GNU ARM Eclipse

A family of Eclipse CDT extensions and tools for GNU ARM development

Using J-Link with the STM32 boards

This page explains how to build a cable to connect the J-Link to STM32 DISCOVERY and NUCLEO boards.


As most vendors do, ST also designed the cheap STM32 DISCOVERY and NUCLEO boards to include an on-board programmer. The protocol used by this programmer is ST-LINK/V2, an ST protocol. Unfortunately ST does not provide a portable GDB server to support the ST-LINK/V2 protocol, and using these STM32 boards out of the box on GNU/Linux or macOS is not possible.

For a few limited tests, the OpenOCD plug-in can be used, but the reliability of OpenOCD is not appropriate for professional use.

Preferably the J-Link debug plug-in would be used, but, for costs reasons, ST did not provide the standard JTAG connector, so it is currently not possible to connect to these boards with an external J-Link probe.

In a very inspired move to address this issue, SEGGER provided an upgrade path, and most of the ST bemo boards can now be converted to J-Link; please follow the SEGGER Converting ST-LINK on-board into a J-Link page.

After the firmware upgrade, you’ll be able to connect the J-Link GDB server via the existing board USB port. However, please note that not all J-Link features are available via this solution, which is somehow a J-Link Lite version.

For a fully featured J-Link solution, a little harware is required.

For those equipped with a soldering iron, it is relatively easy to adapt the existing 6-pin connector to J-Link.

The solution is a custom ribbon cable, to connect the 6-pin to the 20-pin J-Link, and a short strap to bring Vcc to pin 1 of the 6-pin connector, so that J-Link can detect the board is up and running.

The bill of materials includes:

  • a 6 wire ribbon cable, about 15 cm (6”) long
  • a 6-pin 2.54mm (0.1”) female, single raw, in-line connector, to match the STM board header
  • a 20-pin 2.54mm (0.1”) female, dual raw (2x10), in-line connector, to match the J-Link male connector

Solder the ribbon cable to the 6-pin connector, and be sure you mark the first pin with a different colour, to later help you plug the connector in the proper position.

The ST 6-pin SWD connector

The signals on this 6-pin are:

  • 1 - Vcc → 1 (brown)
  • 2 - JTCK/SWCLK → 9 (red)
  • 3 - GND → any of 4,6,8,10,12,14,16,18,20 (orange)
  • 4 - JTMS/SWDIO → 7 (yellow)
  • 5 - NRST → 15 (green)
  • 6 - SWO → 13 (blue)

The signals on the 20-pin are split into two groups, the even number pins are on one raw, and, except pin 2, all are connected to GND, and the odd number pins are:

  • 1 - Viref → 1 (brown)
  • 3 - nRST
  • 5 - TDI
  • 7 - TMS/SWDIO → 4 (yellow)
  • 9 - TCK/SWCLK → 2 (red)
  • 11 - RTCK
  • 13 - TDO/SWO → 6 (blue)
  • 15 - RESET → 5 (green)
  • 17 - DBGQ
  • 19 - 5V out
  • 2 - NC
  • 4 - GND
  • 6 - GND → 3 (orange)
  • 8,10,12,12,16,18,20 - GND

The correspondence between the two connectors is represented on each connector by the second number.

The target detect strap

After building the ribbon cable, the ST board needs a strap to connect the pin 1 of the 6-pin connector to Vcc, to signal J-Link that the target board is present and powered.


The ST boards have a pair of jumpers, that need to be installed, for the programming signals to be connected from the 6-pin connector to the target processor.


The recommended solution is to upgrade the flash, and you’ll be able to connect the J-Link GDB server via the existing board USB port.

For those who need a full J-Link connection, with this cable it is possible to connect to the ST board with an external J-Link probe, to program the flash and run debugging sessions.