Embedded Debugging

Embedded debugging

On-device bugs hide behind missing consoles, optimized builds, and hardware timing. The toolkit: a debug probe for the lab, lightweight trace for timing truth, and crash storage that survives reset for the field.

Method

1. Wire the probe workflow first. SWD/JTAG with an openocd/probe-rs/vendor-IDE stack: breakpoints, watchpoints (break on data change: the tool for "who corrupts this variable"), memory/peripheral-register inspection, and flash-and-halt cycles in seconds. Keep a debug build profile with -Og and asserts; but reproduce release-only bugs in release with map files at hand (see embedded-memory-constraints' linker literacy).
2. Log without a console, cheaply. SWO/ITM or RTT-class channels give printf-grade streams over the debug probe with microsecond overhead; deferred formatting (log IDs + arguments, formatted host-side: defmt-style) cuts cost further (see log-levels' structure, embedded edition). For timing questions, a GPIO toggle read by logic analyzer is the most honest profiler on an MCU (ISR duration, task timing: see interrupt-safe-code, rtos-task-design's tracing).
3. Make hard faults report their origin. A fault handler that captures the stacked PC/LR/registers and fault-status registers (CFSR on Cortex-M decodes to "bus fault at address X"/"usage fault: divide by zero"), stores them in noinit RAM, and logs after reboot: turns "it resets sometimes" into file:line via addr2line on the captured PC. This handler is twenty lines and pays for itself the first week (see stack-trace-reading's ethic, bare-metal).
4. Treat reset causes as first-class telemetry. Read and log the reset-cause register every boot (watchdog, brown-out, software, pin): fleets summarize it (see mobile-observability's crash-rate analog); a rising watchdog-reset rate is a hang epidemic announcing itself. Watchdog design: kicked from a health-checking task (never an ISR or a timer blindly: that guards nothing; see rtos-task-design's watchdog task).
5. Bisect hardware vs firmware deliberately. Same firmware on a second board (board fault?), scope the actual signals at the pins (is the sensor really answering? see sensor-data-handling's garbage-at-ADC rule), check power rails during the failure (brown-outs mimic software bugs: see low-power-design), and swap suspected components. Half of "firmware bugs" on new hardware are electrical; the scope settles arguments the debugger cannot.
6. Build the field-diagnosis path before shipping. Crash records + reset causes + a diagnostic mode uploadable over the normal channel (see iot-messaging, firmware-ota-updates' confirm telemetry); a serial/debug header that survives into production units (fused off only if security demands); and a device-side log ring in flash for the support case that arrives with "it stopped working Tuesday" (see production-debugging's mindset at device scale).

Boundaries

• Debugger-halted timing lies: halting the core mid-protocol breaks every live bus transaction; timing bugs need trace (ITM/ETM, logic analyzer), not breakpoints.
• Optimized-out variables and reordered lines in release builds are the compiler doing its job; debug the release binary with the map and disassembly when the bug lives only there (see zero-cost-abstractions' inspection habit).
• Secure/locked devices (readout protection, sealed debug) trade debuggability for security deliberately; design the field-diagnosis path before enabling the locks, because after is too late.