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Comment by hyperman1 | Hacker News Reader

The different would be that it is never possible to have an unsynthesable design, as long as everything connects together. So the problem designed in the article can't happen if you only have blocks and wires.

Maybe a rephrased question is, what causes Verilog or VHDL to be unsynthesable if they are based on this style of block design

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There are lots of reasons why some HDL could be unsynthesizable:

- multi-driven nets where two or more signals are driving the same node

- the target not supporting certain features like multi-clocked FFs

- missing components or libraries (not uncommon if you're trying to use VHDL 2008, for example)

- timing analysis failing, meaning the HDL isn't fast enough / the critical path is too long for how fast the fpga is going to go

And while not necessarily being unsynthesizable, a lot of tools like Vivado will heavily optimize the HDL, meaning if you forget to connect a part of the HDL to the output or do something with it, it will be removed from the design.

Because the logic inside blocks can contain constructs that do stuff that aren't expressible as physical hardware, but are useful for other purposes/otherwise expressible in Verilog (some people think it's an anti-feature such things are even possible to express, but it's open to debate). You still have to implement the content of the blocks somehow, be it grabbing it from a library or writing the logic yourself. I didn't mean to imply that Verilog/VHDL are purely piecing together functional blocks. It's just one of the main abstractions to allow you to compartmentalize, like implementing functions that you can reuse in a traditional programming language. The other reply also goes into some good examples.

You could of course only build on top of very simple blocks that are trivially correct like say piecing together logic gates and other very simple circuit elements, but that kinda defeats the purpose of using an HDL and doesn't scale well. Some of the blocks you get included with your development tools might be quite complex, e.g. entire complex DSP operations that are designed to exploit specific hardware features of your target.

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