Xilinx SRAM clock-to-out and input constraint with forwarded

[Sylvain Munaut]

Hi,

I have SRAM connected to a FPGA.
The clock is forwarded to the SRAM by the FPGA (using ODDR trick).
The input clock to the system is of another frequency and is
internally multiplied by a DCM.
Lets call the input clock (on the pad) clk_in and the internal DCM
generated clock clk_1x

Now, I'd like to constraint :

- The clock to out from my internal clk_1x to pad for the various
outputs
- The skew between all the pins. (Since I use IOB registers, it
should be constraint by design but I'd like at least a report ...)
- The valid window for capturing the incoming signals.

And somehow I can't figure out how to do these because I don't have a
"pad" that reflects my internal clock.
If I just try to put an OFFSET relative to my clk_in, then it outputs
mostly garbage because the clk_in and clk_1x have different
frequencies and so it tries to consider intemediate edges and stuff
but I don't care about that, all the design is clocked on clk_1x and
not clk_in ...

I looked at the Xilinx example but couldn't find one that fits. Each
time they have the external clock directly feed the internal logic and
not a DCM modifying the frequency in the middle.

Cheers,

Sylvain

 

[Nico Coesel]

"Sylvain Munaut <SomeOne@SomeDomain.com>" <246tnt@gmail.com> wrote:

Hi,

I have SRAM connected to a FPGA.
The clock is forwarded to the SRAM by the FPGA (using ODDR trick).
The input clock to the system is of another frequency and is
internally multiplied by a DCM.
Lets call the input clock (on the pad) clk_in and the internal DCM
generated clock clk_1x

Now, I'd like to constraint :

- The clock to out from my internal clk_1x to pad for the various
outputs

You can't. You must specify FF to PAD. The path between a flipflop and
an output isn't covered by a clock domain.


TIMEGRP "all_ff" = ffs("*");
TIMEGRP "trigger_analog_pad" = pads("trigger_analog_out");
TIMESPEC "TStrigger_analog_out" = FROM "all_ff" TO
"trigger_analog_pad" 7;

- The skew between all the pins. (Since I use IOB registers, it
should be constraint by design but I'd like at least a report ...)

See above. The path between an input and a flipflip isn't covered by a
clock domain.

- The valid window for capturing the incoming signals.

You have to calculate this by hand.


--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
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