of a cycle, and the new connection may not take effect immediately.

Enable and Disable Channel
~~~~~~~~~~~~~~~~~~~~~~~~~~

You can call :cpp:func:`esp_etm_channel_enable` and :cpp:func:`esp_etm_channel_disable` to enable and disable the ETM channel from working.

ETM Channel Profiling
~~~~~~~~~~~~~~~~~~~~~

To check if the ETM channels are set with proper events and tasks, you can call :cpp:func:`esp_etm_dump` to dump all working ETM channels with their associated events and tasks. The dumping format is like:

::

    ===========ETM Dump Start==========
    channel 0: event 48 ==> task 17
    channel 1: event 48 ==> task 90
    channel 2: event 48 ==> task 94
    ===========ETM Dump End============

The digital ID printed in the dump information is defined in the ``soc/soc_etm_source.h`` file.

.. _etm-power-management:

Power Management
^^^^^^^^^^^^^^^^

When power management is enabled, i.e., :ref:`CONFIG_PM_ENABLE` is on, the system may adjust or disable the clock source, and power off the ETM peripheral before going to sleep. As a result, the existing connection between events and tasks will be lost, and the ETM channels can't work correctly after wake up. So by default, the driver will acquire a power management lock internally to forbid the system from powering off the ETM peripheral.

.. only:: SOC_ETM_SUPPORT_SLEEP_RETENTION

    If you want to save more power, you can set :cpp:member:`esp_etm_channel_config_t::etm_chan_flags::allow_pd` to ``true``. Then ETM registers will be backed up before sleep and restored after wake up. Please note, enabling this option will increase the memory consumption for saving the register context.

.. _etm-thread-safety:

Thread Safety
^^^^^^^^^^^^^

The ETM core driver is thread-safe.

- Factory functions (for example, :cpp:func:`esp_etm_new_channel`, :cpp:func:`gpio_new_etm_task`, and other ``*_new_etm_*`` creators) are thread-safe and can be called from different RTOS tasks concurrently.
- Channel control APIs (for example, :cpp:func:`esp_etm_channel_connect`, :cpp:func:`esp_etm_channel_enable`, :cpp:func:`esp_etm_channel_disable`, :cpp:func:`esp_etm_del_channel`) are also thread-safe. Concurrent operations on different channels are safe. Concurrent operations on the same channel are serialized internally; if an operation is incompatible with the current channel state, it will return :c:macro:`ESP_ERR_INVALID_STATE`.
- No functions are allowed to run within the ISR environment.

.. _etm-kconfig-options:

Kconfig Options
^^^^^^^^^^^^^^^

- :ref:`CONFIG_ETM_ENABLE_DEBUG_LOG` is used to enable the debug log output. Enabling this option increases the firmware binary size as well.

API Reference
-------------

.. include-build-file:: inc/esp_etm.inc
.. include-build-file:: inc/gpio_etm.inc
.. include-build-file:: inc/esp_systick_etm.inc

.. [1]
   Different ESP chip series might have different numbers of ETM channels. For more details, please refer to *{IDF_TARGET_NAME} Technical Reference Manual* > Chapter **Event Task Matrix (ETM)** [`PDF <{IDF_TARGET_TRM_EN_URL}#evntaskmatrix>`__]. The driver does not forbid you from applying for more channels, but it will return an error when all available hardware resources are used up. Please always check the return value when doing channel allocation (i.e., :cpp:func:`esp_etm_new_channel`).
