12.3.2 Overview

When enabled, the MTB records the changes in program flow that are reported by the Cortex-M0+ processor over the execution trace interface. This interface is shared between the Cortex-M0+ processor and the CoreSight MTB-M0+. The information is stored by the MTB in the SRAM as trace packets. An off-chip debugger can extract the trace information using the Debug Access Port to read the trace information from the SRAM. The debugger can then reconstruct the program flow from this information.

The MTB stores trace information into the SRAM and gives the processor access to the SRAM simultaneously. The MTB ensures that trace write accesses have priority over processor accesses.

An execution trace packet consists of a pair of 32-bit words that the MTB generates when it detects a non-sequential change of the program pounter (PC) value. A non-sequential PC change can occur during branch instructions or during exception entry. See the CoreSight MTB-M0+ Technical Reference Manual for more details on the MTB execution trace packet format.

Tracing is enabled when the MASTER.EN bit in the Master Trace Control Register is 1. There are various ways to set the bit to 1 to start tracing, or to 0 to stop tracing. See the CoreSight Cortex-M0+ Technical Reference Manual for more details on the Trace start and stop and for a detailed description of the MTB’s MASTER register. The MTB can be programmed to stop tracing automatically when the memory fills to a specified watermark level or to start or stop tracing by writing directly to the MASTER.EN bit. If the watermark mechanism is not being used and the trace buffer overflows, then the buffer wraps around overwriting previous trace packets.