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Connecting a JTAG/SWD Debugger to a Device

JTAG and SWD debuggers are the preferred way to interact with devices in SJSU-Dev2, as they provide two main benefits.

  1. Can be used to program/flash devices, which tends to be faster than serial programming.
  2. Can be used to debug code on device line-by-line, which is a powerful tool in debugging the state and behavior of a program over print statements.

SJSU-Dev2 leverages the programs OpenOCD and GDB for the purposes above, but this combination has 1 caveat, which is that GDB does not have a clean way to accept STDIN from a keyboard.

Prerequisite Knowledge

SWD stands for Serial Wire Debug and is the favored solution for SJSU-Dev2 due to its lower pin count compared to JTAG. SWD uses two pins for debugging, the SWCLK, clock reference pin, and the SWDIO, data input/output pin. Some development boards will omit the SW prefix to save space and will simply show CLK and DIO.

JTAG can also be used in SJSU-Dev2 but has far more pins such as the following TDI, TMS, TCK, and TDO.

With both of these standards, you will need to connect a jumper wire between the SWD pins on the debugger device and the development board.

Step 0: Getting the Parts

This example uses the STM32F10x Blue Pill as an example but this guide can also be used for any other development board so long as the debugging pins are available as connections.

If you do not have the parts you can buy them from Amazon using these links:


There are other types of debuggers out there like the Segger brand of debuggers. These will work with SJSU-Dev2 as well, but they tend to be more expensive.

Step 1: Connecting the Debugger

A connection to ground (GND) must be made between the debugger and the development board in order for the devices to communicate.


DOUBLE AND TRIPLE CHECK YOUR CONNECTIONS! Incorrect connects can result in breaking a board, debugger or possible your computer.

Step 1.1: Connecting JTAG

Connect jumpers from the GND, TDI, TMS, TCK, and TDO pins on the JTAG debugger to the headers on the development board of the same name.

Step 1.2: Connecting SWD

Connect jumpers from GND, SWDIO and SWDCLK to the pins on the board. If the board supports both SWD and JTAG like many arm cortex boards do, then connect the pins in the following way:

  • SWDIO --> TMS
  • SWDCLK --> TCK