Software-Only Hardware Simulation

Simulating embedded hardware in a Windows environment can significantly reduce development time. In this article, Michael Melkonian provides techniques for the software-only simulation of embedded hardware. He presents a simple example of an RTOS-less embedded system that uses memory-mapped I/O to access a UART-like peripheral to serially poll a slave device. The simulator is capable of detecting bugs and troublesome design flaws.

Melkonian writes:

In this article, I will describe techniques for the software-only simulation of embedded hardware in the Windows/PC environment. Software-only simulation implies an arrangement with which the embedded application, or parts of it, can be compiled and run on the Windows platform (host) talking to the software simulator as opposed to the real hardware. This arrangement doesn’t require any hardware or tools other than a native Windows development toolset such as Microsoft Developer Studio/Visual C++. Importantly, the same source code is compiled and linked for both the host and the target. It’s possible and often necessary to simulate more complex aspects of the embedded target such as interrupts and the RTOS layer. However, I will illustrate the basics of simulating hardware in the Windows environment with an example of an extremely simple hypothetical target system (see Figure 1).

Figure 1: There is a parallel between the embedded target and host environment. Equivalent entities are shown on the same level.
Figure 1: There is a parallel between the embedded target and host environment. Equivalent entities are shown on the same level.

Assuming that the source code of the embedded application is basically the same whether it runs in Windows or the embedded target, the simulation offers several advantages. You have the ability to develop and debug device drivers and the application before the hardware is ready. An extremely powerful test harness can be created on the host platform, where all code changes and additions can be verified prior to running on the actual target. The harness can be used as a part of software validation.

Furthermore, you have the ability to test conditions that may not be easy to test using the real hardware. In the vast majority of cases, debugging tools available on the host are far superior to those offered by cross development tool vendors. You have access to runtime checkers to detect memory leaks, especially for embedded software developed in C++. Lastly, note that where the final system comprises a number of CPUs/boards, simulation has the additional advantage of simulating each target CPU via a single process on a multitasking host.

FIRST THINGS FIRST
Before you decide to invest in simulation infrastructure, there are a few things to consider. For instance, when the target hardware is complex, the software simulator becomes a fairly major development task. Also, consider the adequacy of the target development tools. This especially applies to debuggers. The absence, or insufficient capability, of the debugger on the target presents a strong case for simulation. When delivery times are more critical than the budget limitations and extra engineering resources are available, the additional development effort may be justified. The simulator may help to get to the final product faster, but at a higher cost. You should also think about whether or not it’s possible to cleanly separate the application from the hardware access layer.

Remember that when exact timings are a main design concern, the real-time aspects of the target are hard to simulate, so the simulator will not help. Moreover, the embedded application’s complexity is relatively minor compared to the hardware drivers, so the simulator may not be justified. However, when the application is complex and sitting on top of fairly simple hardware, the simulator can be extremely useful.

You should also keep in mind that when it’s likely that the software application will be completed before the hardware delivery date, there is a strong case for simulation …

SOFTWARE DESIGN GUIDE
Now let’s focus on what makes embedded software adaptable for simulation. It’s hardly surprising that the following guidelines closely resemble those for writing portable code. First, you need a centralized access mechanism to the hardware (read_hw and write_hw macros). Second, the application code and device driver code must be separated. Third, you must use a thin operating level interface. Finally, avoid using the nonstandard add-ons that some cross-compilers may provide.

Download the entire article: M. Melkonian, “Software-Only Hardware Simulation,” CIrcuit Cellar 164, 2004.