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High Level Synthesis Architectural Optimization Basics In part 1 of this article we introduced basic filter bank theory and used the Synplify DSP High Level Synthesis (HLS) tool to implement an example filter bank into three alternative architectures. In part 2 we dive deeper into these three architectures to better understand how these filters work. We will also examine the HLS optimizations we applied and the resulting benefits. Example Filter Bank Review Before we proceed, let's quickly review our filter bank example. Our example, shown in Figure 1, is a size 16 DFT filter bank. The color scheme shows the sample rate change where a 16 MHz input sample rate (red) has been chosen and the output sample rate is downsampled by 16 (green).

Rube Goldberg couldn’t have designed a more elegant confluence of convoluted causal relationships.  Start analyzing the perplexing paradox of the FPGA synthesis market and each link of the chain reveals a bizarre force vector that eventually doubles back onto itself into an unlikely equilibrium that miraculously has held stable for a full decade despite disruptive forces of epic proportions. For over a decade now, Synplify has navigated these waters and has continued to survive and thrive through the unlikeliest of conditions.  Now in the hands of EDA giant Synopsys, the Synplify family of FPGA synthesis tools continues to evolve - with a major upgrade this fall.  When you put a digital design into an FPGA, there are two technologies that determine whether your design fits or doesn’t fit, whether it meets your timing constraints or does not, whether the power consumption will be within your limits (or those of the FPGA), or whether it fails completely, leaving your project at the mercy of major mulligans.   Those two technologies are synthesis and place-and-route. 

Have you ever written code that behaves correctly under a simulator only to have intermittent failures in the field? Or maybe your code no longer functions properly when you compile with a newer version of your tool chain. You review your test bench and verify 100 percent complete test coverage and that all tests have passed with no errors--yet the problem stubbornly remains. While designers understandably place great emphasis on coding and simulation, they often have only a nodding acquaintance with the internal workings of the silicon within an FPGA. As a result, incorrect logic synthesis and timing problems, rather than logic errors, are the cause of most logic failures. But writing FPGA code that creates predictable, reliable logic is simple if designers take the right steps. In FPGA design, logic synthesis and related timing closure occur during compilation. And many things, including I/O cell structure, asynchronous logic and timing constraints, can have a big impact on the compilation process, varying results with each pass through the tool chain. Let's take a closer look at ways to eliminate these variances to better and more quickly achieve timing closure.