RE: nRF51LM20 QFN52 SWO pin not assigned

Brutus — Mon, 10 Nov 2025 07:19:09 GMT

Hi PJ !

Thank you for the in depth and detailed answer ! Great suggestions, I will carefully review them and see what fits best to our current design.

RE: nRF51LM20 QFN52 SWO pin not assigned

PjGlasso — Sat, 08 Nov 2025 17:04:04 GMT

Hi there,

So I use the Xiao nRF54L15 with the CSP47 talk about no Extra pins..😅 Any way Here is why...

The absence of the SWO (Serial Wire Output) pin on the nRF54LM20 QFN52 package, while present on the larger CSP98 package, is a result of pin multiplexing and the physical pin constraints of the smaller package. To accommodate extra GPIOs and USB capability within the limited pin count of the QFN52, certain less critical or optional debugging features like SWO are not assigned to a dedicated physical pin.

Reasoning for the unassigned SWO pin

Pin Limitations: The QFN52 package has a fixed number of physical pins. When adding new features like USB (which requires dedicated D+ and D- pins) and increasing the general-purpose I/O count compared to the nRF54L15 QFN48, the available pins for optional debugging interfaces become limited.
Pin Multiplexing: Modern MCUs use pin multiplexing (pin-muxing) to assign different peripheral functions to the same physical pin. In the QFN52 package, the pin that would normally be assigned the SWO function might be shared by other essential peripherals (like the USB or additional GPIOs), and the designers chose to prioritize those other functions for this package variant.
SWO is an Optional Debugging Aid: The SWO pin is used for receiving trace and log data (printf-style debugging) from the device via the Serial Wire Viewer (SWV) or Embedded Trace Macrocell (ETM) features. It is not essential for basic debugging operations like programming, halting, and single-stepping the CPU, which only require SWD (Serial Wire Debug) which uses SWDIO and SWCLK pins.

Workaround to have something similar to SWO

While you cannot have a dedicated SWO pin on the nRF54LM20 QFN52 package, you can use alternative methods to achieve similar debugging functionality:

RTT (Real Time Transfer): This is the most common and recommended alternative to SWO for Nordic devices. RTT is a proprietary Segger technology that uses the standard SWD interface (SWDIO and SWCLK pins) to transfer data between the target MCU and the debugger. It is often faster than SWO, does not require a dedicated pin, and supports multiple data channels for both up-link (target to host) and down-link (host to target) communication. Most modern development environments and the nRF Connect SDK have full support for RTT.
UART (Universal Asynchronous Receiver-Transmitter): You can configure one of the available GPIO pins as a standard UART TX pin and use it to send debug messages to your host PC via a USB-to-serial converter. This approach uses two pins (TX and potentially RX if two-way communication is needed) and is slower than RTT, but offers a simple, universally compatible method of outputting data.
LED/GPIO Toggling: For simple debugging scenarios, toggling a GPIO pin or an LED can signal different states in the code.

For your application, transitioning from SWO on the nRF54L15 to RTT on the nRF54LM20 would be the most seamless and performant solution, as it requires minimal hardware changes (only the standard SWD connection) and provides robust logging capabilities.

I'm waiting on the LM20A, curious how the onboard usb-c works out.

HTH

GL :) PJ ✌