nRF9151 includes an Embedded Trace Macrocell (ETM) and an Instrumentation Trace Macrocell (ITM).
The ETM is used for non-intrusive, step-by-step processor instruction tracing, while the ITM is used for printf()-style debugging. Traces from both ETM and ITM are accessible through General-Purpose Input/Output (GPIO) pins P0.21–P0.25. It is recommended to reserve this interface and make it available in the end product to assist Nordic customer support.
For more information on debug and tracing, see Debug and trace.