Unlock maximum SWO speed on SEGGER's high end debug probes

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Some of SEGGER's high end debug probes like J-Link PRO and J-Trace PRO support higher maximum SWO speeds than they are shipped with. How to unlock the maximum speed potential of the SWO interface for these probes will be described in this article.

Why is the SWO speed not unlocked from the beginning?

Over time SEGGER's debug probes got faster and more capable. With that the maximum supported SWO speed rose as well. Unfortunately not many CPU vendors kept up with SEGGER's debug probes in terms of interface speed.

Whenever a J-Link/J-Trace tries to set up SWO it will negotiate the fastest possible SWO speed that the target device supports. Now some target devices claim to support high SWO speeds. Unfortunately this is not always true which leads to incomplete SWO data. To counter this issue all high end debug probes from SEGGER are shipped with a fixed maximum SWO clock speed of 50 MHz which turned out to be a good sweet spot for all target devices on the market.

However this limitations can be lifted on the high end probes that support higher clocks. How is described in the following steps.

Which debug probes support setting the maximum SWO speed to a higher value?

Maximum SWO speed can be adjusted with the following hardware:

  • J-Link Ultra+ V4 or later.
  • J-Link PRO V4 or later.
  • J-Trace PRO Cortex V2 or later.
  • J-Trace PRO Cortex-M V1 or later.

Later hardware revisions of these models support this feature as well. The maximum SWO clock speed can be set up to 100 MHz.

How to set the maximum supported speed?

  1. Connect your debug probe through USB or Ethernet to your PC.
  2. Open the J-Link Configurator which is part of the J-Link software and documentation package.
  3. Your connected debug probe should be now listed there.
  4. Double click the debug probe you want to edit.
  5. Edit the setting "Max. SWO speed [kHz]" to the SWO clock speed you want to set it to e.g. 100000.
  6. Press OK.
  7. You should now get a prompt telling you to unplug and reconnect the J-Link. Do that.
  8. Press OK.
  9. Verify the newly set value by double clicking your debug probe again. The new value should show up. Alternatively connect to your target device that supports SWO with J-Link Commander and execute command "swospeed".