Intermittent bugs are the worst kind of bugs in embedded systems.
They don’t appear on demand, they don’t follow a pattern, and they often vanish the moment you attach a debugger.
But there’s good news:
Intermittent issues follow predictable root causes, and there is a structured, reliable workflow to uncover them.
This guide walks through the exact workflow I use in real-world firmware for IoT, industrial and automotive systems.
🧩Why Intermittent Issues Are Hard
Intermittent faults usually happen because of:
Most of these cannot be reproduced easily with a traditional debugger.
So we use a layered strategy.
🔍 Step 1 — Confirm the Symptoms
Before diving in, verify:
Is the bug truly intermittent?
If it occurs on every 20th cycle, it’s deterministic but low-frequency, not intermittent.
Can the issue be reproduced under stress?
Thermal stress
RF stress
Power cycling
Heavy I/O
Multi-task load
If stress triggers it, you’re already 50% done.
🧵 Step 2 — Enable Instrumentation Without a Debugger
Intermittent issues hate breakpoints.
As soon as you slow the system, the bug disappears.
Use:
✔ GPIO pin toggles (poor man’s logic trace)
Toggle pins before/after a suspected function.
✔ UART breadcrumbs
Print single-byte markers, not full logs.
✔ Event counters
Counts before crashes help identify patterns.
✔ Internal watchdog logs
Capture:
Reset reason
Fault registers
Last state before reboot
These will tell you if you’re dealing with timing drift, starvation, or corruption.
A $10 Saleae-clone logic analyzer can solve bugs that IDE debuggers can’t.
Use it to:
Almost every real intermittent bug has a timing clue.
🧠 Step 4 — Analyze RTOS Behavior
If your system uses FreeRTOS, Zephyr, or ThreadX, analyze:
80% of intermittent bugs in RTOS systems come from task interactions.
🧪 Step 5 — Create a Reproducible “Bug Amplifier”
Your job is to turn a bug that happens once a day
→ into a bug that happens every minute.
Common amplifiers:
✔ Increase task frequency
Hidden race conditions burst out.
✔ Add induced jitter
Random delays surface timing flaws.
✔ Simulate power dips
Brownouts → corrupted memory → intermittent resets.
✔ Reduce memory heap
If the issue is memory-related, it appears faster.
This is the secret to beating intermittent bugs.
🧱 Step 6 — Watch for Classic Failures
These patterns appear again and again:
1. ISR → Task race condition
Signal posted before task is ready.
2. Buffer overflow of 1–2 bytes
The device behaves “weirdly” but doesn’t crash.
3. Missing timeout
A blocking call eventually starves others.
4. Hardware noise
I2C ACKs dropped
→ retries
→ state machine stuck.
5. Stack overflow in low-memory MCUs
Causes random corruption far away from the overflow.
🛠 Step 7 — Capture Failing State Before It Disappears
Before the system resets, store:
Use battery-backed RAM if possible.
This forms the “black box recorder” of firmware.
🎯 Step 8 — Root Cause Analysis (RCA)
Summarize:
Trigger: What causes the issue?
Fault: What fails internally?
Impact: What user-visible behavior occurs?
Fix: Software, hardware, timing, or spec correction?
If you cannot clearly state all 4, you’re not done.
⭐ Step 9 — Fix, Harden, Prevent
Once solved, prevent future recurrences with:
🧘 Final Thoughts
Intermittent bugs look scary, but with a structured workflow, they almost always point to:
Timing
Memory
Hardware noise
Power
Concurrency
Use the systems above and you’ll solve them faster than 90% of engineers.