Debugging Capabilities of the Cortex-M3 (Explained)

The Cortex-M3 processor from ARM offers a comprehensive set of debugging capabilities to help developers optimize their applications. This includes features like breakpointing, single stepping, watchpoints, and profiling. In this article, we will explore the key debugging features of the Cortex-M3 in detail.

On-Chip Debug Module

The Cortex-M3 contains an on-chip debug module that implements the ARM CoreSight debug architecture. This provides a debug interface compliant with the IEEE-ISTO 5001-2003 Nexus standard. The debug module provides real-time access to the internal state of the Cortex-M3 processor which is useful for debugging code and analyzing system behavior.

Some of the key capabilities offered by the debug module are:

• Breakpointing to stop program execution when specific conditions are met
• Single stepping to incrementally run through code one instruction at a time
• Watchpoints to monitor changes to data variables or memory locations
• Profiling buffers to collect profiling information in real-time
• Trace capabilities to capture instruction and data traces

The on-chip debug module on the Cortex-M3 can be accessed through the JTAG debug interface or the Serial Wire Debug (SWD) interface. JTAG provides a standard 4-wire Test Access Port (TAP) controller interface while SWD uses only 2 wires making it more suitable for space-constrained designs.

Breakpoints

One of the basic debugging capabilities offered by the Cortex-M3 is breakpointing. Breakpoints allow you to stop the execution of a program when a certain event occurs. This helps you analyze the application state when the breakpoint triggers to identify bugs or issues.

There are several types of breakpoints supported on the Cortex-M3:

• Hardware breakpoints – The processor contains dedicated breakpoint comparators that can be configured to trigger on specified conditions like an access to a particular address range, a specific instruction, or a data value match.
• Software breakpoints – These use a special breakpoint instruction to trigger a debug event. When a breakpoint is set at an instruction address, the instruction is replaced with the breakpoint instruction.
• Watchpoints – Breakpoints triggered on access to a watched data address. Useful for monitoring changes to data variables.

The Cortex-M3 supports up to 6 hardware breakpoints. You can set multiple breakpoints via the JTAG/SWD debug interface and continue running the program until a breakpoint triggers. The program halts when a breakpoint matches and you can then examine the system state – register values, memory contents, peripheral states, etc. to debug the issue.

Single Stepping

Single stepping is the process of executing program instructions one at a time. After each instruction executes, the processor halts and you can examine the system state. This allows you to incrementally analyze how the program behaves.

Single stepping is implemented on the Cortex-M3 using a special debug single step mode. In this mode, the processor executes only one instruction and then enters the debug halted state. The debugger handles continously entering and exiting this mode to implement stepping. This method provides seamless single step capabilities without having to set software breakpoints at every instruction.

Single stepping is useful when you want to analyze the effects of individual instructions or trace through program execution in detail. It helps observe register and memory changes at each step.

Watchpoints

Watchpoints are a specialized type of breakpoint that allow you to monitor access to a specific data address and break execution when the data value changes. This allows you to track when a variable or memory location is read or written during program execution.

The Cortex-M3 supports two types of watchpoints:

• Data watchpoints – Break on access to a specified data address
• Address watchpoints – Break on access to a specified address range

Watchpoints are implemented using the hardware breakpoint comparators in the Cortex-M3. Up to 4 watchpoints are supported. When debugging code, you can set a watchpoint on key data variables and the processor will halt when those variables get modified. You can then analyze the code that made the change.

Profiling and Trace

Profiling allows you to collect statistics about code execution performance. The Cortex-M3 debug module includes embedded trace macrocell (ETM) components that enable tracing and profiling capabilities.

The ETM modules provide:

• Instruction trace to capture execution flow
• Data trace to track data reads and writes
• Timestamps for profiling

The trace data collected by the ETM units can be output off-chip via trace ports. This data can then be used to analyze and profile program behavior. For example, you can determine which functions take the most time or which code paths are exercised most frequently.

The profiling features of the Cortex-M3 help you optimize the performance of your applications. The traces give deep insights into how the code executes which is useful for resolving issues and improving efficiency.

Debugging in Low Power Modes

The Cortex-M3 supports debugging even in low power modes. When the processor enters sleep or deep sleep modes, the debug module remains powered on. This allows you to halt execution, set breakpoints, and analyze system state during low power modes.

For ultra low power modes like shutdown, debug is not available. But the Cortex-M3 supports modes like sleep-on-exit to optionally re-enable debugging when the processor wakes up. This is useful to debug low power related issues.

The debug capabilities work independently of the Cortex-M3 power modes. This helps verify that your low power strategies are working correctly by debugging execution in those modes.

Debugging Peripherals

The Cortex-M3 debug architecture provides access to core registers, memory, and peripherals. By halting program execution, you can directly read and write peripheral registers to analyze their state for debug purposes.

Some useful peripheral debugging capabilities include:

• Halting timers or watchdog timers
• Reading interrupt controller registers to identify pending interrupts
• Accessing I/O port registers to check I/O pin states
• Checking status registers of communication peripherals like I2C, SPI, UART etc.

This peripheral access allows you to debug issues related to the Cortex-M3 peripherals and integration with external system components. You can monitor peripheral states in real-time while software executes to identify any anomalies.

Debugging Multicore Systems

For multicore designs using multiple Cortex-M3 processors, the debug architecture provides synchronization capabilities to coordinate debugging across the cores. This allows unified control of all CPU cores during debug.

Some of the multicore debugging features are:

• Simultaneous halting of multiple cores
• Atomic stepping through multicore code execution
• Shared breakpoint management across cores
• Coordinated watchpoint monitoring

These capabilities help debug complex interactions between different processor cores in a synchronized manner. Issues that arise from inter-core communication and synchronization can be identified more easily.

The cross-CPU core debug support in the Cortex-M3 simplifies debugging for multicore embedded systems. Unified control and visibility across cores makes the debug process easier.

Debug Tool Options

There are a range of tool options available to debug Cortex-M3 systems:

• JTAG/SWD debug probes – Dedicated hardware debug probes from vendors that connect via JTAG or SWD to enable debugging.
• In-circuit emulators – Advanced tools that replace the Cortex-M3 processor to provide debugging capabilities along with real-time code execution.
• IDEs/Debuggers – Software tools like GDB or commercial IDEs that interact with probes/emulators and allow control of debugging.
• RTOS-aware debuggers – Debuggers with awareness of RTOS behavior to enable debugging task interactions in multi-threaded applications.

ARM also offers its own DS-5 Development Studio which incorporates a debugger, compiler toolchain, and analysis tools for Cortex-M devices. There are lots of options for debugging tools, so you can choose the best fit for your specific needs and budget.

Debugging Example Code

Let’s look at a simple debugging example to see how we can use these capabilities to identify and fix a bug.

Consider the following pseudo-code for a function that reads data from an ADC channel into a buffer:

read_adc() 
{
  buffer_index = 0;

  while (buffer_index < BUFFER_SIZE) {
    adc_value = read_adc_channel();
    buffer[buffer_index] = adc_value; 

    buffer_index = buffer_index + 1; 
  }
}

This function is supposed to fill the entire buffer with ADC values. However, after running it we find that the buffer only gets partially filled. To debug this issue:

1. Set a breakpoint at the start of the function.
2. Single step through the code and observe the values of buffer_index and BUFFER_SIZE.
3. We find that the loop exits early before buffer_index reaches BUFFER_SIZE.
4. Check the while loop condition and realize that BUFFER_SIZE is incorrect.
5. Fix the BUFFER_SIZE definition to resolve the issue.

By using single stepping and breakpoints, we can isolate the root cause without having to insert lots of temporary debug print statements. This demonstrates how the Cortex-M3 debug features can simplify debugging firmware.

Conclusion

In summary, the Cortex-M3 provides a comprehensive set of hardware debug capabilities aimed at embedded software development. The debug module implements real-time access and control of code execution through features like breakpoints, watchpoints, single stepping, trace and profiling. These features enable you to analyze program flow, data accesses, peripheral states, and system events while debugging. The on-chip debug architecture reduces the need for extra debugging hardware.

Leveraging the integrated debug capabilities early in development can help identify and resolve bugs faster. Proper use of the debugging features makes the software development process smoother on Cortex-M3 embedded systems.