Unbricking A SEGGER J-Link V9 Debug Probe

Last year [Emil] found themselves in the situation where a SEGGER J-link debug probe suddenly just stopped working. This was awkward not only because in-circuit debuggers are vital pieces of equipment in embedded firmware development, but also because they’re not that cheap. This led [Emil] to take the device apart to figure out what was wrong with it.

After checking voltages on the PCB, nothing obvious seemed wrong. The Tag-Connect style JTAG header on the PCB appeared to be a good second stop, requiring only a bit of work to reverse-engineer the exact pinout and hook up an ST-Link V2 in-circuit debugger to talk with the STM32F205RC MCU on the PCB. This led to the interesting discovery that apparently the MCU’s Flash ROM had seemingly lost the firmware data.

Fortunately [Emil] was able to flash back a version of the firmware which was available on the internet, allowing the J-Link device to work again. This was not the end of the story, however, as after this the SEGGER software was unable to update the firmware on the device, due to a missing bootloader that was not part of the firmware image.

Digging further into this, [Emil] found out a whole host of fascinating details about not only these SEGGER J-Link devices, but also the many clones that are out there, as well as the interesting ways that SEGGER makes people buy new versions of their debug probes.

(Thanks Zelea for the tip)

Adding A Debugger To A Teensy 3.5/3.6

The Teensy is a powerful ARM-based development board with loads of features that can do fun stuff with USB as well. Like many dev boards, it uses a less powerful processor as an interface. Teensy designer [Paul Stoffregen] added a debug header to allow direct SWD JTAG access to the main chip, but the interface microcontroller has to be silenced for that to work, and the code to do so is still in progress.

Impatient, [Erich Styger] documents the changes he made to add support for the J-Link SWD protocol by removing the offending NXP Kinetis KL02Z that serves as the as the onboard interface and bootloader that helps the Arduino IDE talk to the K64F which is the main chip. After the KL02Z was removed, [Erich] populated the debug headers and then wired up the Segger J-Link to the board and tested it out with Eclipse, GDB, and standard SWD debug tools.

The end result is a Cortex M4F board that can work with standard tools at a third of the price of the Kinetis’ development board. [Paul Stoffregen] confirms that the debugging functionality will be added to the bootloader code soon but until then, a hardware hack is a working, if brutal, approach to debugging on the platform.

More information on the JTAG interface is available for the interested. And if Teensy isn’t your thing, you might consider an STM32-based development board.