SOPC clock from dynamic reconfigured pll

[Altera_Forum]

Hi all, 

 I try to do the following and would like to ask if this is absolute nonsense before I start . . .:confused:  I have Quartus 9.1 SP2, a Cyclone III FPGA, and a Nios II SOPC up and running at a system speed of 40MHz. Everything works as expected.  Now, I would like to add to the SOPC system a custom streaming component for interfacing an external ADC. The streaming component should be connected to a DMA sink (and does also provide the ADC data on a conduit port) that transfers the ADC data to a SRAM on request by the NIOS II CPU. I have implemented such system already in a different project and all was working as expected. Now, in this project I need a little flexibility in the clocking of the ADC since I would like to sample different signals at different sample rates. The sample rates are 25MHz and 5MHz. I only have 1 ADC at hand so that I have to clock this one ADC at different speeds. So the question is if I can use a reconfigurable PLL external to my SOPC system and provide this clock via a second clock input to my SOPC system. This second clock would drive the ADC component, the DMA component and the SRAM component with its tri state bridge. The first and fixed clock input to the SOPC system drives my NIOS II CPU and peripherials. I would like to be able to read sample data from the SRAM to the NIOS CPU no matter what clock speed the external configurable PLL just provides. I read in the SOPC avalon interconnect manual that SOPC builder automatically inserts clock domain synchronization logic if I do not provide a clock crossing bridge myself (which I intend not to do). So is this clock crossing logic able to be driven by different clock speeds on one clock domain side? Since the manual tells that there is a certain handshake protocoll I would say that it does not matter what clock speed is currently running on one domain. Maybe I have to add the information that I do not want to switch the clock speed every mycrosecond, or so. Once the Clock speed is switched it should usually stay like that for minutes or even hours.  Regards, Maik