The Spartan-3 family User Guide provides the following recommendation for the Extended Spartan-3A family:
"In the Extended Spartan-3A families, when two adjacent DCMs are used, the outputs should be considered valid once both DCMs are LOCKED.
Adjacent DCMs should share the same reset signal."
If an application is using adjacent DCMs without a shared reset, if either DCM loses LOCK, it is possible that there will be an effect on the output clocks of the other running DCM.
This can be seen as up to an additional 500pS of jitter on the output clocks.
The following diagram shows the layout of the DCMs along with the adjacent pairs. For devices with less than 8 DCMs (for example, a Spartan-3A 200 which has 4 DCM located at the top and bottom of the device), each of these pairs are adjacent DCMs: