Watchdogs

Overview

The ESP-IDF has support for two types of watchdogs: The Interrupt Watchdog Timer and the Task Watchdog Timer (TWDT). The Interrupt Watchdog Timer and the TWDT can both be enabled using make menuconfig, however the TWDT can also be enabled during runtime. The Interrupt Watchdog is responsible for detecting instances where FreeRTOS task switching is blocked for a prolonged period of time. The TWDT is responsible for detecting instances of tasks running without yielding for a prolonged period.

Interrupt watchdog

The interrupt watchdog makes sure the FreeRTOS task switching interrupt isn’t blocked for a long time. This is bad because no other tasks, including potentially important ones like the WiFi task and the idle task, can’t get any CPU runtime. A blocked task switching interrupt can happen because a program runs into an infinite loop with interrupts disabled or hangs in an interrupt.

The default action of the interrupt watchdog is to invoke the panic handler. causing a register dump and an opportunity for the programmer to find out, using either OpenOCD or gdbstub, what bit of code is stuck with interrupts disabled. Depending on the configuration of the panic handler, it can also blindly reset the CPU, which may be preferred in a production environment.

The interrupt watchdog is built around the hardware watchdog in timer group 1. If this watchdog for some reason cannot execute the NMI handler that invokes the panic handler (e.g. because IRAM is overwritten by garbage), it will hard-reset the SOC.

Task Watchdog Timer

The Task Watchdog Timer (TWDT) is responsible for detecting instances of tasks running for a prolonged period of time without yielding. This is a symptom of CPU starvation and is usually caused by a higher priority task looping without yielding to a lower-priority task thus starving the lower priority task from CPU time. This can be an indicator of poorly written code that spinloops on a peripheral, or a task that is stuck in an infinite loop.

By default the TWDT will watch the Idle Tasks of each CPU, however any task can elect to be watched by the TWDT. Each watched task must ‘reset’ the TWDT periodically to indicate that they have been allocated CPU time. If a task does not reset within the TWDT timeout period, a warning will be printed with information about which tasks failed to reset the TWDT in time and which tasks are currently running on the ESP32 CPUs. And also there is a possibility to redefine the function esp_task_wdt_isr_user_handler in the user code to receive this event.

The TWDT is built around the Hardware Watchdog Timer in Timer Group 0. The TWDT can be initialized by calling esp_task_wdt_init() which will configure the hardware timer. A task can then subscribe to the TWDT using esp_task_wdt_add() in order to be watched. Each subscribed task must periodically call esp_task_wdt_reset() to reset the TWDT. Failure by any subscribed tasks to periodically call esp_task_wdt_reset() indicates that one or more tasks have been starved of CPU time or are stuck in a loop somewhere.

A watched task can be unsubscribed from the TWDT using esp_task_wdt_delete(). A task that has been unsubscribed should no longer call esp_task_wdt_reset(). Once all tasks have unsubscribed form the TWDT, the TWDT can be deinitialized by calling esp_task_wdt_deinit().

By default CONFIG_TASK_WDT in make menuconfig will be enabled causing the TWDT to be initialized automatically during startup. Likewise CONFIG_TASK_WDT_CHECK_IDLE_TASK_CPU0 and CONFIG_TASK_WDT_CHECK_IDLE_TASK_CPU1 are also enabled by default causing the two Idle Tasks to be subscribed to the TWDT during startup.

JTAG and watchdogs

While debugging using OpenOCD, the CPUs will be halted every time a breakpoint is reached. However if the watchdog timers continue to run when a breakpoint is encountered, they will eventually trigger a reset making it very difficult to debug code. Therefore OpenOCD will disable the hardware timers of both the interrupt and task watchdogs at every breakpoint. Moreover, OpenOCD will not reenable them upon leaving the breakpoint. This means that interrupt watchdog and task watchdog functionality will essentially be disabled. No warnings or panics from either watchdogs will be generated when the ESP32 is connected to OpenOCD via JTAG.

Interrupt Watchdog API Reference

Functions

void esp_int_wdt_init()

Initialize the non-CPU-specific parts of interrupt watchdog. This is called in the init code if the interrupt watchdog is enabled in menuconfig.

Task Watchdog API Reference

A full example using the Task Watchdog is available in esp-idf: system/task_watchdog

Functions

esp_err_t esp_task_wdt_init(uint32_t timeout, bool panic)

Initialize the Task Watchdog Timer (TWDT)

This function configures and initializes the TWDT. If the TWDT is already initialized when this function is called, this function will update the TWDT’s timeout period and panic configurations instead. After initializing the TWDT, any task can elect to be watched by the TWDT by subscribing to it using esp_task_wdt_add().

Return
  • ESP_OK: Initialization was successful
  • ESP_ERR_NO_MEM: Initialization failed due to lack of memory
Note
esp_task_wdt_init() must only be called after the scheduler started
Parameters
  • timeout: Timeout period of TWDT in seconds
  • panic: Flag that controls whether the panic handler will be executed when the TWDT times out

esp_err_t esp_task_wdt_deinit()

Deinitialize the Task Watchdog Timer (TWDT)

This function will deinitialize the TWDT. Calling this function whilst tasks are still subscribed to the TWDT, or when the TWDT is already deinitialized, will result in an error code being returned.

Return
  • ESP_OK: TWDT successfully deinitialized
  • ESP_ERR_INVALID_STATE: Error, tasks are still subscribed to the TWDT
  • ESP_ERR_NOT_FOUND: Error, TWDT has already been deinitialized

esp_err_t esp_task_wdt_add(TaskHandle_t handle)

Subscribe a task to the Task Watchdog Timer (TWDT)

This function subscribes a task to the TWDT. Each subscribed task must periodically call esp_task_wdt_reset() to prevent the TWDT from elapsing its timeout period. Failure to do so will result in a TWDT timeout. If the task being subscribed is one of the Idle Tasks, this function will automatically enable esp_task_wdt_reset() to called from the Idle Hook of the Idle Task. Calling this function whilst the TWDT is uninitialized or attempting to subscribe an already subscribed task will result in an error code being returned.

Return
  • ESP_OK: Successfully subscribed the task to the TWDT
  • ESP_ERR_INVALID_ARG: Error, the task is already subscribed
  • ESP_ERR_NO_MEM: Error, could not subscribe the task due to lack of memory
  • ESP_ERR_INVALID_STATE: Error, the TWDT has not been initialized yet
Parameters
  • handle: Handle of the task. Input NULL to subscribe the current running task to the TWDT

esp_err_t esp_task_wdt_reset()

Reset the Task Watchdog Timer (TWDT) on behalf of the currently running task.

This function will reset the TWDT on behalf of the currently running task. Each subscribed task must periodically call this function to prevent the TWDT from timing out. If one or more subscribed tasks fail to reset the TWDT on their own behalf, a TWDT timeout will occur. If the IDLE tasks have been subscribed to the TWDT, they will automatically call this function from their idle hooks. Calling this function from a task that has not subscribed to the TWDT, or when the TWDT is uninitialized will result in an error code being returned.

Return
  • ESP_OK: Successfully reset the TWDT on behalf of the currently running task
  • ESP_ERR_NOT_FOUND: Error, the current running task has not subscribed to the TWDT
  • ESP_ERR_INVALID_STATE: Error, the TWDT has not been initialized yet

esp_err_t esp_task_wdt_delete(TaskHandle_t handle)

Unsubscribes a task from the Task Watchdog Timer (TWDT)

This function will unsubscribe a task from the TWDT. After being unsubscribed, the task should no longer call esp_task_wdt_reset(). If the task is an IDLE task, this function will automatically disable the calling of esp_task_wdt_reset() from the Idle Hook. Calling this function whilst the TWDT is uninitialized or attempting to unsubscribe an already unsubscribed task from the TWDT will result in an error code being returned.

Return
  • ESP_OK: Successfully unsubscribed the task from the TWDT
  • ESP_ERR_INVALID_ARG: Error, the task is already unsubscribed
  • ESP_ERR_INVALID_STATE: Error, the TWDT has not been initialized yet
Parameters
  • handle: Handle of the task. Input NULL to unsubscribe the current running task.

esp_err_t esp_task_wdt_status(TaskHandle_t handle)

Query whether a task is subscribed to the Task Watchdog Timer (TWDT)

This function will query whether a task is currently subscribed to the TWDT, or whether the TWDT is initialized.

Return
:
  • ESP_OK: The task is currently subscribed to the TWDT
  • ESP_ERR_NOT_FOUND: The task is currently not subscribed to the TWDT
  • ESP_ERR_INVALID_STATE: The TWDT is not initialized, therefore no tasks can be subscribed
Parameters
  • handle: Handle of the task. Input NULL to query the current running task.

void esp_task_wdt_feed()

Reset the TWDT on behalf of the current running task, or subscribe the TWDT to if it has not done so already.

This function is similar to esp_task_wdt_reset() and will reset the TWDT on behalf of the current running task. However if this task has not subscribed to the TWDT, this function will automatically subscribe the task. Therefore, an unsubscribed task will subscribe to the TWDT on its first call to this function, then proceed to reset the TWDT on subsequent calls of this function.

Warning
This function is deprecated, use esp_task_wdt_add() and esp_task_wdt_reset() instead