58 lines
2.7 KiB
Plaintext
58 lines
2.7 KiB
Plaintext
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Synthesis
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#########
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Synthesis is the process of converting input Verilog file into a netlist,
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which describes the connections between different block available on the
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desired FPGA chip. However, it is worth to notice that these are only
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logical connections. So the synthesized model is only a draft of the final
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design, made with the use of available resources.
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RTL Generation
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==============
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the input Verilog file is often really complicated. Usually it is written in
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a way that it is hard to distinguish the digital circuit standing behind
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the implemented functionality. Designers often use a so-called
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*Behavioral Level* of abstraction, in their designs, which means that the whole
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description is mostly event-driven. In Verilog, support for behavioral models
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is made with use of ``always`` statements.
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However, FPGA mostly consist of Look Up Tables (LUT) and flip-flops.
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Look Up Tables implement only the functionality of logic gates.
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Due to that, the synthesis process has to convert the complicated
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Behavioral model to a simpler description.
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Firstly, the design is described in terms of registers and logical operations.
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This is the so-called *Register-Transfer Level* (*RTL*).
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Secondly, in order to simplify the design even more, some complex logic is
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rewritten in the way that the final result contain only logic gates
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and registers. This model is on *Logical Gate level* of abstraction.
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The process of simplification is quite complicated, because of that it often
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demands additional simulations between mentioned steps to prove that the input
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design is equivalent to its simplified form.
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Technology mapping
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==================
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FPGAs from different architectures may have different architecture. For example,
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they may contain some complicated functional blocks (i.e. RAM, DSP blocks)
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and even some of the basic blocks like LUT tables and flip-flops may vary
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between chips. Because of that, there is a need to describe the final design
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in terms of platform-specific resources. This is the next step in the process
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of synthesis. The simplified description containing i.e. logic gates, flip-flops
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and a few more complicated blocks like RAM is taken and used "general" blocks
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are substituted with that physically located in the chosen FPGA.
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The vendor-specific definitions of these blocks are often located
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in a separate library.
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Optimization
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============
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Optimization is the key factor that allows to better utilize resources
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of an FPGA. There are some universal situations in which the design
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can be optimized, for example by substituting a bunch of logic gates
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in terms of fewer, different gates. However, some operations can be performed
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only after certain steps i.e. after technology mapping.
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As a result, optimization is an integral part of most of the synthesis steps.
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