[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.

Sure.

MyHDL defines a number of "event" objects that you can "wait" on using
a yield statement, such as a delay, edges and also signals. (Waiting on
a signal is defined as waiting on a signal value change.) A list of
signals is *not* such an event object.

To specify the sensitivity "list", the syntax with the least amount
of overhead is convenient:

    yield a, b, c

As per the Python definition, this is equivalent to:

    yield (a, b, c)

that is, yielding a tuple.

Often, when a tuple is supported, it is trivial to support a list also,
and that is what MyHDL does. So it is in this indirect way that you
can use a list of signals (or other event objects) as a sensitivity list.

Remaining question: does this have applications? Well, I added list support
when I needed it during a real modelling project. Actually, the concept of
a "dynamic sensitivity list" is, I believe, a unique feature of MyHDL. You
can build lists (and tuples) dynamically, based on dynamic control info,
and use them as a sensitivity list that will work as expected.

I haven't document of advertized this feature though, because I don't yet
know how significant it is.

Regards, Jan

-- 
Jan Decaluwe - Resources bvba - http://jandecaluwe.com
Losbergenlaan 16, B-3010 Leuven, Belgium
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