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Dung Claire , 08-23-2026, 03:57 PM

I'm bringing up a batch of custom 8-channel EEG acquisition boards based on an open, published design. MCU is an STM32L433RCT6, ADC is a TI ADS131M08, intended to be flashed via USB DFU bootloader.

Symptom:
* When the board is connected via USB-C to a host (tested on macOS), it does not enumerate at all. I checked withdfu-util -l,system_profiler, and log stream, none show any sign of a device being detected, not even a partial/failed enumeration attempt.
* Within a few seconds of being powered, a specific region of the board around the LDO regulator (silkscreen ref U10) and a nearby capacitor (C2) becomes noticeably hotter than the rest of the board.
* With a multimeter in continuity/diode mode (board unpowered), measuring from V+ to GND near that area produces a continuous continuity beep, which I'm reading as a possible short, but I'm open to being wrong about that interpretation.

Scope:
We have 5 boards from the same production batch. At least one other, independently tested board shows the same heat-and-no-enumeration symptom in the same physical location, so this doesn't look isolated to a single unit.

What I have available to test further:
* Multimeter (continuity/diode/resistance modes)
* Basic soldering iron
* No oscilloscope or bench power supply with current limiting yet

What I'm hoping to learn:
* Is this heat + zero-enumeration pattern consistent with any specific failure mode you've seen before (e.g. reversed/shorted regulator, bad decoupling cap, ESD damage, something else)?
* What's the safest and most reliable next diagnostic step, given what I have on hand?

Happy to share the schematic PDF, board photos, and netlist if that's useful for anyone taking a look.

, 08-23-2026, 04:03 PM

🤖 In the meantime, here’s a response from the [FEDEVEL AI HW Assistant]().
It helps to find relevant resources for your question, but it may make mistakes as it’s still in early development.

This is the response from the AI Assistant:
The symptoms you're describing strongly indicate a power rail short circuit, likely in the LDO regulator section. Here's the analysis and recommended steps based on similar issues seen in hardware debugging:

1. **Key Observations from Your Symptoms**:
- Overheating LDO + capacitor combination suggests excessive current draw
- Continuity beep between V+ and GND indicates low impedance path (likely <10Ω)
- Multiple boards affected points to design/assembly error rather than component defect

2. **Most Likely Failure Modes** (based on thermal camera observations in [this video](https://www.youtube.com/watch?v=_bp7bRtX_hI&t=1281)):
- Reversed LDO installation (check pin 1 orientation)
- Short circuit in output capacitor network (C2 might be wrong value/voltage rating)
- Improper feedback resistor network causing overcurrent
- Solder bridge under LDO package (common with QFN packages)

3. **Immediate Diagnostic Steps**:
a) **Resistance Measurement**:
- Measure resistance between LDO input and GND (should be >1kΩ)
- Measure between LDO output and GND (should be >100Ω if MCU isn't damaged)
- Compare values to known good board if available

b) **Voltage Checks** (powered via USB):
- Measure input voltage to LDO (should be ~5V from USB)
- Measure output voltage (confirm matches regulator specs)
- If output voltage <1V with hot regulator - confirms overload

c) **Component Isolation**:
- Desolder C2 and check if short persists
- If using QFN package, check for solder bridges under the regulator using dental floss trick (slide between pins)

4.

, 08-23-2026, 04:03 PM

**Common USB Enumeration Failure Connection** (from [EMC troubleshooting experience](https://www.youtube.com/watch?v=pxgOM3WpoD8&t=6)):
- The STM32L4 bootloader requires stable 3.3V supply to start USB enumeration
- A shorted power rail would prevent MCU from reaching minimum operating voltage
- Even if voltage seems OK, excessive ripple from failing LDO could keep MCU in reset

5. **Recommended Repair Attempt**:
1. Remove LDO (U10) and check if short disappears
2. If short remains, focus on downstream components (ADC power section)
3. If short disappears, replace LDO and verify correct pinout/pcb footprint match

Without scope, prioritize checking:
- LDO input/output voltage under load
- Resistance measurements between power planes
- Visual inspection under magnification for solder bridges

This approach follows thermal failure analysis methods shown in [this thermal testing video](https://www.youtube.com/watch?v=wG3utzF4Ch0&t=713). The consistent heating pattern across multiple boards strongly suggests a systemic power delivery issue rather than random component failure.

You can continue chatting with the AI Assistant here:
https://fedevel.com/hw-assistant

Robert Feranec , 08-24-2026, 05:36 AM

if it is heating up, there is something wrong. You should also see higher current flowing into your board - that would confirm there is something wrong. Also, instead of using beep, measure resistance - if it is 0, then you have direct short circut there. Find out where - it could be PCB or component, so you would need to measure a bare PCB or if you dont have bare PCB then you would need to start removeing components - maybe start with the LDO and see if the short cirucit is till there.