SWO (Single Wire Output) in ARM cores is probably one of the most under-used features. Which is surprising, because SWO can be very useful. In a nut shell: SWO is a single wire output pin/signal channel which can provide lots of different data, like PC sampling for coverage information, interrupt tracing data or ‘uart-like’ text packets.
Using SWO
Despite its usefulness, using SWO can be challenging: it requires a physical pin, support in the debugger, plus despite it is a standard ARM feature, using it might be different for each MCU because of clocking. Tool vendors try to make things easier with ‘automatic’ setup of all the SWO hardware registers. That’s OK if that works, but unfortunately this is not always the case. In addition to that it conflicts with SWO configuration and settings done in the application. One might argue that the application should not touch SWO: but handling SWO in the application is my preferred way as that way I know what is going on. And I don’t have to rely on some ‘magic code’ from the tool vendors. Handling SWO in the application and to rely on the tool vendor ‘black box’ configuration allows me to use SWO without any external tools involved: so I can use the Saleae or Sigrok tools to capture the data and process it, without the interference of other tools.
Standalone SWO Viewer
After some debates and discussions, I’m very happy to report that SEGGER has added a ‘standalone’ SWO functionality to their SWO viewer V7.22 and later. From the release notes:
[SWOViewer]: Added command line option “-swoattach <OnOff>” that disables all J-Link side initialization of SWO and relies on the target application to perform SWO init.
SEGGER V6.22 Release Note
💡 Note that I have to use the V6.22b, as at least in the actual V7.50b (both 32bit and 64bit) the feature seems not to work any more.
With this, the SEGGER SWOViewer can be used in combination with application level SWO setup and configuration. How this configuration works: check SWO with ARM Cortex-M33 which describes the McuSWO module which gives you access and configuration of the SWO feature across ARM cores.
Usage
Here is how to use it, using the example presented in SWO with ARM Cortex-M33.
Start the SWO viewer with the following command line (see https://wiki.segger.com/J-Link_SWO_Viewer for the list of options):
JLinkSWOViewerCL -swoattach on -swofreq 64000 -device LPC55S16 -itmport 0x1
This starts the JLinkSWOViewer in standalone mode:
This example above uses the stimulus port 0x1. Note that for SWO ‘printf’ this goes to the stimulus port 0.
The application can now use SWO. I’m using for example the McuSWO:
The application has configured SWO with a baud of 64k baud for testing purposes:
For sending text I’m going to use the stimulus port 0x0:
JLinkSWOViewerCL -swoattach on -swofreq 64000 -device LPC55S16 -itmport 0x0
Using the console I can send data over the SWO pin:
Summary
It is fine if tools configure SWO. I wish that the settings and changed applied by the tools would be more transparent. For the case where the application needs to set up SWO, SEGGER has now added an ‘attach’ option which does not tamper with the application SWO configuration. With this a ‘standalone’ SWO usage can be realized.
💡 Update: the MCUXpresso IDE 11.5.0 has now added standalone SWO functionality too :-).
Happy wiring 🙂
Links
- SEGGER SWO Viewer: https://wiki.segger.com/J-Link_SWO_Viewer
- Tutorial: Using Single Wire Output SWO with ARM Cortex-M and Eclipse
- SWO with ARM Cortex-M33
- SWO with NXP i.MX RT1064-EVK Board
- ARM SWO Performance Counters
- Sigrok Logic Analyzer software with TPIU decoder
MCU on Eclipse 
Interesting. Thank you.
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Yes, it is. I’m always concerned by debuggers or tools trying to setup things in the back. Similar thing with the semihosting too.
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Interesting. One of the first reasons I came across your blog was reading how to setup SWO within mcuxpresso a couple years ago. I think it was using a 1064evk board.
In any case, you are absolutely right, it has not been an easy thing to get going and relying on the tools to automagically set it up hasnt been great either.
I no longer have issues now that I use SES but I occasionally use other tools with my Segger so this will be good to know.
many thanks.
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Hi John,
you might run into issues with any tool which wants to setup SWO directly, especially for newer devices. Vendors do not have all the devices at hand, and it is likely that there are always some hidden changes in newer devices e.g. for the clocking, resulting in ‘blind’ implementation not working: my experience is that if it has not been verified non actual hardware, it might not work or is just luck. Even with SES as I don’t think they have the bandwidth to test things with the number of devices out there. The good thing is that they are usually pretty fast in fixing things fast.
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Yeah and I think that was the case for me initially, it was not reliable! but since I’m only concerned with the IMXRT10xx family, Segger seems to have good support for it now. They have addressed several issues for me., I know for a fact they have EVK boards so as to replicate issues and recently they made changes to their software and firmware to better (aka reliably) support mcu’s like the imxRT which have complex boot requirements complicated by internal 1st stage bootoaders (bootrom) and of course security. SWO setup is easy now and trace and profiling works reliably.
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Is SWO different from SWD? The reference manuals for the ARM processors I have (KL25, KL26, K20 from Freescale, now NXP) mention SWD, but not SWO. The K20 has JTAG, cJTAG, and ARM Real-Time Trace Interface as well.
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The SWO (Single Wire Output) is an extra/optional part of the ARM spec and not available on the lower end cores like the KL. See https://mcuoneclipse.com/2016/10/17/tutorial-using-single-wire-output-swo-with-arm-cortex-m-and-eclipse/
SWD is the spec for the debug (single wire debug) part, while SWO is part of the trace functionality.
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can swo be used without an active debug connection, my understanding is that you can configure it in sw, but you can’t enable it from sw, this must be done from outside via debugger. have you tried it just to capture the output with saleae with no debugger connected at any point?
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Yes, I configure SWO in the application and capture the output without debugger. So you really do *not* need a debugger to set it up and get the data. And yes, I can use Salea and Sirok to capture output too.
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