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Xilinx DCM

Hi all,

i am facing a problem with Xilinx DCM in DFS (not in DLL mode, in which you need to provide a feedback clock for phase alignmen). So my DCM is working in without feedback (internal as well as external) mode.

I am trying to generate a 32Mhz clock from a 16Mhz input clock. Most of the time it works fine but sometimes after giving a reset to FPGA or reprogramming the FPGA the DCM is not able to multiply the clock to give a 32Mhz clock and gives the same input 16Mhz clock as the output.

But according to Xilinx DCM datasheet, in DFS mode we should be able to multiply or divide clocks with frequency > 1 Mhz.

So if anyone has faced any such problem or if there is any synthesis attribute which I need to set etc then please guide me.

Thanks in advance Debashish

Reply to
debashish.hota
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Debashish,

The DFS part of the DCM is more than happy to work with inputs down to 1 MHz as long as the multiplier results in an output of > 24 MHz.

Do you intentionally use the reset pin? Or are you using the internal reset on startup? If you are not resetting intentionally some time after configuration is complete and the input clock is stable, it may be that the input glitches right as the part is trying to lock.

Try a delayed reset to the DCM.

The only way the DCM will not lock is if the input clock is glitching while it is trying to lock. Once it has tried, and failed, it will just output the 1X clock (the input) as you described until it is reset. LOCK remains low throughout.

If the output is correct for a while, and LOCK does go high, and then the DFS fails, that is a different issue which is caused by excessive input jitter.

Aust> Hi all,

Reply to
Austin Lesea

We occasionally have problems with the DCMs not locking. I just made a simple "DCM watchdog". It is a state machine starting at system reset. It first resets the DCM, then waits for the DCM to lock (as long as specified in the data sheet). If the DCM doesn't lock, the state machine starts over (resetting the DCM again).

- Ell> Hi all,

Reply to
Elling Diesen

I discovered that I had to provide feedback from CLK0 to CLKFB to get CLKFX from the DFS.. I dont know if this is a requirement, but at least it gave me the CLKFB I needed.

Reply to
Morten Leikvoll

Morten,

CLKFX is only required if the CLK0, 90, 180, 270, CLK2X, or CLKDV output is also being used (besides the CLKFX output).

I suspect you are using something more than just CLKFX, which has triggered the software to use the DLL which requires CLKFB.

Note that CLKFB min frequency limit is above 24 MHz...

Aust> I discovered that I had to provide feedback from CLK0 to CLKFB to get CLKFX

Reply to
austin

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