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You program and read the ETM trace unit registers using the Debug APB interface.
The core does not have to be in debug state when you program the ETM trace unit registers.
When you are programming the ETM trace unit registers, you must enable all the changes at the same time. Otherwise, if you program the counter, it might start to count based on incorrect events before the correct setup is in place for the trigger condition.
To disable the ETM trace unit, use the TRCPRGCTLR.EN bit.