UART wiring for USB TTL on uconsole

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ChatGPT has some thoughts about it all

I’ll (1) verify the uConsole 40-pin header pinout for GPIO14/15 on Radxa CM5, (2) map which UART they correspond to, and (3) list the common “no logs” causes (wrong console, baud, voltage, flow control, boot config) with quick checks.

Expert: Embedded Linux / SBC hardware debug engineer
Objective: Confirm correct UART-to-USB-TTL wiring for uConsole + Radxa CM5, and pinpoint the most likely reasons you’re seeing no serial output.
Assumptions: You’re trying to read early boot logs (U-Boot + kernel) over a 3.3V USB-TTL adapter; you’re using the uConsole baseboard with a Radxa CM5 installed (not the Radxa CM5 IO board).

Your TX/RX/GND crossover is correct if the header you’re using actually exposes the CM5’s debug UART:

  • Pin 8 (UART TX) → USB-TTL RX

  • Pin 10 (UART RX) → USB-TTL TX

  • Pin 6 (GND) → USB-TTL GND

That mapping (Pin 8 TX / Pin 10 RX / Pin 6 GND) is Radxa’s standard approach for Rockchip-based boards, and Radxa CM5 IO’s 40-pin header explicitly shows Pin 8 = UART2_TX and Pin 10 = UART2_RX. (Radxa Docs)

The two big “no logs at all” culprits

1) Wrong baud rate / terminal settings

For Radxa Rockchip platforms, the default debug UART is typically:

If you’re using 115200 you’ll often see nothing (or garbage), depending on the bootloader/kernel config.

Also: some USB-TTL adapters don’t reliably do 1.5M. Radxa explicitly warns certain CP210x / PL2303 variants can be limited. (Radxa Docs)

Quick checks:

  • Disable RTS/CTS in your serial program (set flow control to None).

  • Try a known-good adapter (CH340 is commonly fine; some FT232RL cables have quirks per Radxa). (Radxa Docs)

2) On uConsole: those pins may not actually be the console UART

This is the common gotcha with uConsole specifically.

ClockworkPi’s own forum discussion about uConsole serial debugging notes:

  • The stock image may specify console=ttyS2,1500000 (so: UART2 at 1.5M) (clockworkpi)

  • People have found the relevant UART signals on the expansion port (GPIO32/GPIO33) on at least one uConsole configuration (clockworkpi)

  • And a ClockworkPi maintainer flat-out says: there isn’t space for an external breakout; you either wire it yourself or use a DT expansion board—“The DT has the UART.” (clockworkpi)

So even though “Pin 8/10 UART” is right on Radxa IO boards, the uConsole baseboard may not route the CM5’s debug UART to the place you’re probing (or it may be shared/repurposed).

What I would do next (fastest path)

  1. Verify your USB-TTL adapter works

    • Do a loopback: short adapter TX↔RX and confirm typed characters echo back.

  2. Use the Radxa serial settings

    • 1500000 8N1, flow control = none (Radxa Docs)

    • Start the terminal before you power-cycle, so you catch early boot.

  3. Confirm what the OS thinks the console is

    • If you can boot at all: cat /proc/cmdline and look for console=... (ClockworkPi uConsole images commonly show ttyS2,1500000). (clockworkpi)

    • If it’s ttyS2, you want the physical pins for that UART, not “whatever resembles Pi GPIO14/15”.

  4. If you need serial on uConsole + CM5

    • Plan on either:

      • using the DT expansion board, or

      • soldering/wiring to the appropriate expansion port UART pins per the uConsole community findings. (clockworkpi)

One more critical wiring note

  • Make sure your USB-TTL is 3.3V logic, and do not connect VCC/5V from the adapter to the board. Radxa calls this out explicitly. (Radxa Docs)

Confidence: 0.78

Suggested searches
Radxa “UART Serial Console” docs site:docs.radxa.com
Radxa CM5 IO 40-pin UART2_TX UART2_RX pin 8 pin 10 site:docs.radxa.com
ClockworkPi uConsole serial debug port ttyS2 1500000 site:forum.clockworkpi.com
uConsole DT expansion board UART site:forum.clockworkpi.com