JLink GDB Server

SEGGER's JLink GDB Server provides excellent performance and reliability for JLink debug probes.

Prerequisites

Download and install the JLink Software and Documentation Pack from SEGGER. This installs JLinkGDBServerCLExe (Linux/macOS) or JLinkGDBServerCL.exe (Windows).

Verify installation:

JLinkGDBServerCLExe --version   # Linux/macOS
JLinkGDBServerCL.exe --version  # Windows

launch.json Configuration

{
  "type": "mcu-debug",
  "request": "launch",
  "name": "Debug (JLink)",
  "servertype": "jlink",
  "executable": "${workspaceFolder}/build/firmware.elf",
  "device": "STM32F407VG",
  "interface": "swd",
  "serverPath": "/opt/SEGGER/JLink/JLinkGDBServerCLExe"
}

Key Properties

Property	Description
device	The JLink device name (e.g. STM32F407VG, nRF52840_xxAA). Exact name must match SEGGER's device database.
interface	Debug interface: "swd" (default) or "jtag"
serverPath	Path to JLinkGDBServerCLExe if not on PATH
serverArgs	Extra arguments to the JLink GDB Server

Finding the Device Name

The device name must exactly match an entry in SEGGER's device database. To find yours:

1. Open J-Flash or J-Link Commander
2. Connect to your target and check the auto-detected device name
3. Or browse the SEGGER device database

Common examples:

MCU	Device Name
STM32F407VG	STM32F407VG
nRF52840	nRF52840_xxAA
LPC1768	LPC1768
ATSAM4S	ATSAM4SD32C

JLink Speed

Default SWD clock speed may be too high for some targets. Reduce it:

"serverArgs": ["-speed", "1000"]

Speed is in kHz. Start with 1000 kHz and increase to find the maximum stable speed.

License Notes

The JLink GDB Server is free to use with genuine SEGGER JLink probes. Using it with third-party probes that identify as JLink (clones) may require a license. See SEGGER's licensing terms for details.

Common Issues

"Cannot connect to target"

• Reduce SWD speed: "serverArgs": ["-speed", "1000"]
• Check that the correct device name is used
• Ensure JLink firmware is up to date (J-Link Commander: connect then check for firmware update prompt)
• Verify the device is powered and the debug connector is correct

"No JLink found"

• JLink is not connected or not recognized
• On Linux: ensure JLink udev rules are installed (/etc/udev/rules.d/99-jlink.rules). SEGGER's installer does this automatically.