Re: [myhdl-list] Re: inference problem?

[Haitao Z. <ha...@gm...>]

Jan,
Thanks! You spotted the problem!

> Yes, I infer that you didn't simulate this :-)

Right I didn't on this one. I want to build a model of my project in
myhdl and before doing it I  wanted to be sure that it can be
converted so I am experimenting a bit to see how things go. I did try
some other sims just not thought of doing it here. As you can tell I
am new to myhdl, and also pretty new to Python. Thanks for all the
good advice.

Haitao

> 
> Here's the problem: your intention is that q1 is a Signal, but it isn't.
> In fact, in MyHDL:
> 
>    q1 = Signal(intbv(0))[n:]
> 
> is equivalent to:
> 
>    q1 = Signal(intbv(0)).val[n:]
> 
> that is, you get a slice of the current underlying value, which is an
> intbv and not a signal.
> 
> What you want is:
> 
>    q1 = Signal(intbv(0)[n:])
> 
> In this case, you get a signal with an intbv (with a defined bit width)
> as its underlying value.
> 
> The reason why conversion currently behaves like it does has to do
> with namespace lookups. Perhaps more could be done to detect flaws,
> BUT in a Python context this is an upfront battle. You really should
> use the run-time (= simulation) to detect errors. I have written
> about this before:
> 
> (from the manual, section 6.5.1)
> """
> In the Python philosophy, the run-time rules. The Python compiler
> doesn't attempt to detect a lot of errors beyond syntax errors, which
> given Python's ultra-dynamic nature would be an almost impossible task
> anyway. To verify a Python program, one should run it, preferably
> using unit testing to verify each feature.
> 
> The same philosophy should be used when converting a MyHDL description
> to Verilog: make sure the simulation runs fine first. Although the
> converter checks many things and attempts to issue clear error
> messages, there is no guarantee that it does a meaningful job unless
> the simulation runs fine.
> """
> 
> In other words, if the simulation works fine, my goal is to guarantee
> that the conversion (if it succeeds) is correct. Otherwise, all bets
> are off (even if conversion succeeds).
> 
> Note that in this case, a trivial test bench would detect the error,
> as q1 in your code cannot have a next attribute.
> 
> Miscellaneous remark: please don't use "private" attributes such as
> _nrbits. An intbv supports the 'len' function (which returns 0 for
> undefined bit widths.)
> 
> Regards, Jan
> 
> --
> Jan Decaluwe - Resources bvba - http://jandecaluwe.com
> Losbergenlaan 16, B-3010 Leuven, Belgium
>     Using Python as a hardware description language:
>     http://jandecaluwe.com/Tools/MyHDL/Overview.html
> 
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