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Observer bvolain
Observer
6,115 Views
Registered: ‎01-22-2009

Clocking wizard doesn't like me

I'm trying to run DCM_ADV clocking wizard 9.1i in ISE 10. I want to generate an 80MHz clock from a 320 MHz input for an XC5VLX30-3FF324.  I enter 320 MHz for the input clock frequency, and 80 MHz for the output frequency, and the wizard says that the specified input is in Low Frequency mode (No it isn't!!!)so the output must be also.  If I specify "Divide Input Clock by 2" in the Advanced window, it then tells me that I'm in High Frequency mode, and everything works okay. Is this a bug, or am I doing something wrong?

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4 Replies
Instructor
Instructor
6,109 Views
Registered: ‎08-14-2007

Re: Clocking wizard doesn't like me

It seems that the wizard's "output clock" only refers to the CLK0 or CLKFX outputs, not the CLKDV,

which is the one you should use for this purpose.  Try setting the output to 320 MHz as well, and

then setting the CLKDV divide to 4.  Otherwise you will end up using the input clock divider and

the CLKFX output.  In the latter case you will be forced to low frequency mode due the the

minimum output clock frequency.

-- Gabor
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Observer bvolain
Observer
6,103 Views
Registered: ‎01-22-2009

Re: Clocking wizard doesn't like me

Well, that works, but I would have thought I should be able to use the CLKFX output as well as the CLKDV. I'm assuming that using the DCM for a simple divide-by-four will yield better results than simply implementing a counter in VHDL and letting the tools take care of the timing. True? 

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Instructor
Instructor
6,092 Views
Registered: ‎08-14-2007

Re: Clocking wizard doesn't like me

You can divide by 4 in the fabric and apply the output to a BUFG, but the tools won't "take care of

the timing".  There will be significant delay between the input clock and the resultant divided clock.

The CLKDV output of the DCM on the other hand is implemented so as to match the rising edge

timing of the input clock.  If you need the phase alignment you're better off using the DCM.  If you

don't need the phase alignment, you could still use the DCM with "no feedback" or "local feedback"

to avoid running a 320 MHz clock onto a global clock net.

-- Gabor
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Historian
Historian
6,076 Views
Registered: ‎02-25-2008

Re: Clocking wizard doesn't like me


bvolain wrote:

I'm trying to run DCM_ADV clocking wizard 9.1i in ISE 10. I want to generate an 80MHz clock from a 320 MHz input for an XC5VLX30-3FF324.  I enter 320 MHz for the input clock frequency, and 80 MHz for the output frequency, and the wizard says that the specified input is in Low Frequency mode (No it isn't!!!)so the output must be also.  If I specify "Divide Input Clock by 2" in the Advanced window, it then tells me that I'm in High Frequency mode, and everything works okay. Is this a bug, or am I doing something wrong?


Skip the wizard. Instantiate the DCM and the necessary BUFGs by hand. It's actually not as hard as it sounds. You WILL have to read the user guide to understand how to set all of the generics. Put your DCM and associated stuff into a clocks.vhdl module for ease in integrating with the rest of your design.

 

-a

----------------------------Yes, I do this for a living.
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