Cortex Trace support

CPU initialization code support Cortex-M trace through ITM/DWT/TPIU.

ITM is user-facing instrumentation cell. It allows the running software to output trace events (mostly messages) through trace infrastructure.

DWT is CPU-tied trace logic. It allows to issue CPU event traces (data load/stores, PC events, exceptions, etc.). DWT is also able to issue periodic messages for timstamping and synchronization.

TPIU is a formatter/multiplexer cell. It may be optionally enabled/disabled. It is responsible for multiplexiing streams from ITM, DWT and other sources. It is mandatory if more than one source is enabled.

Traces can be outputted on either:

• SWO (Serial Wire Output), which is a self-clocked signal, either manchester or UART-like.

• A parallel trace bus, including a clock and 1, 2 or 4 DDR data signals.

• ETB (Embedded Trace Buffer), a dedicated RAM buffer that can be read either from usual debugger interface (JTAG/SWD) or platform CPUs. This is usually only available in SMP setups.

In most architectures, enabling the trace infrastructure requires Mem-AP DBGEN to be set, therefore it requires an initialized debugger session.

All the trace logic is clocked through a TRACECLKIN (Trace clock input), provided by the SoC. Enabling this clock properly is implementation dependant.

Trace pins are usually shared/muxed with other SoC peripherals. Setting up pin muxing is implementation dependant.

Currently, only ITM output may be automatically enabled. It requires the following configuration tokens:

• CONFIG_CPU_ARM32M_CORESIGHT and CONFIG_CPU_ARM32M_TRACE to enable trace

• CONFIG_CPU_ARM32M_TRACE_PARALLEL, set to 0 (swo), 1, 2, or 4 (parallel with clock)

• CONFIG_CPU_ARM32M_ITM and CONFIG_CPU_ARM32M_ITM_PRINTK to get printk output in ITM flow

• CONFIG_CPU_ARM32M_TRACE_RATE to set SWO baud rate

Other architecture-specific tokens may be needed. See related architecture support documentation and configuration tokens.

Orbuculum project is a good start. For a basic setup where an UART is used with UART-compatible SWO output, one may use following MutekH configuration:

Matching orbuculum instance, responsible for reading SWO data from UART:

Matching orbuculum dumper helper connecting to orbuculum: