Top Debugging Techniques Used In Embedded Systems

Embedded systems design combines the highly technical disciplines of hardware design and firmware and application software development. Embedded systems engineers face significant challenges throughout the design process, especially when it comes to the integration and debugging of hardware and software systems. 

As an embedded systems project matures and grows in complexity, it becomes increasingly hard to track down and isolate bugs in the code. One study conducted in 2013 by researchers at the University of Cambridge identified that software developers spend up to 50% of their time and budget on each project debugging code. This amounts to billions of dollars each year in developer salaries and overhead, funds that could easily be allocated elsewhere if there was a more efficient way to debug your product.

To help bring relief to developers in debug purgatory, we're offering up some of our preferred debugging techniques that actually work. We'll review some of the traditional run-time debugging techniques, describe the benefits of integration testing, and explain how developers can implement real-time trace debugging to discover and rectify software bugs, bringing products to market faster and with fewer errors.

Traditional Debugging Techniques

Our exploration of debugging techniques for embedded systems begins with traditional debugging techniques. These techniques can all be used to identify and isolate coding errors for removal from your firmware or software application, but the efficacy of each method depends on the unique circumstances of your project. Some of these methods include automation while several others are heavily manual processes. More complex embedded systems projects will typically benefit from more sophisticated debugging methodologies.

Debugging Method #1: "Print Method"

Print debugging is probably the simplest and most basic way of debugging an embedded system. The method is carried out by watching live print statements that are written on the screen as the code is executed. To achieve this, the developer must intersperse print statements throughout the code that will be printed on the screen when specific portions of code are executed. In this way, it is possible to visualize when specific parts of the code are executed.

While the print method can provide transparency into how the code is behaving, it is not typically ideal for more complex projects. Printed statements may confirm that a specific piece of code has executed, but they fail to provide a complete view of the system state that would be useful (and sometimes required) for tracking down more elusive errors. While the print method is useful for simple applications and for developers who lack access to a complete debugging environment, a more technical solution is required to meet the needs of more complex embedded systems projects. 

Debugging Method #2: "Run-time Methods"

The print debugging method is really just the simplest version of a run-time debugging technique. Run-time debugging methods share a common characteristic: they monitor the live execution of a process or a piece of code while the developer uses either manual techniques or debugging software to debug the process. In addition to the print method, run-time debugging methods include remote debugging and communication-based debugging.

Remote debugging describes a debugging technique where the process or code that is being debugged is executed in an environment that is separate from the debugger itself. Remote debugging is useful for embedded systems that do not have a full operating system implemented on them, making it easier for the developer to implement a small debug platform in the software that facilitates debugging from a remote location. In remote debugging, the debugger communicates with the debugged process over a network instead of directly initiating actions or waiting for events.

Communication-based debugging is a technique where the debugger is hooked into the communication between various processes on the embedded system. This allows for effective monitoring of data and messages that are passed between the processes, giving the debugger insight into the function of the application. A developer can set up a proxy process where all messages are sent through, then monitor the messages under various conditions to ensure that all processes are behaving as expected.

Without the right tools, debugging can prove an expensive and time-consuming process. Techniques like integration testing and real-time trace can help embedded systems engineers troubleshoot code errors more quickly than traditional, manual debugging methods.

Integration Testing Technique

Integration testing, sometimes referred to as System Integration Testing (SIT), describes a fundamentally different approach to testing and debugging embedded systems throughout the development process. This method borrows wisdom from new working methods of software development such as Agile and DevOps that promote periodic testing throughout the development process instead of a single, lengthy debugging process in the late stages of product development.

The wisdom behind integration testing can be summarized as follows:

In traditional software development, engineers develop working specifications, program individual modules and combine the modules to complete the program before testing begins. This process may produce a large number of errors. The detection and isolation of errors is complicated by the fact that the code has already been integrated. It may be difficult to identify and isolate bugs once all code modules have already been integrated.

To improve on this method, integration testing begins with unit testing. Before code modules are integrated, they must be subjected to unit tests in isolation to ensure they operate bug-free on their own. Unit testing ensures that each software component functions as expected on its own before being integrated with the whole.

Once a module has passed unit testing, it can be integrated with other modules to begin developing a system. Developers can start by combining low-level modules that implement a common functionality and establishing test cases and procedures to verify their correct functioning. Developers must write a battery of test cases that will be used to verify the build is working correctly. When two or more modules are integrated successfully and pass all relevant tests, developers can go on to incorporate more modules, write new test cases and repeat testing until the final bits of code are added to the system. 

Successful integration means that the software worked correctly on the target hardware and that the software performed as expected according to the requirements specified during the design phase of the project plan.

There are several ways that developers may choose to organize integration testing, but the most important points are:

  1. Conduct unit testing before integrating the new code to ensure it works properly.
  2. Write test cases for each new integration and ensure that a newly integrated module is functional before creating additional complexity.
  3. Test early and often to avoid a lengthy and complicated debugging process towards the end of the development process.

Debugging with Real-Time Trace

The final debugging technique that we want to mention here, and the most powerful, is known as a real-time trace debugging.  With real-time trace, developers implement a hardware device such as a protocol analyzer that records information about the execution of processes in the code. This information can include sequences of program counter values, register reads and writes, and changes in device memory and associated data values.

When a developer notices that a process is behaving unexpectedly, real-time trace debugging provides complete and total insight into the functioning of the process within the code. The developer can produce a log that indicates exactly how the system changed while the process was executed, making it significantly easier to isolate, identify, and correct software bugs.

Real-time trace debugging can be implemented at any stage of development, as a complement to integration testing or when the software and hardware for the embedded system has been fully integrated. Trace information can also be captured non-intrusively, meaning that the system's timing and performance will not be affected by the trace. Real-time trace also enables developers to more easily capture, record and collect test results for future analysis of bugs.

Summary

While debugging can consume a large number of development resources, embedded systems engineers can choose to take advantage of debugging techniques and products that expedite the process and make it significantly easier to detect and diagnose coding errors in embedded systems.

At Total Phase, we offer industry-leading diagnostic tools like our Beagle USB 480 Protocol Analyzer, a non-intrusive bus monitoring solution that makes it easy to monitor activity and communications on the USB bus and isolate unexpected events. Building a product using SPI or I2C protocols? Our Beagle I2C/SPI Protocol Analyzer is perfect for engineers working in the lab or the field on SPI or I2C-based projects.

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