About DCM, can you take multiple clocks from one?...can you dinamically change the speed? I dont think that is possible...I was thinking on using 2 clocks, a fixed one for control and a external voltage controlled oscillator for capturing data.

The Spartan 250E chip has four DCM's so you can generate four different clocks. This example project shows how to dynamically change the speed of a DCM. It is not normal to do this from what I gather though. http://www.xilinx.com/products/boards/s3estarter/files/s3esk_frequency_generator.pdf

Jack.