Re: gEDA: Icarus NetAssign, NetAssignNB

[Steven Wilson <stevew-at-NOSPAMintrinsix_dot_com>]

I may have completely mis-understood Hendrik's question -

I'm likely not able to answer what I now perceive the question. Let me 
restate it just for clarities sake and Hendrik can say yes or no.

He asking about the use of NetAssign/NetAssignNB within icarus and the 
followup was a question whether blocking/non-blocking might be their use?

Hendrik - does that capture the question correctly?

Steve

Steven Wilson wrote:

> I'll take this - Steve isn't around for the moment.
>
> This is one of those very basic confusions of verilog -
>
> Two examples - one of each:
>
> reg a, b,c ;
>
> // Assume that we start at before the clock with a = 1, b = 0, c = 0.
>
> always @ ( posedge clk)  // Blocking
>  begin
>       b = a;
>       c = b;
>   end
>
> always @(posedge clk) // Non-blocking
>  begin
>    b <= a;
>    c <= b;
>  end
>
> So what is the difference -   after the rising clock edge occurs -
>
> Blocking: a = 1, b = 1, c = 1.
> Non-blocking a = 1, b = 1, c = 0.
>
> Why?
>
> Well - blocking means wait till the current step is completed before 
> moving to the next statement. Non-blocking means perform all 
> statements in parallel. 
> Applying these definitions to the examples we find that in the 
> blocking code. First the value of a is assigned to b, then the value 
> of b is assigned to c. Thus the value of a propagates to both of the 
> other variables.
>
> In the non-blocking case, the value of a is ONLY assigned to the first 
> statement, while the current value of b is assigned to c SIMULTANEOUSLY.
>
> As a coding convention - a way to keep out of trouble is to use 
> Blocking ONLY for combinational logic (think muxes..) while you only 
> use non-blocking for describing registered logic.  So anywhere you 
> have a "posedge/negedge" in the always statement - no blocking code.
>
> Hope this helps.
>
> Steve
>
>
>
>
>
> Hendrik Greving wrote:
>
>> Hendrik Greving wrote:
>>
>>> Hi,
>>>
>>> I'm Hendrik, new to this group.. Currently, I'm working with Icarus 
>>> and modifying it according my purpose. I've got a short question, 
>>> could anybody tell me, what's the difference between above?
>>>
>>> Regards,
>>> Hendrik
>>>
>>>
>> blocking/non-blocking?
>>
>> Regards,
>> Hendrik Greving
>
>
>