The ESP-IDF has support for multiple types of watchdogs, with the two main ones being: The Interrupt Watchdog Timer and the Task Watchdog Timer (TWDT). The Interrupt Watchdog Timer and the TWDT can both be enabled using Project Configuration Menu, 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.
ESP-IDF has support for the following types of watchdog timers:
Interrupt Watchdog Timer (IWDT)
Task Watchdog Timer (TWDT)
Crystal 32K Watchdog Timer (XTWDT)
The various watchdog timers can be enabled using the Project Configuration Menu. However, the TWDT can also be enabled during runtime.
Interrupt Watchdog Timer (IWDT)
The purpose of the IWDT is to ensure that interrupt service routines (ISRs) are not blocked from running for a prolonged period of time (i.e., the IWDT timeout period). Blocking ISRs from running in a timely manner is undesirable as it can increases ISR latency, and also prevents task switching (as task switching is executed form an ISR). The things that can block ISRs from running include:
Critical Sections (also disables interrupts)
Other same/higher priority ISRs (will block same/lower priority ISRs from running it completes execution)
The IWDT utilizes the watchdog timer in Timer Group 1 as its underlying hardware timer and leverages the FreeRTOS tick interrupt on each CPU to feed the watchdog timer. If the tick interrupt on a particular CPU is not run at within the IWDT timeout period, it is indicative that something is blocking ISRs from being run on that CPU (see the list of reasons above).
When the IWDT times out, the default action is to invoke the panic handler and display the panic reason as
Interrupt wdt timeout on CPU0 or
Interrupt wdt timeout on CPU1 (as applicable). Depending on the panic handler’s configured behavior (see CONFIG_ESP_SYSTEM_PANIC), users can then debug the source of the IWDT timeout (via the backtrace, OpenOCD, gdbstub etc) or simply reset the chip (which may be preferred in a production environment).
If for whatever reason the panic handler is unable to run after an IWDT timeout, the IWDT has a secondary timeout that will hard-reset the chip (i.e., a system reset).
The IWDT is enabled by default via the CONFIG_ESP_INT_WDT option.
The IWDT’s timeout is configured by setting the CONFIG_ESP_INT_WDT_TIMEOUT_MS option.
Note that the default timeout is higher if PSRAM support is enabled, as a critical section or interrupt routine that accesses a large amount of PSRAM will take longer to complete in some circumstances.
The timeout should always at least twice longer than the period between FreeRTOS ticks (see CONFIG_FREERTOS_HZ).
If you find the IWDT timeout is triggered because an interrupt or critical section is running longer than the timeout period, consider rewriting the code:
Critical sections should be made as short as possible. Any non-critical code/computation should be placed outside the critical section.
Interrupt handlers should also perform the minimum possible amount of computation. Users can consider deferring any computation to a task by having the ISR push data to a task using queues.
Neither critical sections or interrupt handlers should ever block waiting for another event to occur. If changing the code to reduce the processing time is not possible or desirable, it’s possible to increase the CONFIG_ESP_INT_WDT_TIMEOUT_MS setting instead.
Task Watchdog Timer (TWDT)
The Task Watchdog Timer (TWDT) is used to monitor particular tasks, ensuring that they are able to execute within a given timeout period. The TWDT primarily watches the Idle task, however any task can subscribe to be watched by the TWDT. By watching the Idle task, the TWDT can detect instances of tasks running for a prolonged period of time wihtout yielding. This can be an indicator of poorly written code that spinloops on a peripheral, or a task that is stuck in an infinite loop.
The TWDT is built around the Hardware Watchdog Timer in Timer Group 0. When a timeout occurs, an interrupt is triggered. Users can redefine the function esp_task_wdt_isr_user_handler in the user code, in order to receive the timeout event and handle it differently.
The following functions can be used to watch tasks using the TWDT:
esp_task_wdt_init()to initialize the TWDT and subscribe the idle tasks.
esp_task_wdt_add()subscribes other tasks to the TWDT.
esp_task_wdt_reset()should be called from the task to feed the TWDT.
esp_task_wdt_delete()unsubscribes a previously subscribed task
esp_task_wdt_deinit()unsubscribes the idle tasks and deinitializes the TWDT
In the case where applications need to watch at a more granular level (i.e., ensure that a particular functions/stub/code-path is called), the TWDT allows subscription of “users”.
esp_task_wdt_add_user()to subscribe an arbitrary user of the TWDT. This function will return a user handle to the added user.
esp_task_wdt_reset_user()must be called using the user handle in order to prevent a TWDT timeout.
esp_task_wdt_delete_user()unsubscribes an arbitrary user of the TWDT.
The default timeout period for the TWDT is set using config item CONFIG_ESP_TASK_WDT_TIMEOUT_S. This should be set to at least as long as you expect any single task will need to monopolize the CPU (for example, if you expect the app will do a long intensive calculation and should not yield to other tasks). It is also possible to change this timeout at runtime by calling
Erasing large flash areas can be time consuming and can cause a task to run continuously, thus triggering a TWDT timeout. The following two methods can be used to avoid this:
Increase CONFIG_ESP_TASK_WDT_TIMEOUT_S in menuconfig for a larger watchdog timeout period.
You can also call
esp_task_wdt_init()to increase the watchdog timeout period before erasing a large flash area.
For more information, you can refer to SPI Flash.
The following config options control TWDT configuration at startup. They are all enabled by default:
CONFIG_ESP_TASK_WDT - the TWDT is initialized automatically during startup. If this option is disabled, it is still possible to initialize the Task WDT at runtime by calling
CONFIG_ESP_TASK_WDT_CHECK_IDLE_TASK_CPU0 - Idle task is subscribed to the TWDT during startup. If this option is disabled, it is still possible to subscribe the idle task by calling
XTAL32K Watchdog Timer (XTWDT)
One of the optional clock inputs to the ESP32-S2 is an external 32 KHz crystal or oscillator (XTAL32K) that is used as a clock source (
XTAL32K_CLK) to various subsystems (such as the RTC).
The XTWDT is a dedicated watchdog timer used to ensure that the XTAL32K is functioning correctly. When
XTAL32K_CLK works as the clock source of
RTC_SLOW_CLK and stops oscillating, the XTWDT will detect this and generate an interrupt. It also provides functionality for automatically switching over to the internal, but less accurate oscillator as the RTC_SLOW_CLK source.
Since the switch to the backup clock is done in hardware it can also happen during deep sleep. This means that even if
XTAL32K_CLKstops functioning while the chip in deep sleep, waiting for a timer to expire, it will still be able to wake-up as planned.
XTAL32K_CLK starts functioning normally again, you can call
esp_xt_wdt_restore_clk to switch back to this clock source and re-enable the watchdog timer.
When the external 32KHz crystal or oscillator is selected (CONFIG_RTC_CLK_SRC) the XTWDT can be enabled via the CONFIG_ESP_XT_WDT configuration option.
The timeout is configured by setting the CONFIG_ESP_XT_WDT_TIMEOUT option.
The automatic backup clock functionality is enabled via the ref:CONFIG_ESP_XT_WDT_BACKUP_CLK_ENABLE configuration option.
JTAG & 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-S2 is connected to OpenOCD via JTAG.
A full example using the Task Watchdog is available in esp-idf: system/task_watchdog
esp_err_t esp_task_wdt_init(const esp_task_wdt_config_t *config)
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 current configuration. This funciton will also subscribe the idle tasks if configured to do so. For other tasks, users can subscribe them using esp_task_wdt_add() or esp_task_wdt_add_user().
esp_task_wdt_init() must only be called after the scheduler is started
config – [in] Configuration structure
ESP_OK: Initialization was successful
Other: Failed to initialize TWDT
Deinitialize the Task Watchdog Timer (TWDT)
This function will deinitialize the TWDT, and unsubscribe any idle tasks. Calling this function whilst other tasks are still subscribed to the TWDT, or when the TWDT is already deinitialized, will result in an error code being returned.
ESP_OK: TWDT successfully deinitialized
Other: Failed to deinitialize TWDT
esp_err_t esp_task_wdt_add(TaskHandle_t task_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.
task_handle – Handle of the task. Input NULL to subscribe the current running task to the TWDT
ESP_OK: Successfully subscribed the task to the TWDT
Other: Failed to subscribe task
esp_err_t esp_task_wdt_add_user(const char *user_name, esp_task_wdt_user_handle_t *user_handle_ret)
Subscribe a user to the Task Watchdog Timer (TWDT)
This function subscribes a user to the TWDT. A user of the TWDT is usually a function that needs to run periodically. Each subscribed user must periodically call esp_task_wdt_reset_user() to prevent the TWDT from elapsing its timeout period. Failure to do so will result in a TWDT timeout.
user_name – [in] String to identify the user
user_handle_ret – [out] Handle of the user
ESP_OK: Successfully subscribed the user to the TWDT
Other: Failed to subscribe user
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.
ESP_OK: Successfully reset the TWDT on behalf of the currently running task
Other: Failed to reset
esp_err_t esp_task_wdt_reset_user(esp_task_wdt_user_handle_t user_handle)
Reset the Task Watchdog Timer (TWDT) on behalf of a user.
This function will reset the TWDT on behalf of a user. Each subscribed user must periodically call this function to prevent the TWDT from timing out. If one or more subscribed users fail to reset the TWDT on their own behalf, a TWDT timeout will occur.
user_handle – [in] User handle
ESP_OK: Successfully reset the TWDT on behalf of the user
Other: Failed to reset
esp_err_t esp_task_wdt_delete(TaskHandle_t task_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().
task_handle – [in] Handle of the task. Input NULL to unsubscribe the current running task.
ESP_OK: Successfully unsubscribed the task from the TWDT
Other: Failed to unsubscribe task
esp_err_t esp_task_wdt_delete_user(esp_task_wdt_user_handle_t user_handle)
Unsubscribes a user from the Task Watchdog Timer (TWDT)
This function will unsubscribe a user from the TWDT. After being unsubscribed, the user should no longer call esp_task_wdt_reset_user().
user_handle – [in] User handle
ESP_OK: Successfully unsubscribed the user from the TWDT
Other: Failed to unsubscribe user
esp_err_t esp_task_wdt_status(TaskHandle_t task_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.
task_handle – [in] Handle of the task. Input NULL to query the current running task.
ESP_OK: The task is currently subscribed to the TWDT
ESP_ERR_NOT_FOUND: The task is not subscribed
ESP_ERR_INVALID_STATE: TWDT was never initialized
Task Watchdog Timer (TWDT) configuration structure.
typedef struct esp_task_wdt_user_handle_s *esp_task_wdt_user_handle_t
Task Watchdog Timer (TWDT) user handle.