200MHz ucf constraints for Xilinx DA Decimation by 2

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Hi folks,

I have generated a DA filter in coregen with the following specs:

Decimation by 2
40 taps symmetric
18 bit coes (signed)
12 bit input
2 clock cycles per output

The input <sampling> rate is 100MHz down to 50MHz.

However, I want to clock it at 200MHz down to 100MHz to save area - hence
the 2 clocks per output.

I am targetting an xc2v3000fg676-5 and I am pretty sure this is achievable
since the polyphases need only run at 100MHz - only the data demuxing needs
to run at 200MHz.

Constraining the whole lot via the CLK signal to 5ns (200MHz) doesn't work -
gets to about 177MHz with 16 logic levels across what is clearly a carry
chain (which needs only to run at 100MHz).

Here is what I tried in the ucf to try and tell ISE 5.2.03i what to do:

NET "CLK*" TNM_NET = "CLK";
TIMESPEC "TS_CLK" = PERIOD "CLK" 5 ns HIGH 50 %;
NET "ND*" TNM_NET = "ND";
TIMESPEC "TS_CLK_ND" = FROM "CLK" TO "ND" "TS_CLK" * 2;

This doesn't work though - the timing report still moans about a carry chain
failing the 5ns constraint even though it would meet the 10ns constraint it
should be tied to.  I think the logic will work at these rates but it would
be nice to know for sure via correct contraints.

So, can someone suggest some ucf details I can use to tell the tools what to
do please?  Do I need to specify some internal signal rather than ND?

Thaks for your time,

Ken


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Re: 200MHz ucf constraints for Xilinx DA Decimation by 2
Hi Ken,

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needs
work -
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chain
it
would
to

What you've described is known as a multi-cycle path (data updates only
every X clock cycles allowing for the operation to span multiple clock
cycles).
I don't remember the exact ucf syntax, but I think it's a generic device to
device constraint (the example in the Xilinx documentation is very good).
Please note that you may specify this time as "X * <clock_period_name>",
which allows your design to be easily scaled for different frequencies.

Hope this helps,
Alvin.



Re: 200MHz ucf constraints for Xilinx DA Decimation by 2

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hence
achievable
what
address.
to


Thanks for the response Alvin.

I am aware that I am trying to specify a multicycle path - I have used the
syntax I posted above (which does have an X * BTW) in other similar
situations and successfully specified a multicycle path - that was for VHDL
that I wrote however and not for an edif generated by coregen.

Cheers,

Ken



Re: 200MHz ucf constraints for Xilinx DA Decimation by 2
Hi Ken, <BR>
When I was working on a part of my project that had a clk and a clk2x, I didn't
even need to specify the 2x period, as my signal was comming from the DCM, and
ISE found that one itself.  However, I did need to insert a timing ignore
constraint (TIG) on the path that crossed my clock domain boundary.  I'm always
queasy about doing such things, and I haven't tested it out physically yet, so
take it for what it's worth, but it got rid of my error messages, at least.
<p>Pierre-Olivier

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