Re: Virtex-4FX DCM autoshutdown failure, any suggestions

Austin Lesea schrieb:

Antti,

The chaining rule is there somewhere, and it should state that a CLKIN
is not allowed to be driven from a CLKFX of a previous DCM.

Also no more than two DCM shall be cascaded.

This an old restriction, since V2, and has not changed (as the jitter on
CLKFX has not changed).

I have heard about the DCM NBTI macro causing headaches before, and that
may be the problem. I do not know how to disable it. Maybe a quick
email to the hotline would point you to how to disable it.

It may be the 12 MHz is too slow and trips the macro "loss of clock"
circuit?

What value of M do you use to multiply the 12 MHz by?

Austin

Antti wrote:
Austin Lesea schrieb:

Antti,

[snip]

the min input frequency. Thus the first DCM should not be using the
CLKFB pin. If the CLKFB pin is connected, then the software will also
be trying to get the DLL part of the DCM to lock, and it will fail to do
so (with an input below its lock range).

Austin

weird - the thing is that the DCM chaining works fine, in ISE
and in EDK sometimes, and the single DCM in DFS mode works
in EDK also always with CLKFB no connected.

but with an PPC design in EDK when the CLKFB is not connected
then I think the DCM_STANDBY macro jumps in and shuts down
the DCM, well Xilinx docs say the standby macro tracks idle state
longer 100ms, hm the 360 milliseconds sounds still plausible time
for the shutdown macro to activate

connecting the 12MHz input clock that *IS* defenetly clean
and stable to both CLKIN _and_ CLKFB fixed the issue.

surpsingly I have EDK microblaze systems with similarly chained DCMs
where CLKFB of first DCM is not connected that work.

BTW after reading xilinx AR´s I did not find a clear note that DCM
chaining from FX is not allowed, there was note that Arch wizard does
not support it yet, but not that it want work or can not be used.

Antti


50, 72 and 100MHz out from first DCM all had the 360 milliseconds
shutdown. for the moment the connection of input clock to both CLKIN
AND CLKFB of the first DCM seems to fix the issue.

If there is another better workaround solution, well I will possible
open webcase or something, for tonight I got it working, what is needed
right now.

Antti

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