Digital Protocols: How does SWD work?

SWD stands for Serial Wire Debug and is similar to JTAG. It is an ARM protocol specifically developed for the debugging processes. Just like JTAG, the purpose here is to test the board during production and check for any errors. For example, an update that should take place OTA (Over The Air) may not be possible or may fail. In this case, the manufacturer has the option to recall their devices and manually patch them through these interfaces.

With SWD, it is sometimes possible to gain complete control over a target. The simple design is also one of the major advantages of SWD. Basically, only two pins are needed.

SWDIO and SWCLK.

Sometimes SWDO is also used.

SWDIO - SWD Input Output - Bidirectional data line

SWDCLK - SWD Clock - Clock

SWDO - SWD Output - for unidirectional output from the controller

To capture and decode the signals of the protocols, certain hardware adapters are used. There are inexpensive TTL to USB converters or I2C or SPI to USB converters. In many cases, you can also find combined devices online that can capture all common protocols. With some luck, you can also find hardware adapters that not only know the individual protocols but also have an integrated logic analyzer. I will cover in another blog post, what a logic analyzer is and how you can use it.

OpenOCD can be used as software to interfere with SWD.