i.MX6ULL Porting Log 02: Project Layout, a Serial Port Trap, and the Current Board Baseline

The Goal

Build a clean project structure and capture the current board boot baseline through the serial console.

The Problem

At first, this level looked simple:

• create folders
• initialize Git
• connect the serial cable
• save the boot log

But the real system was not that simple.

This board is not in confirmed factory-default state. Its serial output already contains traces from a previous project. So I could not call this log a factory baseline. I had to treat it as a current board baseline.

I also hit a second problem: my CH340 serial adapter was detected by Linux, but the serial port disappeared immediately.

The Fix

Step 1: Build the project layout

I created a clean working structure:

mkdir -p ~/imx6ull-porting/{src,build,out,logs,docs}
cd ~/imx6ull-porting
git init

This keeps source code, build output, final output, logs, and documents separated.

Step 2: Record the board state first

Before trusting the serial log, I wrote down the truth:

• the board is not in confirmed factory-default state
• the current boot output contains traces from an older project
• this log must be treated as a current board baseline

This was important because future debugging must compare against the real current state, not an imagined clean state.

Step 3: Find the serial trap

I checked lsusb and confirmed the CH340 adapter was detected.

But the kernel log showed the real problem:

ch341-uart converter now attached to ttyUSB0
usbfs: interface 0 claimed by ch341 while 'brltty' sets config #1
ch341-uart ttyUSB0: ch341-uart converter now disconnected from ttyUSB0

So the problem was not the cable. It was brltty, a background accessibility service in Ubuntu. It hijacked the CH340 device and disconnected it.

Step 4: Remove the conflicting service

I fixed it by removing brltty:

sudo apt-get remove --purge -y brltty

After that, /dev/ttyUSB0 became stable.

Step 5: Open the serial console and capture the baseline

I used picocom and rebooted the board.

The captured log showed important real facts:

• U-Boot 2016.03
• Board: I.MX6U ALPHA|MINI
• DRAM: 512 MiB
• Net: FEC1

It also showed that the current board still had an old network boot path:

FEC1 Waiting for PHY auto negotiation to complete......... TIMEOUT !
TFTP from server 169.254.82.125
Filename 'zImage'
ARP Retry count exceeded; starting again
Filename 'imx6ull-alientek-emmc.dtb'
Bad Linux ARM zImage magic!

That means the board is currently trying to boot by TFTP, but the path is not working correctly.

The Test

This level passes if:

• the project folders exist
• board_state.md exists
• current_board_boot_baseline.log exists
• the serial console works
• the current board state is clearly documented

The Debug Log

• Issue 1: The CH340 device was detected, but /dev/ttyUSB0 was unstable
• Fix: Found that brltty hijacked the device, then removed it
• Issue 2: Normal user access gave Permission denied
• Fix: Used sudo picocom first, then prepared to fix group permissions later
• Issue 3: The board was not in clean factory state
• Fix: Recorded that truth first, and saved the log as a current board baseline instead of a factory baseline

What I Learned

This level was not about code. It was about discipline.

I learned two important things:

1. Always record the real board state before trusting any boot log
2. When serial devices behave strangely on Linux, check dmesg first

In low-level work, the real system state is always more important than assumptions.