Problem cascading 2 DCMs

MNiegl · 2007-04-27T18:07:46+00:00

Hi everyone,I have a problem that is bugging me for 2 days now and I was hopingsomeone here might be able to help me out.The problem is as follows:I want to implement a DDR2 RAM Controller in a Xilinx Virtex 4 FX FPGAon the Xilinx ML410 eval board. It's supposed to run at 200 MHz, whichI want to derive from the on-board 100 MHz oscillator. For this I needto use 2 DCMs cascaded (1 to get to 200 MHz, and the second one forall the other frequencies the RAM controller [generated with MIG 1.6]needs (main problem is the 200 MHz shifted by 90 deg). The first oneworks perfectly fine, only the second one never locks. Even in amodule with just the 2 DCMs and Clock Buffers it fails to work. Allthe clocks fall well within the ranges of the modes I use andeverything is connected as recommended by Xilinx. I use the CLK2Xoutput of the first one to specifically avoid excessive jitter, Idelay the config done flag until the first one has locked, I use theinverted lock output of the first one to reset the second one with ashift register in between. I already tried to manually place the DCMsat specific locations and impose timing constraints on the signal inbetween.The interesting thing is that the CLKFX output seems to work fine, Ichecked that on a scope, only the others fail to work. And even moreinteresting, 1 week ago I implemented a DDR RAM Controller in which Iused exactly the same structure except that it was running at 160 MHzand I didn't have any problems at all. If I try to do same now, itstill doesn't work!!So after about 20 hours of trying I just ran out of ideas. Maybesomeone of you has another idea.Cheers,Michael

Apr 27, 2007 21 Replies

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austin 19 years ago

Michael,

Thanks. I am trying to keep track of DCM issues, so that we can either make the documentation better, or the hardware better (in future chips).

Austin

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MNiegl 19 years ago

I just ran it just as successfully with the auto-cal enabled so the issue doesn't seem to be related to that. Seems more like a slight jitter problem when the DCM comes out of config without another reset.

Cheers, Michael

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