Wojtek proposed that idea. However, after some testing, I have confirmed that doesn't provide the information I desire. I want to know when the soft device is running. I don't care if the RF is transmitting or receiving.
Most of the times (longest blocking period) that the softdevice occupy CPU are when we have BLE events. So radio notification could basically tell the period that the CPU is occupied.
Another option you can do is to try flipping a GPIO pin in your main application or inside a timer "TICK" handler with Application High priority. This way you can have a look at the logic trace of the pin and see the period when the softdevice occupy the CPU.
