embedded-debug

Testing & Quality

Firmware crash analysis, stack trace decoder, and register dump interpreter for ESP32/ARM/AVR platforms. Use when debugging device crashes, panics, guru meditation errors, hard faults, or analyzing core dumps.

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How to use this skill

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I want to install this Agent Skill for this project in Codex.

Source SKILL.md: https://github.com/FastLED/FastLED/blob/HEAD/.claude/skills/embedded-debug/SKILL.md

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First, summarize what it does, its dependencies, license status if identifiable, and any risks. Show the exact files you propose to add under .agents/skills/embedded-debug/. Do not write files or run scripts until I approve.

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Copying this prompt does not install or run the skill. Review third-party files before use. Codex skill guide

Analyze firmware crash output, decode stack traces, and diagnose embedded system failures.

$ARGUMENTS

What This Skill Does

Crash Analysis: Parse panic handlers, guru meditation errors, hard faults, watchdog resets
Stack Trace Decoding: Decode raw addresses to function names using ELF binaries
Register Dump Interpretation: Analyze CPU register state at crash time
Root Cause Identification: Correlate crash data with source code to find the bug
Fix Recommendations: Suggest specific code changes to prevent the crash

Supported Platforms

Platform	Crash Types
ESP32 (all variants)	Guru Meditation, LoadProhibited, StoreProhibited, InstrFetchProhibited, watchdog reset, brownout, cache errors
ARM Cortex-M (STM32, Teensy, nRF)	HardFault, MemManage, BusFault, UsageFault, watchdog reset
AVR (Arduino Uno/Mega)	Stack overflow (silent corruption), watchdog reset, undefined opcode
RISC-V (ESP32-C3/C6/H2)	Illegal instruction, load/store fault, breakpoint

How To Use

With crash output

/embedded-debug
Guru Meditation Error: Core  0 panic'ed (LoadProhibited). Exception was unhandled.
Core 0 register dump:
PC      : 0x400d1234  PS      : 0x00060030  A0      : 0x800d5678
...

With a description

/embedded-debug Device reboots every 30 seconds, serial shows "rst:0x3 (SW_RESET)"

With a core dump

/embedded-debug Analyze the core dump from the last crash

What You'll Get

Decoded stack trace with source file and line numbers
Explanation of the crash type and what triggered it
Identification of the faulty code path
Specific fix recommendations with code examples
Prevention strategies (stack size tuning, watchdog config, null checks)

ESP32 Crash Quick Reference

Exception Type → Root Cause

Exception	EXCVADDR hint	Likely Root Cause
`LoadProhibited`	~0x00000000	NULL pointer dereference (uninitialized LED strip?)
`LoadProhibited`	Other	Read from freed or out-of-bounds memory
`StoreProhibited`	Any	Write to read-only or invalid memory address
`InstrFetchProhibited`	Any	Jump to invalid/NULL function pointer or corrupt vtable
`IllegalInstruction`	Any	Stack overflow corrupting code, or corrupt heap
`IntegerDivideByZero`	N/A	Division by zero in LED index math
`Unaligned`	Any	Misaligned memory access (DMA buffer not 4-byte aligned)
`Cache disabled but cache memory range accessed`	Any	ISR accessing flash — add `IRAM_ATTR` to ISR function

Key Register Meanings (ESP32 Xtensa)

Register	Meaning
`PC`	Program Counter — where the crash happened
`EXCVADDR`	Address that caused the fault (NULL pointer, invalid write target)
`A1`	Stack pointer — compare to task stack base to detect overflow
`A0`	Return address — calling function

ISR Violation Pattern

Symptoms: Cache disabled but cache memory range accessed or crash inside FreeRTOS API

Cause: FastLED ISR handler calling blocking or flash-accessing code.

Diagnosis: Look for crash inside xSemaphoreTake, vTaskDelay, printf, or any non-ISR-safe function.

Fix:

Add IRAM_ATTR to all ISR-related functions
Replace blocking calls with ISR-safe variants (xSemaphoreGiveFromISR, xQueueSendFromISR)
Move logging out of ISR to a deferred task

Watchdog Reset Pattern

Symptoms: rst:0x7 (TG0WDT_SYS_RESET) or rst:0x8 (TG1WDT_SYS_RESET)

Cause: Task or interrupt handler not yielding — common with:

FastLED show() blocking on DMA in a task with watchdog enabled
High LED counts causing long encoding loops without yield
Spinning in a loop without vTaskDelay(0) or taskYIELD()

Fix:

Call esp_task_wdt_reset() in long loops, or
Move FastLED show() to a dedicated task exempt from watchdog, or
Reduce LED count to bring encoding time under watchdog timeout

Stack Overflow Pattern

Symptoms: StackOverflow + task name, or IllegalInstruction with A1 near stack limit

Diagnosis: Check task stack size in xTaskCreate or xTaskCreatePinnedToCore. FastLED show() with large LED arrays can use significant stack.

Fix: Increase task stack size (minimum 4096 bytes for tasks calling FastLED show with encoding)

Memory Corruption Pattern

Symptoms: IllegalInstruction at random address, corrupted register values, intermittent crashes

Diagnosis: Enable diagnostics:

// In sdkconfig or menuconfig:
CONFIG_HEAP_POISONING_COMPREHENSIVE=y
CONFIG_HEAP_TASK_TRACKING=y
CONFIG_FREERTOS_WATCHPOINT_END_OF_STACK=y

Common FastLED Cause: Writing past end of CRGB array (index out of bounds in user sketch)

Backtrace Decoding Commands

# ESP32 (Xtensa)
xtensa-esp32-elf-addr2line -pfiaC -e build/firmware.elf 0xADDR1 0xADDR2

# ESP32-S3
xtensa-esp32s3-elf-addr2line -pfiaC -e build/firmware.elf 0xADDR1

# ESP32-C3/C6 (RISC-V)
riscv32-esp-elf-addr2line -pfiaC -e build/firmware.elf 0xADDR1

# Live decode via idf.py
idf.py -p /dev/ttyUSB0 monitor