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Choose the trace path by the question you need to answer. RTT and ITM carry firmware-generated events. ETM records executed instruction addresses. Coverage derives function and source-line execution from ETM statistics.

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Compare the capture types

RTT and ITM sessions contain decoded events. ETM sessions contain instructions. Event query and summary tools do not interpret an ETM session as an event log.

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Record RTT events

Select a J-Link target in the serial view before you start. Without an explicit transport, Embedder chooses:
  • A discovered RTT up channel marked ctf or binary
  • A discovered RTT down channel for control
  • Data channel 2 and control channel 1 as fallbacks
The target firmware must emit Embedder compact trace packets on the data channel. At start, Embedder sends an ETRC control request on the down channel and waits for the firmware’s trace descriptor. Plain RTT log text is not a trace event stream. Use the serial transport workflow for ordinary logs. Stop the session when you have reproduced the behavior:

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Record ITM events

ITM event recording uses one stimulus port as a binary packet stream:
Configure OpenOCD and the target before you start. The firmware must emit compact trace packets, not line-oriented ITM_SendChar text, on the selected port. Use the ITM serial monitor when you want human-readable stimulus output. Use trace recording only when the firmware implements the packet format and descriptor expected by the decoder.

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Capture an ETM instruction window

instruction_trace requires:
  • A SEGGER J-Trace, not a plain J-Link
  • SEGGER J-Link host software
  • A fine-pitch CoreSight-20 trace cable
  • A target that exposes and drives its ETM trace pins
  • The matching ELF and SEGGER device name
The tool stops active GDB and RTT sessions before it takes the probe. It resets and runs the target, then reads the probe’s full 65,536-instruction window. The print limit changes only the returned table.
The window is usually the tail of a longer run. ETM gives instruction order, not elapsed time. A CPU frequency can produce an estimate, but it does not add real timestamps to the trace.
The stored session includes symbolized instructions and reconstructed call frames. Open frames indicate that the function entry or exit fell outside the retained window.

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Stream ETM from a known start

Use trace_start with transport.kind="etm" when the capture must begin at a known point and continue until you stop it. Pass firmware.elfPath at start; addresses are symbolized as they arrive and cannot be resolved from an ELF attached later.
Streaming runs the target in short slices and halts it between slices to drain the probe. This changes wall-clock timing and can break interrupts, timeouts, and peripheral handshakes. Do not use a sliced stream to draw timing conclusions. Check the session’s continuity summary. Gaps or lost instructions mean the probe buffer overran; call-frame reconstruction across a gap is incomplete.

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Run source coverage

coverage_run uses J-Trace instruction statistics for the whole bounded run:
The report contains:
  • Covered and total functions
  • Covered and total source lines
  • Covered and total instructions
  • Function entry run counts
  • Per-line rows when you request funcs+lines
Coverage counts describe the whole run even when the readable instruction window contains only its tail. Reports persist and appear in the Coverage tab. List or load an existing report instead of rerunning the target when you only need a different view.

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Inspect and export sessions

Trace sessions persist under .embedder/traces/<session-id>/. Use:
  • trace_status to find the active or latest session
  • trace_list_sessions to compare RTT, ITM, and ETM sessions
  • Event query and summary tools for RTT or ITM sessions
  • The Trace tab or instruction_trace output for ETM instructions
  • trace_export with raw to copy a complete session
NDJSON export contains event data and applies only to RTT and ITM sessions. ETM instructions live in instructions.json; export the raw session directory so you do not drop them.

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Interpret capture quality

Every trace result reports provenance:
  • Firmware match state and ELF hash when available
  • Resolved and unresolved symbols
  • Dropped records, resynchronizations, and transport errors
  • Trace format and confidence
Treat a firmware mismatch or dropped records as low-confidence evidence. A resynchronization or unresolved symbol count makes the result partial.

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Fix trace problems

Confirm the data and control channel indexes and the firmware’s compact trace integration. Plain RTT logging does not answer the trace-start control request.
Verify OpenOCD, SWO clocks, the stimulus port, and firmware packet output. Test human-readable ITM separately in the serial view.
Confirm that the probe is J-Trace, the target drives TRACECLK, and the fine-pitch CoreSight-20 cable carries the trace signals.
Stop active hardware scripts and trace sessions. The coverage tool already stops managed GDB and RTT connections during its handoff.
Check continuity, window truncation, and the matching ELF. Do not reconstruct across reported gaps.

J-Trace setup

Check the probe, cable, target, and scripted SDK path.

Serial transports

Configure RTT and ITM before recording events.
Last modified on August 24, 2026