feedback input with combinational logic! 
Author Message
 feedback input with combinational logic!

hi everyone!
I'm very confuse to use feedback input with combinational logic.

assign a = sel ? b : a ;

I want to know whether this assign statement has no problem.
If there is any problem, please let me know.

thank you!



Sun, 23 Mar 2003 03:00:00 GMT  
 feedback input with combinational logic!

Quote:

> hi everyone!
> I'm very confuse to use feedback input with combinational logic.

> assign a = sel ? b : a ;

> I want to know whether this assign statement has no problem.

There is nothing in the language that says that you can't, but some
tools will probably freak out on this one. This is certainly no way to
make a latch, though.

This should oscillate when sel == 0:

        assign #5 a = sel ? b : ~a;

--
Steve Williams                "The woods are lovely, dark and deep.


http://www.picturel.com       And lines to code before I sleep."



Sun, 23 Mar 2003 03:00:00 GMT  
 feedback input with combinational logic!
Are you latching "a" to some clock edge after that?

Like

assign a = sel ? b : aa ;


        aa <= a;

If so, then I don't see any problem.

Note: I have changed the "assign" statement a little bit.

SH



Quote:
> hi everyone!
> I'm very confuse to use feedback input with combinational logic.

> assign a = sel ? b : a ;

> I want to know whether this assign statement has no problem.
> If there is any problem, please let me know.

> thank you!

Sent via Deja.com http://www.deja.com/
Before you buy.


Sun, 23 Mar 2003 03:00:00 GMT  
 feedback input with combinational logic!

Quote:

> hi everyone!
> I'm very confuse to use feedback input with combinational logic.

> assign a = sel ? b : a ;

> I want to know whether this assign statement has no problem.
> If there is any problem, please let me know.

yes you can get away with this to implement things like simple
latches - I've seen extracted netlists end up having these structures.

However I personally think they're not that good an idea for example
there's a trap here - if you are using them to create
thing like master-slave flops - ie I've seen people create
structures like:

        assign m_q = (~clk ? d : m_q);
        assign s_q = (clk ? s_q : m_q);

and then complain that they don't work on both VCS and XL (frankly they
were lucky to get them to work on one - and the occasional hand
inserted delay  sprinkled through their code showed they had to
work at that).

In short just like a real world flop you need some internal
delay to make it work - it's best to avoid this sort of structure
if you possibly can

        Paul Campbell



Sun, 23 Mar 2003 03:00:00 GMT  
 feedback input with combinational logic!

: hi everyone!
: I'm very confuse to use feedback input with combinational logic.
: assign a = sel ? b : a ;
: I want to know whether this assign statement has no problem.
: If there is any problem, please let me know.
: thank you!

  combinational feed back is an interesting issue in digital design, many
  text books advise avoiding using combinational feed back logical because
  verification tools that count on no combinational logic feed back (cycle-based
  simulatiors, boolean equivalence checkers, timing verifies) diagnose such feed
  back loops.

  here is an example

  module m(a,d);
  input a;
  output d;
  reg b,d;
  wire c;

    begin
      b = a;
      d = c;
    end
  assign c = b;
  endmodule

  regards,
--
  pigStand (|?  ?)

  Tel: 886-3-5952097                



Mon, 24 Mar 2003 11:33:26 GMT  
 
 [ 5 post ] 

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