SWO

caution

SWO support is limited and may not work with all targets. RTT is recommended as a more reliable alternative for most use cases. SWO is currently not supported in CLI mode

Overview

SWO (Single Wire Output) uses the ITM (Instrumentation Trace Macrocell) and TPIU (Trace Port Interface Unit) peripherals built into Cortex-M3, M4, and M7 to output trace data over a dedicated SWO pin on the debug connector.

Customizations

We try to initialize SWO from the outside with GDB commands. But if the adderess don't match your Silicon it may not work. Clocking topoloties also may not be standard. There may be other HW that needs to be inilized besides the standard ARM ones. In the installation directory ~/.vscode/extensions/mcu-debug.mcu-debug.#.#.#/support directory there is a gdbsupport.init file. You can copy and modify this file and added add it to launch.json the preLaunchCommands. It will override existing definitions.

"preLaunchCommands": [
  "source \"mygdbsupport.init\""
]

Doing the SWO setup in firmware is another alternative.

Limitations

Before choosing SWO, consider these limitations:

• Requires SWO pin exposure: the debug connector must expose the SWO pin. Many PCBs omit this pin even when the MCU supports SWO. Many USB debug probes do not expose SWO even when the connector has it.
• Requires correct CPU clock setup: the SWD/SWO divider must be configured with the exact CPU clock frequency at session start. If the firmware changes the clock (e.g. PLL setup in SystemInit), SWO output may become garbled.
• Vendor-specific peripheral map: slight differences in ITM/TPIU memory map across vendors can prevent correct automatic setup.
• Not supported on Cortex-M0/M0+: these cores do not have ITM/TPIU.
• Not compatible with multi-core configurations: SWO is a single global channel.

When to Use RTT Instead

RTT is strongly preferred for new projects:

	RTT	SWO
Extra pins	None	SWO pin
Cortex-M0/M0+	Yes	No
Multi-core	Yes	No
Throughput	High	Low
Clock dependency	None	Yes
Probe support	All	Probe and PCB dependent

launch.json Configuration

If SWO is the only option for your setup:

"swoConfig": {
  "enabled": true,
  "cpuFrequency": 168000000,
  "swoFrequency": 2000000,
  "decoders": [
    {
      "type": "console",
      "label": "ITM",
      "port": 0
    }
  ]
}

Key Properties

Property	Description
cpuFrequency	CPU clock in Hz at the time the debug session starts. Must match the actual running frequency.
swoFrequency	Desired SWO output frequency in Hz. Lower values are more reliable.
decoders	ITM stimulus port decoders — each ITM channel (0–31) can have its own decoder.

Firmware Setup

Enable ITM in firmware to use SWO:

// Enable ITM stimulus port 0 for character output
CoreDebug->DEMCR |= CoreDebug_DEMCR_TRCENA_Msk;
ITM->LAR = 0xC5ACCE55;  // unlock
ITM->TER = 0x1;         // enable port 0

// Write a character
ITM_SendChar('A');

Or use the ITM_SendChar function from CMSIS core_cm4.h.