RTL Quality Effect on Synthesis

How High-Quality RTL Impacts Synthesis: A Closer Look

In this post, we’re diving into how the quality of your RTL (Register Transfer Level) design directly affects the synthesis process. Think of it this way: the better your RTL, the smoother and more efficient the synthesis will be.

When your RTL is written with precision and offers the flexibility to adapt to design requirements, it results in a well-optimized synthesized netlist. A solid RTL foundation means fewer headaches down the road and a circuit design that aligns perfectly with your goals.

Let’s break this down with some examples. Below, we’ll look at two pieces of code and their corresponding circuits to see how this plays out in action.

Example 1:

SUM1 = A + B;
SUM2 = C + D;

if (SEL == 1’b1)
SUM = SUM1;
else
SUM = SUM2;
end

 

Example 2:

if (SEL == ‘1’) then
SUM <= A + B;
else
SUM <= C + D;
endif;

Key Takeaways from the Two RTL Coding Styles

From the examples above, it’s clear that the RTL coding style has a significant impact on the resulting circuit design.

• First Coding Style:
  In this approach, SUM1 and SUM2 are explicitly defined. This rigid definition prevents the synthesis tool from re-mapping or optimizing the circuit structure during the process. While this ensures consistency, it also limits flexibility and may lead to suboptimal results in terms of area and power consumption.
• Second Coding Style:
  Here, the conditions allow the synthesis tool the flexibility to optimize the circuit as needed. This approach enables the tool to generate an efficient circuit that minimizes cell area and power consumption. The tool intelligently selects the best option, which, in this case, aligns with the second circuit example—an optimized design that prioritizes resource efficiency.

Why Flexible RTL Coding Matters

By using a flexible coding style, we avoid potential issues such as:

1. High Cell Area and Power Consumption:
   Large cells like adders and subtractors can lead to congestion in later design stages if they are densely packed in the same area.
2. Congestion in Further Stages:
   A rigid design may result in bottlenecks during placement and routing, increasing overall complexity.

With a flexible RTL coding style, the synthesis tool has the freedom to explore optimized configurations, ensuring an efficient design that conserves power and reduces congestion. Ultimately, this approach leads to better results and a more robust synthesized netlist.