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walala 09-08-2003 11:28 PM

Are all the signals read in the process should appear in the sensitivity list of the process?
 
Dear all,

I have the following questions about sensitivity list of the process:

1. In the first one, the modelsim simulator says that "Synthesis
WARNING: count is read in the process but not in the sensitivity
list"...

But I read a lot of online tutorials, they do not put count in the
sensitivity list
--------------------------------
p0: PROCESS(rst, clk)
BEGIN
if rst = '1' then
count <= "000";
elsif (clk'event and clk = '1') then
count <= count + '1';
end if;
END PROCESS p0;

---------------------------------


2. This one is a totally combinatory logic -- a MUX, but do I need to
put so many signals in my senstivity list? (where count is dertermined
by the above counter, t1, t2, ... t23 were computed before as
temporary signals...)

p1: PROCESS(count, t1, t2, t3, t4, t5, t6, t7, t8, t9, t12, t13,
t14, t15, t16, t17, t18, t19, t20, t21, t22, t23)
BEGIN
case count is
when "000" =>
temp1<=t1+t6+t22;
temp2<=t2+t12;
temp3<=t3+t13;
temp4<=t4+t14;
temp5<=t5+t15;

when "001" =>
temp1<=t1+t7+t23;
temp2<=t2+t13;
temp3<=t3+t16;
temp4<=t4+t17;


....

Thanks a lot,

-Walala

Allan Herriman 09-09-2003 12:49 AM

Re: Are all the signals read in the process should appear in the sensitivity list of the process?
 
On 8 Sep 2003 16:28:16 -0700, mizhael@yahoo.com (walala) wrote:

>Dear all,
>
>I have the following questions about sensitivity list of the process:
>
>1. In the first one, the modelsim simulator says that "Synthesis
>WARNING: count is read in the process but not in the sensitivity
>list"...
>
>But I read a lot of online tutorials, they do not put count in the
>sensitivity list
>--------------------------------
> p0: PROCESS(rst, clk)
> BEGIN
> if rst = '1' then
> count <= "000";
> elsif (clk'event and clk = '1') then
> count <= count + '1';
> end if;
> END PROCESS p0;
>
>---------------------------------



The template from the online tutorial is right and Modelsim is wrong.
I suggest you turn off the "CheckSynthesis" option in your
modelsim.ini file, as it results in misleading warnings like this one.


>2. This one is a totally combinatory logic -- a MUX, but do I need to
>put so many signals in my senstivity list? (where count is dertermined
>by the above counter, t1, t2, ... t23 were computed before as
>temporary signals...)
>
> p1: PROCESS(count, t1, t2, t3, t4, t5, t6, t7, t8, t9, t12, t13,
>t14, t15, t16, t17, t18, t19, t20, t21, t22, t23)
> BEGIN
> case count is
> when "000" =>
> temp1<=t1+t6+t22;
> temp2<=t2+t12;
> temp3<=t3+t13;
> temp4<=t4+t14;
> temp5<=t5+t15;
>
> when "001" =>
> temp1<=t1+t7+t23;
> temp2<=t2+t13;
> temp3<=t3+t16;
> temp4<=t4+t17;
>



Yes, you need this many signals in the sensitivity list. Yes, this is
a PITA (lots of typing for little value, hard to understand, easy to
make mistakes that describe latches). This feature of the language
may change in VHDL-200x, and there will be a magic way of indicating
that all signals on the right hand side of assignment statements are
automatically in the sensitivity list.
(Verilog grew a similar feature in its 2001 release, for similar
reasons.)

Presumably the signal "temp" is used as an input to a flip flop. In
this case, you could put all the logic that drives temp inside a
clocked process, and the sensitivity list would only need to contain
clock and reset.

Regards,
Allan.

walala 09-09-2003 06:10 AM

Re: Are all the signals read in the process should appear in the sensitivity list of the process?
 
Hi Allan,

Thank you very much for your posting...

But I don't understand the comment you made in the last paragraph:

>
> >2. This one is a totally combinatory logic -- a MUX, but do I need to
> >put so many signals in my senstivity list? (where count is dertermined
> >by the above counter, t1, t2, ... t23 were computed before as
> >temporary signals...)
> >
> > p1: PROCESS(count, t1, t2, t3, t4, t5, t6, t7, t8, t9, t12, t13,
> >t14, t15, t16, t17, t18, t19, t20, t21, t22, t23)
> > BEGIN
> > case count is
> > when "000" =>
> > temp1<=t1+t6+t22;
> > temp2<=t2+t12;
> >
> > when "001" =>
> > temp1<=t1+t7+t23;
> > temp2<=t2+t13;
> >

>
> Presumably the signal "temp" is used as an input to a flip flop. In
> this case, you could put all the logic that drives temp inside a
> clocked process, and the sensitivity list would only need to contain
> clock and reset.
>


t1, t2, t3, ..., t23 are computed before the "p1" process:

t1<=32*X(0);
t2<=28*X(1);
....

where X(0), X(1), etc. are my inputs...

The "temp"s are computed after the "p1" process:

Y(0)<=temp1+temp2;
Y(1)<=temp1-temp2;
....
where Y(0), Y(1), etc. are my outputs...

I did not put the assignments to t1, t2, ..., t23s into process and clock
it; and I also did not put the assignments to Y(0), Y(1), etc. into clocked
process either...

I had thought about that: I can put assignments to t's and assignments to
Y's into clocked processes and also add a "Reset" to the processes...in this
way all my circuit will be clocked:

a t-assignment process, a counter process, a "temp"-assignment process based
on counter, and a final Y-assignment process... all are clocked...

My question is: which is the best? The clocked process approach? Or
non-clocked t and Y-assignments?

Can you clarify a little for me?

Thanks a lot,

-Walala



Allan Herriman 09-09-2003 07:47 AM

Re: Are all the signals read in the process should appear in the sensitivity list of the process?
 
On Tue, 9 Sep 2003 01:10:53 -0500, "walala" <mizhael@yahoo.com> wrote:

>Hi Allan,
>
>Thank you very much for your posting...
>
>But I don't understand the comment you made in the last paragraph:
>
>>
>> >2. This one is a totally combinatory logic -- a MUX, but do I need to
>> >put so many signals in my senstivity list? (where count is dertermined
>> >by the above counter, t1, t2, ... t23 were computed before as
>> >temporary signals...)
>> >
>> > p1: PROCESS(count, t1, t2, t3, t4, t5, t6, t7, t8, t9, t12, t13,
>> >t14, t15, t16, t17, t18, t19, t20, t21, t22, t23)
>> > BEGIN
>> > case count is
>> > when "000" =>
>> > temp1<=t1+t6+t22;
>> > temp2<=t2+t12;
>> >
>> > when "001" =>
>> > temp1<=t1+t7+t23;
>> > temp2<=t2+t13;
>> >

>>
>> Presumably the signal "temp" is used as an input to a flip flop. In
>> this case, you could put all the logic that drives temp inside a
>> clocked process, and the sensitivity list would only need to contain
>> clock and reset.
>>

>
>t1, t2, t3, ..., t23 are computed before the "p1" process:
>
>t1<=32*X(0);
>t2<=28*X(1);
>...
>
>where X(0), X(1), etc. are my inputs...
>
>The "temp"s are computed after the "p1" process:
>
>Y(0)<=temp1+temp2;
>Y(1)<=temp1-temp2;
>...
>where Y(0), Y(1), etc. are my outputs...
>
>I did not put the assignments to t1, t2, ..., t23s into process and clock
>it; and I also did not put the assignments to Y(0), Y(1), etc. into clocked
>process either...
>
>I had thought about that: I can put assignments to t's and assignments to
>Y's into clocked processes and also add a "Reset" to the processes...in this
>way all my circuit will be clocked:
>
>a t-assignment process, a counter process, a "temp"-assignment process based
>on counter, and a final Y-assignment process... all are clocked...
>
>My question is: which is the best? The clocked process approach? Or
>non-clocked t and Y-assignments?
>
>Can you clarify a little for me?


It sounds like your entire design is combinatorial from input 'X' to
output 'Y'. Is that right? If so, please ignore my comment about
flip flops.

....
Yet another way of avoiding large sensitivity lists is to use
concurrent statements instead of processes. Look up the syntax for a
concurrent assignment statement in a text book for ideas.

Regards,
Allan.


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