This article describes device specifics of the Silicon Labs EFM32 series devices
Usually, the SWO output frequency of a device is directly dependent on the CPU speed. The SWO speed is calculated as: <CPUFreq> / n. On the EFM32 series this is not the case: The SWO related units (ITM, TPIU, ...) are chip-internally wired to a fixed 14 MHz clock (AUXHFRCO).
This will cause the auto-detection of J-Link to not work by default for these devices, if the CPU is running at a different speed than AUXHFRCO. All utilities that use SWO speed auto-detection, like the J-Link SWOViewer, need to be told that the CPU is running at 14 MHz, to make SWO speed auto-detection work, no matter what speed the CPU is really running at.