DCM not locking in XC2V4000

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I have already used DCMs in real hw : a DDR controller on an Avnet board and
I got it to work. It used 3 DCM (2 for the DDR and another for something
There were even cascaded.

Now, on a simpler design, I can't get it to lock. It simulates ok, synthesis
place and route ok, but the DCM does not lock. I can see that that's the
since I connected the lock signal to the LEDs on the board.

Basically, I have an SDRAM addition to the Avnet board. Because the clock
can only be provided by the FPGA, I need to make sure that clock and data
going out to the SDRAM have some reasonable skew to avoid set up and hold
time problems.
So, I simply anticipate the outgoing clock of about 1 ns using a DCM.
The output of the DCM is fed back to the CLK_FBK input through a
BUFG. It also goes out to a pad (after BUFG) through a FDDRSE primitive and
To see if there is a clock at all, this skewed clock is divided and
connected to a
LED on the board. The clock is there as the LED flashes.
However, it does not lock. The clock goes nowhere else : only to the divider
to the OBUF.

The clock is 40 MHz which is within the range specs for XC2V4000-4
(24-180 MHz) in low frequency mode.

This is much simpler than the DDR code and I'm going mental trying to find
out why.
Any ideas ? The code is pretty small and I could even post it here, there's
secret about it.

Please help,


Re: DCM not locking in XC2V4000
Look at signal connected to feedback in DCM.
Try to connect output signal from DCM (that You use for feedback) to
ordinary BUF and then to feedback input (CLKFB). Maybe feedback needs little

Best regards

Jerzy Gbur

"Everything is simple until it'll be comprehensible" - furia

Re: DCM not locking in XC2V4000
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Who knows, I tried your suggestion and unfortunately it didn't work.

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Re: DCM not locking in XC2V4000
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I had similar problem with DLL so from here was that advice, nothing else I
can say, maybe this is stupid mistake with signal names or something like
that. I'm afraid I can't help more :(


"Everything is simple until it'll be comprehensible" - furia

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