gsteemso <
gste...@gmail.com> writes:
> Hi all,
>
> I am teaching myself VHDL, and have reached the limits of Ashenden's "Student's
> Guide to VHDL 2nd ed." It is good at introductory concepts but falls short soon
> thereafter; now that I am trying to look up aspects of the language to implement
> a real design, I keep finding that it glosses over the details I need.
>
Really? I keep referring back to it to *find* the details I haven't used for a while!
> Research online has revealed that resolved logic allows you to drive a signal
> from multiple sources, as with the data bus in the average computer, and
> unresolved logic is considered better for simulation because (a) it's faster and
> (b) it will complain about multiple drivers when you don't want them. Is that
> accurate?
Yes
>
> Unfortunately, the same research reveals that you end up with a hideous stew of
> type conversions when you intermix them in the same design, unless you have
> godlike powers of foresight.
No foresight required: just use the unresolved type throughout
- except on the top-level IOs where you need to tristate the output.
Then, (IIRC) the only time it's a pain is (pre 2008) when you have to
use an IP core which uses resolved vectors. Then an intermediate signal
is required.
And if you want to do a post-synth/PAR simulation as the model they
generate will be resolved types on the IOs. A quick wrapper can fix
that easily though. Or you can choose to use resolved types throughout
your top-level as a half-way house.
> I see that VHDL-2008 allows them to mix more easily, but I rather
> suspect there must be pitfalls to that semantic change that I have not
> found any discussion of yet. Will incautiously written code silently
> permit conflicting signals if some of them are declared resolved and
> some are not?
I'd be surprised, I believe all that happens is an implicit resolved
signal is formed for the connection of an unresolved signal to a
resolved one.
Cheers,
Martin
--
martin.j...@trw.com
TRW Conekt - Consultancy in Engineering, Knowledge and Technology
http://www.conekt.co.uk/capabilities/39-electronic-hardware