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Use RTT when firmware includes SEGGER RTT buffers. The local path uses J-Link; the hardware-bridge workflow uses probe-rs. RTT does not require a separate UART wire.
  1. Install the SEGGER J-Link Software and Documentation Pack for the hardware host’s operating system and architecture.
  2. Connect and power the target, and wire the probe for SWD or JTAG as required by the board.
  3. Use a firmware build that initializes RTT and writes to the intended up channel.
  4. In the Monitor’s Serial tab, select the exact J-Link device name, interface, probe speed, and RTT channel. Identify the probe when several are connected.
Use the same target and interface as an existing J-Link debugger session. Changing the selected device or interface may disconnect an active debugger. Probe startup can affect target state; review the attachment precautions before investigating a live fault.

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Use RTT through a bridge

Bridge RTT requires a matching firmware artifact flashed through Embedder to the selected bridge during the current session. It then uses probe-rs attachment to read RTT from that firmware. The bridge path manages probe frequency; do not supply a J-Link frequency override. If the target’s existing state is evidence, do not flash it just to enable monitoring. Preserve captured output and use an already available observation path until a new firmware run is appropriate.

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Read an up channel

Up channels carry output from the target. Select the channel used by the firmware:
Replace the device and channel with those for your target. Confirm that the expected firmware text appears. A listed channel with no output can mean the firmware has not written to it yet. If discovery fails, provide the matching build’s map file containing _SEGGER_RTT, or its control-block address. Use the current build’s address; another firmware revision may place the buffers elsewhere. Down channels carry host input to firmware that reads them. Select a down channel that the firmware implements and send a command with its expected newline convention. Up and down channel numbers refer to separate directions; an up channel used for logging does not establish that an input channel exists. Check the firmware response on the corresponding up channel. A successful send means the input was submitted, not that the firmware accepted the command.

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Reuse captured output

Ask Embedder to read the history of the active RTT channel when the relevant event has already occurred. Keep the current connection and target configuration when preserving a fault. For structured event recordings, the firmware needs compatible trace instrumentation. Ordinary RTT text and arbitrary trace formats are not interchangeable; see Coverage and trace.

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Troubleshoot

  • J-Link software missing: install the host package, then retry readiness checks.
  • Target connection fails: verify target power, exact device name, SWD/JTAG wiring, probe selection, and speed.
  • No channels: confirm RTT initialization and use the current map file or control-block address.
  • Channel is silent: check the selected up channel and whether the firmware reaches its logging code.
  • Input fails: select a down channel and confirm the firmware reads it.
  • Probe is busy: close competing probe applications or finish the active debugger/trace task before reconnecting.
See Serial transports for other logging paths and shared history workflows.
Last modified on September 12, 2026