Cortex-M0 Debugging Capabilities (Explained)

The Cortex-M0 is an ultra low power 32-bit ARM processor core designed for microcontroller applications. It provides a range of debugging capabilities to help developers optimize and troubleshoot their code. This article will provide an overview of the key debugging features available with the Cortex-M0 and how they can be utilized.

On-Chip Debugging

The Cortex-M0 includes an on-chip debugging module known as the CoreSight Debug Access Port (DAP). This provides a debug interface to the processor core, allowing you to control execution and access registers and memory while your code is running. The DAP is accessed through the SWD (Serial Wire Debug) protocol, using only two pins on the MCU package – a clock pin and a data pin.

Through the SWD interface and DAP, you can perform the following debug operations:

• Halt and resume code execution
• Set hardware and software breakpoints
• Single step through instructions
• Access core and peripheral registers
• Read and write data and program memory

This on-chip debugging capability allows you to non-intrusively debug code without the need for extra hardware like debug headers or JTAG connectors. The simple 2-wire SWD protocol reduces pin count making the Cortex-M0 ideal for space-constrained designs.

Breakpoints

One essential debugging tool is the ability to set breakpoints. The Cortex-M0 supports up to 8 hardware breakpoints. These will cause the core to halt execution whenever a certain memory address is accessed, either for reading or writing data. Breakpoints are set by configuring CoreSight’s Breakpoint Control Registers (BCR) and Comparator Registers (BVR) through the SWD interface.

For example, to set a breakpoint on an instruction address:

1. Write the address to BVR0
2. Set BCR0 to enable breakpoint on instruction fetch

Now the core will halt whenever it attempts to fetch an instruction from this address. The breakpoint remains set even if the program counter changes, it will halt whenever returning to this address.

In addition to breakpoint exceptions, the Cortex-M0 can generate debug halts from other sources like the Debug Monitor program counter sampling, or a configured Halt Request system control register.

Single Stepping

Single stepping is the process of executing instructions one-by-one to analyze how the program flows. The Cortex-M0 debugging system supports single stepping in either Instruction Stepping or Cycle Stepping mode.

In Instruction Stepping, the processor halts after each instruction. To step through instructions:

1. Halt execution
2. Read program counter to see current instruction address
3. Set breakpoint at next instruction address
4. Resume execution

The core will execute one instruction then halt on the breakpoint. Rinse and repeat to step through each instruction.

Cycle stepping mode halts the core after each clock cycle. This allows you to observe the effects of single cycle events that may not trigger an instruction breakpoint.

Watchpoints

Watchpoints allow halting program execution when a particular data address is accessed, either for reading or writing. This allows you to monitor memory to track down unexpected changes.

There are 2 watchpoint comparators which you can configure to halt on data access. For example, to monitor writes to a global variable:

1. Write the variable’s address to DWT_COMP0
2. Set DWT_MASK0 and DWT_FUNCTION0 to trigger on write

Now the processor will halt before any instruction that writes to this address. Watchpoints are an essential tool for identifying memory corruption issues.

Trace Support

The Cortex-M0 includes Embedded Trace Macrocell (ETM) support for instruction trace capabilities. This allows recording a trace of program execution over time, providing insight into code flow and optimization opportunities.

Tracing is configured through the ETM registers. You can filter tracing to specific addresses or exception levels. The trace itself is captured through a Trace Port Analyzer tool connected to the chip’s Trace Port Interface.

Trace data is compressed to reduce bandwidth requirements. The amount of trace history stored depends on the system – this could range from kilobytes to megabytes for more complex tracing scenarios.

Debugging Cortex-M0 Designs

Now that we’ve covered the key capabilities, how do we go about debugging an actual Cortex-M0 system? Here are some tips:

• Use IDE debugger integrations – Many IDEs like Keil MDK-ARM or IAR Embedded Workbench have built-in debug probes. This is the easiest way to get started.
• Choose a SWD probe – Dedicated SWD debug probes from vendors like STLink, J-Link, and CMSIS-DAP provide robust debugging.
• Scour the startup code – Make sure clocks, memory, and peripherals are all configured properly even before main().
• Set breakpoints at key main() stages – Pause execution at beginning, between functions, before infinite loops.
• Single step through handled exceptions – See the exception handler context like stacked registers.
• Monitor peripheral register accesses – Use watchpoints to check for invalid configurations.
• Trace early code flow – Instruction trace the vector table, clocks, and memory initialization.

These are just a sample of techniques for utilizing the Cortex-M0’s debug features. Creative use of breakpoints, single stepping, and trace capture can uncover most firmware issues.

Limitations to Consider

While the Cortex-M0 debug module provides extensive capabilities, there are some limitations to note:

• Only 8 hardware breakpoints – Use them wisely for critical program locations.
• No execution profiling or cycle counting – Requires more advanced Cortex-M4 features.
• Limited trace buffer size – Capturing long trace lengths requires high bandwidth.
• Debug increases power consumption – Keep debug sessions short to avoid excess power draw.
• SWD pins shared with GPIO – May conflict with board routing.

Despite these limits, the Cortex-M0 delivers robust debugging on an ultra efficient processor footprint. Minimize power andpins by using SWD interface, and focus hardware breakpoints on only the most critical program addresses.

Debugging Cortex-M0 with GDB

The GNU Debugger (GDB) provides another option for debugging Cortex-M0 designs through the SWD interface. Here are some key steps for getting started with GDB:

1. Build code with debug symbols (-g compiler switch)
2. Launch GDB, connect to target board via SWD
3. Load debug symbols using symbol-file command
4. Halt execution on start-up or main using break command
5. Inspect registers, local variables, memory
6. Set breakpoints and watchpoints
7. Step through code line-by-line or instruction-by-instruction

GDB hooks into the standard DAP/SWD debug interface, so all the underlying Cortex-M0 hardware breakpoints and single step capabilities are available. Debug symbol information contained in the ELF binary allows inspection of code flow, variables, and memory content.

A key benefit of GDB is its adaptability across many different embedded targets. The same GDB instance running on a Linux PC can debug a Cortex-M0, RISC-V core, or other architectures. This provides a flexible, open source debugging option for Cortex-M0 projects.

Debugging Hard Faults

One of the most common issues users face is hard faults crashes or lockups. These occur when the processor encounters an exception it cannot safely handle. The Cortex-M0 fault checking will escalate this to a HardFault, halting execution.

When a hard fault occurs, key steps for debugging include:

1. Check stacked PC to locate fault site
2. Inspect stacked LR to identify return location
3. Enable BusFault/MemFault debugging to catch access issues
4. Set breakpoints around last good instruction location
5. Single step to recreate fault and locate root cause

Hard faults are a great case for using single step debugging. Step back through the code path leading up to the site of the crash. Monitor stack pointer changes and memory accesses to identify the operation triggering the fault.

Debugging Low Power Code

Minimizing power consumption is critical in Cortex-M0 microcontroller designs. This leads developers to utilize low power modes like Sleep and Deep Sleep. But these can also complicate debugging.

To debug low power modes:

• Halt entry/exit to identify wake up sources – Use breakpoints on WFI/WFE instructions.
• Check clock configurations match mode requirements – Watch peripheral register accesses.
• Single step suspend sequences – Verify proper register, peripheral, and memory handling.
• Measure current draw with power monitor – Captures consumption in different modes.
• Use oscilloscope to monitor sleep mode entry/exit – Helps identify wake timing issues.

Debugging low power operation requires correlating instruction flow with physical behaviors like clocks, current draw, and wake up events. Utilize all available tools to match software execution with hardware behavior.

Debugging Intermittent Issues

Intermittent bugs that cannot be easily reproduced are some of the hardest to debug. They may occur due to race conditions, thermal fluctuations, or external interfaces.

Some tips for chasing down intermittent issues include:

• Capture first occurrence with breakpoints – Halt on data aborts or assertions.
• Trace instruction flow leading up to failure – May uncover race conditions.
• Stress test the system – Heat, voltage, peripherals, I/O.
• Increase diagnostic logging – Help reconstruct events leading up to failure.
• Make use of debug timestamping – Correlate code with external events.

Having the failure occur once while debugging is essential. Trace and breakpoint capabilities in the Cortex-M0 allow capturing that first instance. From there, detailed analysis of the pre-failure execution flow can uncover the root cause.

Real-Time Debug Support

Debugging applications with real-time requirements brings additional challenges. Intrusive debugging – halting execution and single stepping – makes it hard to recreate real-time property violations.

Some real-time debug tips for Cortex-M0:

• Minimize execution halts – Use breakpoints sparingly.
• Capture traceback on violations – To identify call sequence.
• Trace execution over a window – Monitor RTOS state changes.
• Generate runtime diagnostics – Task switching, stacks, timing.
• Simulate peripherals and I/O – Eliminate external dependencies.

The Cortex-M0 supports some amount of non-invasive debugging with trace capture, profiling, and diagnostics output. But it is also important to recreate real-time behavior with simulated stimuli when debugging.

Debugging Multicore Cortex-M0 Systems

Complex microcontroller designs may integrate multiple Cortex-M0 cores for greater performance. This presents additional debugging challenges:

• Simultaneous SWD connections – Using multiple debug probes.
• Correlate trace data between cores – Requires timestamp alignment.
• Shared memory visibility – Coordinating watchpoints and breakpoints.
• Cross-triggering execution halts – Cores halt in a matched state.
• Inter-core communication – Message passing, flags, and semaphores.

Multicore Cortex-M0 debugging requires tracing execution across cores and maintaining synchronization when halting cores. Use multiple debug probes and align trace captures to get composite system view.

Conclusion

The Cortex-M0 provides a robust set of debugging capabilities despite its low power optimized design. The CoreSight DAP enables control of program execution while SWD provides non-intrusive access. Hardware breakpoints, watchpoints and trace give detailed insight into code flow and memory operations.

To take full advantage, utilize debugger integration in your IDE for easy access to SWD interface. Focus breakpoints on startup, main program phases, exceptions, and areas of interest. Trace key execution sequences and use single step techniques to recreate hard faults. Follow these guidelines and the Cortex-M0 debugging features will help accelerate your embedded development.