[myhdl-list] Re: always_comb / signal array

[Jan D. <ja...@ja...>]

David Brochart wrote:
> Selon David Brochart <dav...@fr...>:
> 
> 
>>It seems that "always_comb" doesn"t work when an input is an array of
>>signals,
>>while it works fine with the "while 1 / yield" structure.
>>
>>David.
>>
> 
> 
> 
> Actually, according to previous threads I read, I am not sure if it should even
> work with the "while 1 / yield" structure, because generators only deal with
> signals, not lists of signals.
> Jan, could you clarify this point?

Sure.

MyHDL has a number of objects on which you can "wait" using a yield
statements, such as a delay, edges and also signals. (Waiting on
as signal is defined as waiting on a signal value change.)
A list of signals is *not* one of these objects.

To specify the sensitivity "list" in the yield clause, the syntax
with the smallest overhead is convenient:

    yield a, b, c, d

As per the Python definition, this is equivalent to:

    yield (a, b, c, d)

that is, yielding a tuple.

As often, when a tuple is supported, it is trivial to support a list
also, and that's what MyHDL does. So it is in this indirect way that
you can use a list of signals (or any other "event" object) as a
sensitivity list in the yield clause.

Remaining question: does all this have applications? Well, I added
list support for a real modelling project. In fact, the concept of
a "dynamic sensitivity list" is, I believe, a unique feature of
MyHDL. You can build your sensitivitly lists (and tuples) dynamically,
depending on dynamic control info, and it will all work as expected.
I haven't explicitly documented or advertized this feature though,
because I don't yet know how significant it actually is.

Regards, Jan

-- 
Jan Decaluwe - Resources bvba - http://jandecaluwe.com
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