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basic vhdl queries

 
 
sundar
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Posts: n/a
 
      11-25-2008
hi all,

1) how does it matter for a cpld when i declare a constant or signal
2) what is the difference if i mention default value while
declaration /without declaration
3)
difference between below piece of tristate code
output <= a when enable= '1' else ''z';
and using primitive
tri port map
(
a_in <= a,
oe <= enable,
a_out <= output
)

which is better why?
4) output <= a when (expression1)
else b when (expr2)
else....
...
else n when (exprn)

what is the diff if this combo logic is inside a process block

5) whats the diff between
if ( a and b and c and d)
case x
1:
2:
3:
4:
default

and

if (a and b and c and d and x1)
elseif (a and b and c and d and x2)
elseif (a and b and c and d and x3)
elseif (a and b and c and d and x4)
else

note that internal case x is used as x1,x2,x3 and x4 respectively

pls share your thots

rgs,
sundar
 
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Tricky
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Posts: n/a
 
      11-25-2008
On 25 Nov, 12:17, sundar <(E-Mail Removed)> wrote:
> hi all,
>
> 1) how does it matter for a cpld when i declare a constant or signal
> 2) what is the difference if i mention default value while
> declaration /without declaration
> 3)
> difference between below piece of tristate code
> output <= a when enable= '1' else ''z';
> and using primitive
> tri port map
> (
> a_in <= a,
> oe <= enable,
> a_out <= output
> )
>
> which is better why?
> 4) output <= a when (expression1)
> * * else b when (expr2)
> * * else....
> * * ...
> * * else n when (exprn)
>
> * * what is the diff if this combo logic is inside a process block
>
> 5) whats the diff between
> if ( a and b and c and d)
> * * case x
> * * 1:
> * * 2:
> * * 3:
> * * 4:
> * * default
>
> and
>
> if (a and b and c and d and x1)
> elseif (a and b and c and d and x2)
> elseif (a and b and c and d and x3)
> elseif (a and b and c and d and x4)
> else
>
> note that internal case x is used as x1,x2,x3 and x4 respectively
>
> pls share your thots
>
> rgs,
> sundar


Oh dear, I detect homework.

Its a shame that this homework seems to be about 10+ years out of
date.
 
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KJ
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Posts: n/a
 
      11-25-2008
On Nov 25, 7:17*am, sundar <(E-Mail Removed)> wrote:
> hi all,
>
> 1) how does it matter for a cpld when i declare a constant or signal


cplds get quite upset about declaring constants, they think they rule
the universe and so they try to destroy all constants. fpgas on the
other hand, being a bit more advanced are quite tolerant of
constants...in regards to signals, they both like to signal ahead, but
occasionally signal to the left or right.

> 2) what is the difference if i mention default value while
> declaration /without declaration


If you mention a default value without a declaration then it's a
secret.

> 3)
> difference between below piece of tristate code
> output <= a when enable= '1' else ''z';
> and using primitive
> tri port map
> (
> a_in <= a,
> oe <= enable,
> a_out <= output
> )
>


The first piece has a syntax error...you find it.

> which is better why?


The second one is better, because it has no syntax errors. I consider
that to be a major plus, others may disagree. There have been several
heated arguments in this forum about the advantages and disadvantages
of correct syntax code, I hope my meer mention of the topic doesn't
ignite yet another flame war. Google for it.

> 4) output <= a when (expression1)
> * * else b when (expr2)
> * * else....
> * * ...
> * * else n when (exprn)
>
> * * what is the diff if this combo logic is inside a process block
>


Inside a process block this combo logic won't compile, again a syntax
error (see above comments).

I've also used "..." in the past, but have usually found that I have
to replace it with coherent logic before the design is done. I've
never tried the "...." approach, looks interesting.

> 5) whats the diff between
> if ( a and b and c and d)
> * * case x
> * * 1:
> * * 2:
> * * 3:
> * * 4:
> * * default
>
> and
>
> if (a and b and c and d and x1)
> elseif (a and b and c and d and x2)
> elseif (a and b and c and d and x3)
> elseif (a and b and c and d and x4)
> else
>
> note that internal case x is used as x1,x2,x3 and x4 respectively
>


Note also that neither one will pass a basic syntax check.

Note further that mutually exclusive things as expressed in the first
code are not the same as priority encoding as expressed in the second
code. Personally, I like mutually exclusive things that cannnot
overlap, it just bothers me no end when the gravy from my mashed
potatoes encroaches on the veggies destroying the exclusivity that I
tried to set up. In those situations, I'm forced to use a defined
priority and eat the veggies before the gravy encroaches even though
my preference is for the taters.

> pls share your thots
>


I'm all out of thots.

> rgs,
> sundar


rgs back atcha

KJ
 
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M. Norton
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Posts: n/a
 
      11-26-2008
On Nov 25, 4:11*pm, Brian Drummond <(E-Mail Removed)>
wrote:
> >rgs back atcha

>
> What is it with all these unreadable abbreviations young folks are using
> today, anyway?


I'm puzzled too. Not even sure what rgs was supposed to be. Best I
could come up with was "root green square" which is nonsensical, but
had a pleasant sound to it .

Mark
 
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pankaj.goel pankaj.goel is offline
Junior Member
Join Date: Nov 2008
Posts: 8
 
      11-27-2008
Hi friends...what a cool stuff on VHDL forum...But don't u think it is too much... u r getting to critical...
anyways enjoy urself...
 
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KJ
Guest
Posts: n/a
 
      11-27-2008

"Brian Drummond" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Tue, 25 Nov 2008 07:27:05 -0800 (PST), KJ <(E-Mail Removed)>
> wrote:
>
>>> pls share your thots
>>>

>>
>>I'm all out of thots.
>>
>>> rgs,
>>> sundar

>>
>>rgs back atcha

>
> What is it with all these unreadable abbreviations young folks are using
> today, anyway?
>


Maybe sundar just couldn't afford to purchase vowels. In any case, I just
sent the 'rgs' back to him in case he needed them...whatever they
are...maybe they were 'regards'....or 'rugs'.

> 73's OM,
> - Brian


Abbreviations like "OM"?? Hmmmm

KJ


 
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