When programming the ETM registers you must enable all the changes at the same time. For example, if the counter is reprogrammed, it might start to count based on incorrect events, before the trigger condition has been correctly set up.

You can use the ETM programming bit in the ETMCR to disable all trace operations during programming. See ETM Main Control Register. To do this follow the procedure shown in Figure 3.1.

The processor does not have to be in the debug state while you program the ETM registers.