Inverter Chain Synthesis Problem

Hi,

  I work on Xilinx ISE, and my synthesis tool is XST and synplify.
  I use verilog to write a Inverter Chain (delay ) like out =
~(~(~(~...in)).
  But the circuit be synthesised cancel all the invorter.

  How to synthesis out all the inverter chain I want?

Any suggestions will be appreciated!
Best regards,
Davy

Davy wrote:

>   How to synthesis out all the inverter chain I want?
> Any suggestions will be appreciated!

I would use a counter or shifter
to generate the delay.

         -- Mike Treseler

Hi Mike,

I mean a comb logic delay.

Davy

Davy wrote:
> Hi Mike,
> 
> I mean a comb logic delay.
> 
> Davy

A combinatorial chain of inverters is probably a bad idea.  The timing 
isn't easily guaranteed within a tight range especially since routing 
isn't constrained to specific paths.  If you *must* try to keep the 
inverters, synplify would use the syn_keep attribute on the 
combinatorial elements you don't want removed.  Even then, you may need 
similar constraints in place & route.

Please don't go there.

actually whaen u r generating a netlist if u r using series of
inverters but using only first input and the final output the synthesis
tool will won't consider and it will think as redundant terms during
translation phase ............... i think it's easy to use a counter to
generate delay .............if there is any ways of generating delay
just let me know

Hi,

Thank you for your help :-)

Do you think what's the best way to generate delay smaller than a clock
period?

Best regards,
Davy

Davy schrieb:
> Hi,
> 
> Thank you for your help :-)
> 
> Do you think what's the best way to generate delay smaller than a clock
> period?
> 
> Best regards,
> Davy
> 
Hi Davy,
unfortunately you didn't tell why you want to create such a delay.
That would help to give you useful help.

One solution may be to feed the signal into a DFF that is clocked by the 
falling edge of your clock signal (assuming that the rest of your 
circuit uses the rising edge). Thus you get a delay of half a clock period.

Another way could be constraints. While max_delay constraints are 
available I'm not sure about min_delay constraints.

The problem with combinatorical delays in FPGAs is that they are 
somewhat meaningless, since the routing delays between the LUTs are much 
higher and (unless propperly constrained) not predictable. So this 
approach is probably the worst choice, sice it is wasting LUTs for no 
reason.

Have a nice synthesis

    Eilert

Davy wrote:
> Hi,
> 
> Thank you for your help :-)
> 
> Do you think what's the best way to generate delay smaller than a clock
> period?
> 
> Best regards,
> Davy

If you have a steady clock in the right frequency range for your device, 
you can use a PLL, DLL, or DCM to get a different delay than 1.0 or 0.5 
clock periods.

The DCM especially allows you to set up a shifted clock with 1/256th of 
a clock resolution (or about 50 ps, whichever is greater) of delay. 
This precisely delayed clock can give you the precision control over 
skew that today's designers need.

If you're in Virtex-4 devices, the inputs have per-pin controllable 
delays that are calibrated to give you 78 ps delay resolution on 
unclocked signals.

So - is your device able to supply the fun stuff?  More application 
details can bring out the info on the "best" feature set to use.

Davy wrote:
> Hi,
> 
>   I work on Xilinx ISE, and my synthesis tool is XST and synplify.
>   I use verilog to write a Inverter Chain (delay ) like out =
> ~(~(~(~...in)).
>   But the circuit be synthesised cancel all the invorter.
> 
>   How to synthesis out all the inverter chain I want?
> 
> Any suggestions will be appreciated!
> Best regards,
> Davy
> 

For synplify tools,

wire [3:0] inv_chain/*synthesis syn_keep=1*/;

assign inv_chain[3:0] = ~{inv_chain[2:0], din};
assign dout = inv_chain[3];

I'm not use what XST uses in equivalence to syn_keep. It might be the same.

Like other ppl pointed out, you shouldn't expect to use invertor chains 
for any precision timing control. There are just too many uncertainties 
in FPGA. But if you just want to insert some combinational delay to 
solve some racing conditions, it might come in handy.

cheers,

jz

Davy wrote:
> Hi,
>
>  I work on Xilinx ISE, and my synthesis tool is XST and synplify.
>  I use verilog to write a Inverter Chain (delay ) like out =
> ~(~(~(~...in)).
>  But the circuit be synthesised cancel all the invorter.
>
>  How to synthesis out all the inverter chain I want?

Again, noting other's comments about generating delays, the simplest way to 
do this is simply to instantiate them (and don't flatten).

not_gate u_not_1 (
  .in  (input),
  .out (sig1)
);

not_gate u_not_2 (
  .in  (sig1),
  .out (output)
);

etc.

-- 
John Penton, posting as an individual unless specifically indicated
otherwise.