Debug unit would not power up

The chip refused to power its debug unit. Usually the board is browning out, or the probe is reading the wrong target voltage.

CORTEX_M_DEBUG_POWER_FAILED

Symptoms

  • Connecting fails with a debug power error, before any target information appears.
  • The failure comes and goes with what else the board is driving — a motor, a radio, a display backlight.
  • The board is powered only from the probe’s 3V3 pin.

Likely causes

  • Brown-out: the request to power the debug domain arrives while the rail is sagging, and the chip cannot honour it.
  • VTref is not connected, so the probe drives the SWD lines at the wrong level or refuses to drive them at all.
  • The board draws more than the probe can supply. An ST-Link’s 3V3 pin is not a bench supply.
  • On a few parts, the debug domain stays off until the chip leaves a low-power or secure state.

How to fix it

  1. Power the board from its own supply

    Use the board’s normal power input and leave the probe to drive only the signal lines. Keep the ground connection.

  2. Connect VTref

    Wire the probe’s VTref (pin 1 on the 10-pin Cortex Debug header) to the board’s 3V3 so the probe matches the target’s logic level.

  3. Watch the rail while connecting

    Measure VDD during the connect attempt. A dip below the chip’s minimum at that moment explains an error that otherwise looks random.

  4. Reduce the load and retry

    Disconnect what the board drives — motors, radios, displays — and connect again. If it succeeds, the supply, not the debug link, is the problem to solve.

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