Target held in reset

The target MCU is being held in reset and cannot communicate.

CORTEX_M_TARGET_IN_RESET

Symptoms

  • The debug port reports the target is held in reset.
  • NRST measures near 0 V continuously.
  • The firmware never appears to start (no LED, no UART output).

Likely causes

  • An external reset supervisor or watchdog IC is asserting NRST.
  • Brown-out: the supply sits below the reset threshold, so the supervisor never releases.
  • A stuck or shorted reset button, or another device (second debugger, programmer) driving NRST.
  • On Nucleo boards: the on-board ST-Link’s reset line jumpered to the target while another tool drives it.

How to fix it

  1. Measure NRST

    With a multimeter, check NRST at the debug header. Solid ~VDD means reset is fine (look elsewhere); near 0 V confirms something is asserting it.

  2. Find the driver of the line

    Disconnect candidates one at a time: unplug the second debugger, lift the supervisor/watchdog output, check the reset button for shorts.

  3. Check supply voltage

    Measure VDD against the supervisor’s threshold — a marginal 3.0 V rail with a 3.08 V-threshold supervisor holds the system in permanent reset.

  4. Reconnect

    Once NRST is released, connect again from MCUHex; no SDK restart is needed.

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