Comments,
If you are using the DLL only (CLK0, CLK90, CLK180, CLK270, CLK2X, CLKDV), then the output is a delayed version of the input. If you want to stop the DCM from moving its taps, there is the "FREEZE_DLL" pin on the test primitive which when asserted just stops all tap updates.
If this is something you think would be useful, contact your local FAE for assistance. Before you do, the tap value is ~22 ps in V2P, and ~15 ps in V4, so stopping all tap movement will be a really difficult thing to even detect that you have done (as the rest of the jitter tends to mask the tap jitter completely). Not being able to track temeprature then becomes the major source of error. Tap movement is not the source of anyone's problems.
If you are using the DFS part of the DCM (the CLKFX output), then stopping the tap movement will result in no clock out at all. Stopping the input will also stop the output (until RESET is asserted again).
The question of frequency accuracy is not applicable: a delay line can not change the incoming frequency, so frequency in = frequency out. The CLKFX synthesis is hard sync'ed every concurrence, so it, too, is exact as far as frequency goes.
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