How do they make Verilog code for unknown ICs?

Swift Griggs swiftgriggs at
Mon Jun 20 15:56:52 CDT 2016

On Mon, 20 Jun 2016, Paul Koning wrote:
> used to how C or similar languages work.  For example, in this C code:
> 	a = 1;
> 	b = a;
> a and b will both equal 1 at the end.  But in the VHDL code:
> 	a <= 1;
> 	b <= a;

Whoa. That makes total sense, though. In the real world, I'm guessing the 
"less than" just reflects that a signal might not have the level you 

> But it fits hardware, where signals have to propagate and new things 
> happen as a result of previous actions at previous points in time.

It makes me wonder what kind of person had to figure out all those effects 
and enshrine them in a deterministic language. That sounds very 
challenging. Hats off to them.

> VHDL and Verilog can be used to model hardware operation; they can also 
> be used to describe hardware.  These are not quite the same.

I've got a tab in my browser with an article about the differences. I like 
strongly typed languages, so I'd probably start with VHDL if I can get 
that far.

> A model can, for example, talk about actual delays.  A hardware 
> description does not; such a "synthesizable" model is a subset of the 
> full language.

Ah, okay. I had wondered what that meant when Ian had mentioned it.

> This is a common way to design what goes into an FPGA.  A hardware model 
> can be used to replicate what old hardware did; for example, I have a 
> partial CDC 6600 model that shows how it boots, and that model includes 
> propagation delays on some signals (which are critical to correct 
> operation in certain spots).

Wow. All I can say is "I wish I could do something like that, too." :-)

> For example, if all you have is a complex IC spec sheet, it is likely to 
> be rather difficult.  If you have internals, it becomes more feasible.

Hehe, I keep up this page a lot:
 It's concerning how to read spec sheets. 

... because reading these sheets is about like reading a man page for 
non-UNIX users, it's tough at first, but then becomes very valuable!

> There are plenty of textbooks on the topic.  I would recommend the 
> (large) book by Peter Ashenden on VHDL.  He also has a book on Verilog; 
> given how he treated VHDL I expect that one is good too but I don't have 
> it.

I'm not there yet, but I've bookmarked a copy at Half-Price Books and I'll 
come back to it if I make it through my digital logic course with brain 
cells to spare. 


PS: Thanks to you and Ian for the nice explanations and answers. 

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