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.
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.
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 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 **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` decides which edge to trigger the event, 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.
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 task is the most common task type, one GPIO task 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` decides what GPIO action would be taken by the ETM task. Supported actions are listed in the :cpp:type:`gpio_etm_task_action_t`.
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 :doc:`GPTimer </api-reference/peripherals/gptimer>` for how to get the ETM task handle from GPTimer.
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.
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.
The factory functions like :cpp:func:`esp_etm_new_channel` and :cpp:func:`gpio_new_etm_task` are guaranteed to be thread-safe by the driver, which means, you can call them from different RTOS tasks without protection by extra locks.
Other functions that take :cpp:type:`esp_etm_channel_handle_t`, :cpp:type:`esp_etm_task_handle_t` and :cpp:type:`esp_etm_event_handle_t` as the first positional parameter, are not treated as thread-safe, which means you should avoid calling them from multiple tasks.
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`).
