Testing software as early as possible in the development cycle is a widely recognized best practice: the sooner a defect is caught, the less costly it is to fix. This is the essence of the shift-left approach.
But when it comes to embedded systems, such as those used in industrial, Internet of Things (IoT), or automotive applications, "early" takes on a unique meaning. Imagine being able to test your electronic control unit (ECU) software as soon as the device is available, even if the environment it will interact with, like a vehicle, factory line, or cloud service, is not yet ready.
That’s where hardware-in-the-loop (HIL) testing comes in. HIL testing is a real-time testing method where embedded software runs on the target hardware as it interacts with a simulated environment that reproduces real-world signals and behavior. This enables system-level validation before physical prototypes are available.
Understanding HIL testing
For embedded devices, you can use HIL testing to test your software application in a simulated and controlled environment, at system level, before moving to production and testing it on the real system. It simulates the real-world environment where the embedded device operates. Depending on the application under test, this environment includes simulated sensors, actuators, and network interfaces in order to interact with the ECU in real time, just like it would in production.
An HIL setup is a test configuration where the real embedded hardware is connected to a simulator that replicates the operating environment with which the hardware will interact. This allows the device under test to exchange realistic signals with simulated sensors, actuators, and networks, enabling system-level validation before the complete real-world system is available.
Imagine you're building a smart thermostat for a connected home system. Your embedded software is almost ready, but the cloud backend is still under development.
With an HIL setup connected to the device under test, you can simulate temperature changes and sensor failures, mock cloud responses, and introduce communication delays. This allows you to validate system behavior in real time, even before the full infrastructure is in place, helping catch bugs before they become customer issues.
The tester typically interacts with the HIL system via a dedicated host PC connected to the HIL simulator over Ethernet. This setup allows for both manual exploratory testing, where the tester can create simulated drive scenarios that closely mirror real-life conditions, and automated testing via dedicated scripts.
The benefits of HIL testing
HIL testing enables validation of correct behavior in edge cases, fault conditions, and safety-critical behaviors that would be too dangerous, expensive, or simply impractical to test in a real system. Think of simulating a sensor failure in a car traveling at highway speeds. That’s not something you would want to test outside of a controlled lab.
HIL testing not only supports the early detection of system-level discrepancies, but also aligns well with continuous integration practices. You can run daily regression tests that assess the impact of recent code changes. Most modern HIL platforms, whether commercial or based on open-source components, can be effectively integrated with popular CI / CD solutions.
The underlying skills for test automation, regression testing design, and failure analysis remain the same across testing domains. Whether you're running tests against an API or an HIL system, you're still asking “What could go wrong?” You design smart tests, interpret failures, and drive quality upstream. In some ways, HIL setups do at the system level what mocks and stubs do at the unit level: they simulate real-world components to test device behavior in a controlled environment. But instead of simulating a function or service call, you're simulating real-world signals, physical interfaces, and full system interactions.
The costs of HIL testing and how to mitigate them
HIL setups come at a cost, and not just in the financial sense. Commercial HIL platforms can be extremely expensive, and even in large organizations, it’s common to see a single HIL system shared across multiple projects and teams. This often becomes a resource bottleneck. Multiple testers or developers may need to use the system at the same time. What's more, testing requires a lot of time because it includes not just execution, but also analysis and debugging of failed runs.
So how do we prevent a single HIL system from becoming the bottleneck of an entire team's testing bandwidth?
One strategic approach is to reserve access to the HIL system for testing of critical regression builds, such as release candidate validation. Meanwhile, for daily testing activities like debugging test logic, writing new test cases, or handling pull-request-level regressions, we can rely on virtualized solutions.
These virtual environments can simulate the embedded device itself (sometimes referred to as software-in-the-loop, or SIL). Depending on testing needs, they can include a virtualized full software stack: operating system, controller area network (CAN) or Ethernet interfaces, and digital and analog inputs and outputs. The simulations drive signals to and from the virtualized ECU, allowing the system to verify how it behaves under realistic conditions, all from a standard laptop.
By complementing your HIL setup with these lightweight virtualized test environments, your team can reduce the demand on your HIL resource, accelerate early feedback, and adopt a more balanced testing architecture.
Open-source tools make HIL testing widely accessible
A promising trend is the growth of communities working to democratize HIL testing through open-source tools and frameworks. These ecosystems are expanding fast, providing cost-effective platforms that support test automation and CI / CD practices while also enabling interaction with real hardware.
This movement is helping small and mid-sized teams, and even some large corporations, to build lean, flexible HIL setups. In fact, even DIY solutions with open-source tools can be powerful, especially when the budget is tight. It's no longer just about million-dollar labs. It's about smarter, more accessible testing infrastructure that scales with your needs.
In my experience, even the most basic HIL setups, commercial or home-built, have helped uncover edge cases and timing issues that wouldn’t show up in software-only testing that relies solely on unit and integration testing. Validating functionality and behavior at the system level before integration with the real environment has saved my product teams time, effort, and quite a few hard-to-trace bugs.
To wrap up
Here are the major points we have covered:
- HIL testing enables system-level validation in a simulated environment, even before real-world components like vehicles, cloud services, or industrial equipment are available
- You can simulate edge cases, failures, and dangerous conditions in a safe and repeatable way, supporting fault injection and safety verification
- A hybrid testing strategy, combining HIL and virtualized solutions, helps optimize limited resources and reduce test bottlenecks
- Modern HIL systems can integrate into CI / CD pipelines, making daily regression testing and automation possible even for embedded systems
- Open-source HIL testing tools are gaining traction, making HIL testing accessible to teams and organizations of many sizes and budgets
If you work with embedded software, whether in industrial automation, IoT, or any other domain, it’s worth rethinking how system-level testing fits into your Agile development strategy. Start by evaluating which parts of your current process could be shifted earlier in the cycle, and whether HIL or virtualized environments can help you catch issues before they reach the real world.
Reserve HIL for your critical regression tests, and offload daily feedback loops to faster, more scalable virtual setups. And if you're looking for inspiration, explore what the open-source community is building. You might find exactly what you need without breaking the budget.
