Event Task Matrix (ETM)

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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.

ETM channels Overview

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.

  • ETM Channel Allocation - describes how to install and uninstall the ETM channel.

  • ETM Event - describes how to allocate a new ETM event handle or fetch an existing handle from various peripherals.

  • ETM Task - describes how to allocate a new ETM task handle or fetch an existing handle from various peripherals.

  • ETM Channel Control - describes common ETM channel control functions.

  • 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.

ETM Channel Allocation

There are many identical ETM channels in ESP32-C6 1, and each channel is represented by 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 esp_etm_new_channel() and delete it when you call esp_etm_del_channel(). All requirements needed for allocating a channel are provided in esp_etm_channel_config_t.

Before deleting an ETM channel, please disable it by esp_etm_channel_disable() in advance or make sure it has not been enabled yet by esp_etm_channel_enable().

ETM Event

ETM Event abstracts the event source, masking the details of specific event sources, and is represented by 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 esp_etm_channel_connect() with a NULL event handle to disconnect it and then call 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 gpio_new_etm_event() to create a GPIO event handle, with the configurations provided in gpio_etm_event_config_t:

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

Other Peripheral Events

ETM Task

ETM Task abstracts the task action and is represented by 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 esp_etm_channel_connect() with a NULL task handle to disconnect it and then call esp_etm_del_task() to free the task resource.

GPIO Tasks

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 gpio_new_etm_task() to create a GPIO task handle, with the configurations provided in gpio_etm_task_config_t:

To build a connection between the GPIO ETM task and the GPIO number, you should call 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 gpio_new_etm_task() can manage a GPIO. Calling this function with other kinds of ETM tasks returns ESP_ERR_INVALID_ARG error. Needless to say, this function does not help with the GPIO initialization, you still need to call gpio_config() to set the property like direction, pull up/down mode separately.

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

Other Peripheral Tasks

  • Refer to GPTimer for how to get the ETM task handle from GPTimer.

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 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 esp_etm_channel_enable() and 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 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.

Thread Safety

The factory functions like esp_etm_new_channel() and 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.

No functions are allowed to run within the ISR environment.

Other functions that take esp_etm_channel_handle_t, esp_etm_task_handle_t and 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.

Kconfig Options

API Reference

Header File

Functions

esp_err_t esp_etm_new_channel(const esp_etm_channel_config_t *config, esp_etm_channel_handle_t *ret_chan)

Allocate an ETM channel.

Note

The channel can later be freed by esp_etm_del_channel

Parameters

  • config -- [in] ETM channel configuration

  • ret_chan -- [out] Returned ETM channel handle

Returns

  • ESP_OK: Allocate ETM channel successfully

  • ESP_ERR_INVALID_ARG: Allocate ETM channel failed because of invalid argument

  • ESP_ERR_NO_MEM: Allocate ETM channel failed because of out of memory

  • ESP_ERR_NOT_FOUND: Allocate ETM channel failed because all channels are used up and no more free one

  • ESP_FAIL: Allocate ETM channel failed because of other reasons

esp_err_t esp_etm_del_channel(esp_etm_channel_handle_t chan)

Delete an ETM channel.

Parameters

chan -- [in] ETM channel handle that created by esp_etm_new_channel

Returns

  • ESP_OK: Delete ETM channel successfully

  • ESP_ERR_INVALID_ARG: Delete ETM channel failed because of invalid argument

  • ESP_FAIL: Delete ETM channel failed because of other reasons

esp_err_t esp_etm_channel_enable(esp_etm_channel_handle_t chan)

Enable ETM channel.

Note

This function will transit the channel state from init to enable.

Parameters

chan -- [in] ETM channel handle that created by esp_etm_new_channel

Returns

  • ESP_OK: Enable ETM channel successfully

  • ESP_ERR_INVALID_ARG: Enable ETM channel failed because of invalid argument

  • ESP_ERR_INVALID_STATE: Enable ETM channel failed because the channel has been enabled already

  • ESP_FAIL: Enable ETM channel failed because of other reasons

esp_err_t esp_etm_channel_disable(esp_etm_channel_handle_t chan)

Disable ETM channel.

Note

This function will transit the channel state from enable to init.

Parameters

chan -- [in] ETM channel handle that created by esp_etm_new_channel

Returns

  • ESP_OK: Disable ETM channel successfully

  • ESP_ERR_INVALID_ARG: Disable ETM channel failed because of invalid argument

  • ESP_ERR_INVALID_STATE: Disable ETM channel failed because the channel is not enabled yet

  • ESP_FAIL: Disable ETM channel failed because of other reasons

esp_err_t esp_etm_channel_connect(esp_etm_channel_handle_t chan, esp_etm_event_handle_t event, esp_etm_task_handle_t task)

Connect an ETM event to an ETM task via a previously allocated ETM channel.

Note

Setting the ETM event/task handle to NULL means to disconnect the channel from any event/task

Parameters

  • chan -- [in] ETM channel handle that created by esp_etm_new_channel

  • event -- [in] ETM event handle obtained from a driver/peripheral, e.g. xxx_new_etm_event

  • task -- [in] ETM task handle obtained from a driver/peripheral, e.g. xxx_new_etm_task

Returns

  • ESP_OK: Connect ETM event and task to the channel successfully

  • ESP_ERR_INVALID_ARG: Connect ETM event and task to the channel failed because of invalid argument

  • ESP_FAIL: Connect ETM event and task to the channel failed because of other reasons

esp_err_t esp_etm_del_event(esp_etm_event_handle_t event)

Delete ETM event.

Note

Although the ETM event comes from various peripherals, we provide the same user API to delete the event handle seamlessly.

Parameters

event -- [in] ETM event handle obtained from a driver/peripheral, e.g. xxx_new_etm_event

Returns

  • ESP_OK: Delete ETM event successfully

  • ESP_ERR_INVALID_ARG: Delete ETM event failed because of invalid argument

  • ESP_FAIL: Delete ETM event failed because of other reasons

esp_err_t esp_etm_del_task(esp_etm_task_handle_t task)

Delete ETM task.

Note

Although the ETM task comes from various peripherals, we provide the same user API to delete the task handle seamlessly.

Parameters

task -- [in] ETM task handle obtained from a driver/peripheral, e.g. xxx_new_etm_task

Returns

  • ESP_OK: Delete ETM task successfully

  • ESP_ERR_INVALID_ARG: Delete ETM task failed because of invalid argument

  • ESP_FAIL: Delete ETM task failed because of other reasons

esp_err_t esp_etm_dump(FILE *out_stream)

Dump ETM channel usages to the given IO stream.

Parameters

out_stream -- [in] IO stream (e.g. stdout)

Returns

  • ESP_OK: Dump ETM channel usages successfully

  • ESP_ERR_INVALID_ARG: Dump ETM channel usages failed because of invalid argument

  • ESP_FAIL: Dump ETM channel usages failed because of other reasons

Structures

struct esp_etm_channel_config_t

ETM channel configuration.

Type Definitions

typedef struct esp_etm_channel_t *esp_etm_channel_handle_t

ETM channel handle.

typedef struct esp_etm_event_t *esp_etm_event_handle_t

ETM event handle.

typedef struct esp_etm_task_t *esp_etm_task_handle_t

ETM task handle.

Header File

Functions

esp_err_t gpio_new_etm_event(const gpio_etm_event_config_t *config, esp_etm_event_handle_t *ret_event)

Create an ETM event object for the GPIO peripheral.

Note

The created ETM event object can be deleted later by calling esp_etm_del_event

Note

The newly created ETM event object is not bind to any GPIO, you need to call gpio_etm_event_bind_gpio to bind the wanted GPIO

Parameters

  • config -- [in] GPIO ETM event configuration

  • ret_event -- [out] Returned ETM event handle

Returns

  • ESP_OK: Create ETM event successfully

  • ESP_ERR_INVALID_ARG: Create ETM event failed because of invalid argument

  • ESP_ERR_NO_MEM: Create ETM event failed because of out of memory

  • ESP_ERR_NOT_FOUND: Create ETM event failed because all events are used up and no more free one

  • ESP_FAIL: Create ETM event failed because of other reasons

esp_err_t gpio_etm_event_bind_gpio(esp_etm_event_handle_t event, int gpio_num)

Bind the GPIO with the ETM event.

Note

Calling this function multiple times with different GPIO number can override the previous setting immediately.

Note

Only GPIO ETM object can call this function

Parameters

  • event -- [in] ETM event handle that created by gpio_new_etm_event

  • gpio_num -- [in] GPIO number that can trigger the ETM event

Returns

  • ESP_OK: Set the GPIO for ETM event successfully

  • ESP_ERR_INVALID_ARG: Set the GPIO for ETM event failed because of invalid argument, e.g. GPIO is not input capable, ETM event is not of GPIO type

  • ESP_FAIL: Set the GPIO for ETM event failed because of other reasons

esp_err_t gpio_new_etm_task(const gpio_etm_task_config_t *config, esp_etm_task_handle_t *ret_task)

Create an ETM task object for the GPIO peripheral.

Note

The created ETM task object can be deleted later by calling esp_etm_del_task

Note

The GPIO ETM task works like a container, a newly created ETM task object doesn't have GPIO members to be managed. You need to call gpio_etm_task_add_gpio to put one or more GPIOs to the container.

Parameters

  • config -- [in] GPIO ETM task configuration

  • ret_task -- [out] Returned ETM task handle

Returns

  • ESP_OK: Create ETM task successfully

  • ESP_ERR_INVALID_ARG: Create ETM task failed because of invalid argument

  • ESP_ERR_NO_MEM: Create ETM task failed because of out of memory

  • ESP_ERR_NOT_FOUND: Create ETM task failed because all tasks are used up and no more free one

  • ESP_FAIL: Create ETM task failed because of other reasons

esp_err_t gpio_etm_task_add_gpio(esp_etm_task_handle_t task, int gpio_num)

Add GPIO to the ETM task.

Note

You can call this function multiple times to add more GPIOs

Note

Only GPIO ETM object can call this function

Parameters

  • task -- [in] ETM task handle that created by gpio_new_etm_task

  • gpio_num -- [in] GPIO number that can be controlled by the ETM task

Returns

  • ESP_OK: Add GPIO to the ETM task successfully

  • ESP_ERR_INVALID_ARG: Add GPIO to the ETM task failed because of invalid argument, e.g. GPIO is not output capable, ETM task is not of GPIO type

  • ESP_ERR_INVALID_STATE: Add GPIO to the ETM task failed because the GPIO is used by other ETM task already

  • ESP_FAIL: Add GPIO to the ETM task failed because of other reasons

esp_err_t gpio_etm_task_rm_gpio(esp_etm_task_handle_t task, int gpio_num)

Remove the GPIO from the ETM task.

Note

Before deleting the ETM task, you need to remove all the GPIOs from the ETM task by this function

Note

Only GPIO ETM object can call this function

Parameters

  • task -- [in] ETM task handle that created by gpio_new_etm_task

  • gpio_num -- [in] GPIO number that to be remove from the ETM task

Returns

  • ESP_OK: Remove the GPIO from the ETM task successfully

  • ESP_ERR_INVALID_ARG: Remove the GPIO from the ETM task failed because of invalid argument

  • ESP_ERR_INVALID_STATE: Remove the GPIO from the ETM task failed because the GPIO is not controlled by this ETM task

  • ESP_FAIL: Remove the GPIO from the ETM task failed because of other reasons

Structures

struct gpio_etm_event_config_t

GPIO ETM event configuration.

Public Members

gpio_etm_event_edge_t edge

Which kind of edge can trigger the ETM event module

struct gpio_etm_task_config_t

GPIO ETM task configuration.

Public Members

gpio_etm_task_action_t action

Which action to take by the ETM task module

Enumerations

enum gpio_etm_event_edge_t

GPIO edges that can be used as ETM event.

Values:

enumerator GPIO_ETM_EVENT_EDGE_POS

A rising edge on the GPIO will generate an ETM event signal

enumerator GPIO_ETM_EVENT_EDGE_NEG

A falling edge on the GPIO will generate an ETM event signal

enumerator GPIO_ETM_EVENT_EDGE_ANY

Any edge on the GPIO can generate an ETM event signal

enum gpio_etm_task_action_t

GPIO actions that can be taken by the ETM task.

Values:

enumerator GPIO_ETM_TASK_ACTION_SET

Set the GPIO level to high

enumerator GPIO_ETM_TASK_ACTION_CLR

Clear the GPIO level to low

enumerator GPIO_ETM_TASK_ACTION_TOG

Toggle the GPIO level

Header File

Functions

esp_err_t esp_systick_new_etm_alarm_event(int core_id, esp_etm_event_handle_t *out_event)

Get the ETM event handle of systick hardware's alarm/heartbeat event.

Note

The created ETM event object can be deleted later by calling esp_etm_del_event

Parameters

  • core_id -- [in] CPU core ID

  • out_event -- [out] Returned ETM event handle

Returns

  • ESP_OK Success

  • ESP_ERR_INVALID_ARG Parameter error

1

Different ESP chip series might have different numbers of ETM channels. For more details, please refer to ESP32-C6 Technical Reference Manual > Chapter Event Task Matrix (ETM) [PDF]. 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., esp_etm_new_channel()).

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