Debug Probes

A debug probe is special hardware which allows you to control execution of a Zephyr application running on a separate board. Debug probes usually allow reading and writing registers and memory, and support breakpoint debugging of the Zephyr application on your host workstation using tools like GDB. They may also support other debug software and more advanced features such as tracing program execution. For details on the related host software supported by Zephyr, see Flash & Debug Host Tools.

Debug probes are usually connected to your host workstation via USB; they are sometimes also accessible via an IP network or other means. They usually connect to the device running Zephyr using the JTAG or SWD protocols. Debug probes are either separate hardware devices or circuitry integrated into the same board which runs Zephyr.

Many supported boards in Zephyr include a second microcontroller that serves as an onboard debug probe, usb-to-serial adapter, and sometimes a drag-and-drop flash programmer. This eliminates the need to purchase an external debug probe and provides a variety of debug host tool options.

Several hardware vendors have their own branded onboard debug probe implementations: NXP LPC boards have LPC-Link2, NXP Kinetis (former Freescale) boards have OpenSDA, and ST boards have ST-LINK. Each onboard debug probe microcontroller can support one or more types of firmware that communicate with their respective debug host tools. For example, an OpenSDA microcontroller can be programmed with DAPLink firmware to communicate with pyOCD or OpenOCD debug host tools, or with J-Link firmware to communicate with J-Link debug host tools.

Debug Probes & Host Tools
Compatibility Chart

Host Tools

J-Link Debug



Debug Probes

LPC-Link2 J-Link


OpenSDA J-Link

J-Link External


some STM32 boards

Some supported boards in Zephyr do not include an onboard debug probe and therefore require an external debug probe. In addition, boards that do include an onboard debug probe often also have an SWD or JTAG header to enable the use of an external debug probe instead. One reason this may be useful is that the onboard debug probe may have limitations, such as lack of support for advanced debuggers or high-speed tracing. You may need to adjust jumpers to prevent the onboard debug probe from interfering with the external debug probe.