Frequency synthesizer.

[chris]

Hi,

I'm using the output CLKFX of a DCM in a virtexII pro with CLKIN = 10MHz.
The M factor is 27 and the D factor is 10 so that I get CLKFX = 27MHz. Since the
PLL mode does not support CLKIN < 24 MHz should I still keep the feedback loop
between CLK0 and CLKFB.

Thanks. Christophe.

 

[Tarmo Palm]

I'm using the output CLKFX of a DCM in a virtexII pro with CLKIN = 10MHz.
The M factor is 27 and the D factor is 10 so that I get CLKFX = 27MHz. Since the
PLL mode does not support CLKIN < 24 MHz should I still keep the feedback loop
between CLK0 and CLKFB.

Hi,

I had with the Spartan 3 DCM same problem. If you use Feedback, then
input freq. can't be below ca. 24 MHz. But if you do not use Feedback,
then input freq. can't be below ca. 1 MHz. Without Feedback is only
problem Phase.

Tarmo Palm

 

[Peter Alfke]

Do not feed back CLK0 to CLKFB. Then your input frequency can be as low as 1
MHz, if your output frequency is above 24 MHz. So you are o.k.
You do not have a defined phase relationship between input and output, but
that should not be a real loss, considering the ratio of your frequencies.
10 MHz and 27 MHz coincide only once every microsecond...
Peter Alfke

From: tpalm@harmanbecker.com (Tarmo Palm)
Organization: http://groups.google.com
Newsgroups: comp.arch.fpga
Date: 21 Jun 2004 06:52:40 -0700
Subject: Re: Frequency synthesizer.

I'm using the output CLKFX of a DCM in a virtexII pro with CLKIN = 10MHz.
The M factor is 27 and the D factor is 10 so that I get CLKFX = 27MHz. Since
the
PLL mode does not support CLKIN < 24 MHz should I still keep the feedback
loop
between CLK0 and CLKFB.

Hi,

I had with the Spartan 3 DCM same problem. If you use Feedback, then
input freq. can't be below ca. 24 MHz. But if you do not use Feedback,
then input freq. can't be below ca. 1 MHz. Without Feedback is only
problem Phase.

Tarmo Palm