104 lines
4.7 KiB
ReStructuredText
104 lines
4.7 KiB
ReStructuredText
Watchdogs
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=========
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Overview
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--------
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The ESP-IDF has support for two types of watchdogs: The Interrupt Watchdog Timer
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and the Task Watchdog Timer (TWDT). The Interrupt Watchdog Timer and the TWDT
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can both be enabled using ``make menuconfig``, however the TWDT can also be
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enabled during runtime. The Interrupt Watchdog is responsible for detecting
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instances where FreeRTOS task switching is blocked for a prolonged period of
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time. The TWDT is responsible for detecting instances of tasks running without
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yielding for a prolonged period.
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Interrupt watchdog
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^^^^^^^^^^^^^^^^^^
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The interrupt watchdog makes sure the FreeRTOS task switching interrupt isn't blocked for a long time. This
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is bad because no other tasks, including potentially important ones like the WiFi task and the idle task,
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can't get any CPU runtime. A blocked task switching interrupt can happen because a program runs into an
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infinite loop with interrupts disabled or hangs in an interrupt.
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The default action of the interrupt watchdog is to invoke the panic handler. causing a register dump and an opportunity
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for the programmer to find out, using either OpenOCD or gdbstub, what bit of code is stuck with interrupts
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disabled. Depending on the configuration of the panic handler, it can also blindly reset the CPU, which may be
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preferred in a production environment.
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The interrupt watchdog is built around the hardware watchdog in timer group 1. If this watchdog for some reason
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cannot execute the NMI handler that invokes the panic handler (e.g. because IRAM is overwritten by garbage),
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it will hard-reset the SOC.
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Task Watchdog Timer
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^^^^^^^^^^^^^^^^^^^
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The Task Watchdog Timer (TWDT) is responsible for detecting instances of tasks
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running for a prolonged period of time without yielding. This is a symptom of
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CPU starvation and is usually caused by a higher priority task looping without
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yielding to a lower-priority task thus starving the lower priority task from
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CPU time. This can be an indicator of poorly written code that spinloops on a
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peripheral, or a task that is stuck in an infinite loop.
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By default the TWDT will watch the Idle Tasks of each CPU, however any task can
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elect to be watched by the TWDT. Each watched task must 'reset' the TWDT
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periodically to indicate that they have been allocated CPU time. If a task does
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not reset within the TWDT timeout period, a warning will be printed with
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information about which tasks failed to reset the TWDT in time and which
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tasks are currently running on the ESP32 CPUs and.
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The TWDT is built around the Hardware Watchdog Timer in Timer Group 0. The TWDT
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can be initialized by calling :cpp:func:`esp_task_wdt_init` which will configure
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the hardware timer. A task can then subscribe to the TWDT using
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:cpp:func:`esp_task_wdt_add` in order to be watched. Each subscribed task must
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periodically call :cpp:func:`esp_task_wdt_reset` to reset the TWDT. Failure by
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any subscribed tasks to periodically call :cpp:func:`esp_task_wdt_reset`
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indicates that one or more tasks have been starved of CPU time or are stuck in a
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loop somewhere.
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A watched task can be unsubscribed from the TWDT using
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:cpp:func:`esp_task_wdt_delete()`. A task that has been unsubscribed should no
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longer call :cpp:func:`esp_task_wdt_reset`. Once all tasks have unsubscribed
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form the TWDT, the TWDT can be deinitialized by calling
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:cpp:func:`esp_task_wdt_deinit()`.
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By default :envvar:`CONFIG_TASK_WDT` in ``make menuconfig`` will be enabled causing
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the TWDT to be initialized automatically during startup. Likewise
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:envvar:`CONFIG_TASK_WDT_CHECK_IDLE_TASK_CPU0` and
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:envvar:`CONFIG_TASK_WDT_CHECK_IDLE_TASK_CPU1` are also enabled by default causing
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the two Idle Tasks to be subscribed to the TWDT during startup.
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JTAG and watchdogs
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^^^^^^^^^^^^^^^^^^
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While debugging using OpenOCD, the CPUs will be halted every time a breakpoint
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is reached. However if the watchdog timers continue to run when a breakpoint is
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encountered, they will eventually trigger a reset making it very difficult to
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debug code. Therefore OpenOCD will disable the hardware timers of both the
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interrupt and task watchdogs at every breakpoint. Moreover, OpenOCD will not
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reenable them upon leaving the breakpoint. This means that interrupt watchdog
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and task watchdog functionality will essentially be disabled. No warnings or
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panics from either watchdogs will be generated when the ESP32 is connected to
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OpenOCD via JTAG.
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Interrupt Watchdog API Reference
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--------------------------------
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Header File
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^^^^^^^^^^^
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* :component_file:`esp32/include/esp_int_wdt.h`
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Functions
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---------
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.. doxygenfunction:: esp_int_wdt_init
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Task Watchdog API Reference
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----------------------------
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A full example using the Task Watchdog is available in esp-idf: :example:`system/task_watchdog`
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.. include:: /_build/inc/esp_task_wdt.inc
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