04-20-2012 12:31 AM - edited 04-20-2012 04:11 AM
I'm working on a design for a Spartan3A board and used ISE10.1 for a while successfully . Then I had to switch to Win7 and so 10.1 didn't work any more. Since then I tried different version of ISE 11, 12 and 13 and even in the newest 13.4 there is the problem that I get the placer warning about a badly placed DCM :
Warning : Place:1013 - A clock IOB / DCM component pair have been found that are not placed at an optimal clock IOB / DCM site pair. ...
I'm using two DCMs and I already tried changing their location with the floorplanner but whichever DCM is not placed in the nearest location to the MAIN_CLK pin, genrates an error.
How can I resolve that ? Why is that still an issue ? I know the chip is rather old but still ...
Thank you very much for every helpful hint,
04-20-2012 05:51 AM
solved this by connecting the second DCM to the first one as CLK source, instead of connecting both to MAIN_CLK.
But shouldn't it be possible to connect two (or more) DCMs directly to the main clk source independently ? Or is that a problem of architecture in Spartan3A ?
04-20-2012 07:20 AM
You can connect a clock pin to more than one DCM. Spartan 3A has a lot of shared
routing resources, so it is possible that the use af another pin has limited the
connectivity of your MAIN_CLK input. You might want to look through the user guide
to see the clock connectivity.
If you don't want the additional jitter you get from cascading DCM's, you are probably
better off just allowing ISE to use non-optimal routing by adding a CLOCK_DEDICATED_ROUTE
constraint as indicated by the error message. Usually if you are using a DCM to change the
frequency of the clock, you are not concerned with the phase relationship to the input
clock and the non-dedicated clock routing will be OK.
04-30-2012 02:14 AM