Transitioning from C/C++ to SystemC in high-level design

John Sanguinetti

May 31, 2010

John SanguinettiMay 31, 2010

In high-level design, high-level code is put through a series of steps on its way to becoming register transfer level (RTL) code. The first step, algorithm design, is usually done in C or C++, where the high-level code that describes how the system will function is created. To be implemented in hardware, this high-level code must be converted to RTL code, using a synthesis tool. It's almost never the case, however, that high-level synthesis using the result of the algorithm design phase will produce a desirable RTL implementation. An architecture design phase that precedes high-level synthesis is required in order to produce RTL code with the desired characteristics.

Making a translation to SystemC for this step has become the preferred high-level design method. In this article, I'll give some examples of steps taken in the architecture design phase that can help you achieve good RTL code.

High-level design has many advantages over the more commonplace design flow that begins with RTL code. Among the most compelling advantages is the improved verification efficiency that a higher level of abstraction offers. It's apparent to the point of being self-evident that when the source code of a design is created, fewer errors occur if the source is at a higher abstraction level than if it is at a lower level. However, a process is still required to verify the transformations that are applied to the design description as it proceeds through the design flow from creation to final realization.

Figure 1 shows the initial steps in the high-level design (HLD) flow. Figure 2 shows the flow with the verification steps and the design loops added. A verification step after each design step can result in a loop back to fix a design error. Another loop after each of the two synthesis steps can return you to the architecture design step, where you'll improve whatever relevant design criteria have not been met. This loop back to the architecture design step is a key part of the high-level design process.

Click on image to enlarge.

Click on image to enlarge.

Architecture design
The architecture design step in Figure 1 is the fundamental "hardware design" step in a high-level design flow. High-level design, particularly when done using "Plain old C," is often presented as a process where the designer simply takes a C algorithm, runs it through a high-level synthesis tool, and gets high-quality RTL. This high-quality result, however, almost never happens.

The architecture design step is where hardware design is done for a fundamental reason. Algorithms developed in C are software representations, and there are very different costs and benefits between hardware and software implementations.

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