Connection timed out

The device did not respond in time.

CONNECT_TIMEOUT

Symptoms

  • The probe connects but the target MCU never responds.
  • Connection attempts hang for a few seconds, then fail with a timeout.
  • The same probe works fine on a different board.

Likely causes

  • The target board has no power, or its debug header is wired incorrectly (SWDIO/SWCLK swapped, missing GND).
  • The MCU is in a deep low-power mode that gates the debug interface.
  • Firmware reconfigured the SWD pins as GPIO right after boot, so the debug port disappears milliseconds after reset.
  • Very long or noisy wiring between probe and target.

How to fix it

  1. Verify target power and ground

    Measure the target’s VDD and confirm probe GND is connected to target GND. An external probe also needs the VTref pin wired so it senses target voltage.

  2. Check SWD wiring

    SWDIO and SWCLK must not be swapped; keep wires short (under ~15 cm for jumper wires). On Nucleo boards, verify the ST-Link jumpers (CN2) are fitted when using the on-board probe.

  3. Try connect-under-reset

    If firmware disables SWD pins or enters low-power mode instantly, hold the MCU in reset while attaching: keep NRST asserted (press and hold the reset button), start the connect, then release. This catches the debug port before firmware runs.

  4. Recover the firmware side

    Once connected under reset, flash a build that keeps SWD enabled (avoid remapping PA13/PA14 on STM32) so future connects work normally.

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