DCM Simulation Error

Feb 24, 2004 3 Replies
T

I am using Modelsim SE 5.8 to simulate cascaded DCMs. I have two DCMs cascaded, the first with CLKIN running at 10Mhz. I am using the CLK2X of the first as the CLKIN for the second DCM. The desired output is CLK2X of the second DCM (I am trying to generate a 40Mhz clock from a 10Mhz clock). When I simulate, I recieve the following warning:



# ** Warning: Timing Violation Error : Input Clock Period Jitter on instance * exceeds 0.001 ns Locked CLKIN Period = 0.1 ns Current CLKIN Period = 0.3 ns # Time: 675 ns Iteration: 2 Instance: /dcmtest224/xlxi_1037



I cannot get the second DCM to lock as well. Any suggestions would be most appreciated.



Best Regards, T. Justin Campbell


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A

Timothy,

1) You must hold the cascaded DM in reset until the first DCM has locked

2) The tool is correct, cascading DCM CLK2X is not recommended, as the jitter out of the first exceeds the specs of the input of the second.

They will work, but the resulting jitter out of the second one is not pretty (probably close to 400 to 600 ps P-P, which may be alright for your application).

Oh, and one last item, 10 MHz is too low a frequency to go into a DCM with (Fin min is > 24 MHz).

Austin

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K

I tried simulating two cascaded DCM, also not successful...some postings said cascading DCMs is no good...

Just my 2cent...

Best Regards, Kelvin

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J

If you connect the inverted "lock" output from the first DCM to the reset input of the second one it will work fine, provided you are within the frequency range of the DCMs.

jakab
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