What to consider for source synchronous clocking?

Hi everyone, pardon me but I'm pretty new to this.

I'm using a Spartan 3 right now, and I'm trying to provide source synchronous clock clocking to a SDR SDRAM using DDR register. The component looks something like this:

OFDDRCPE sdram0OutputClock ( .Q(sdram0_clk), .C0(sdram_clk), .C1(~sdram_clk), .CE(1'b1), .CLR(1'b0), .D0(1'b1), .D1(1'b0), .PRE(1'b0) );

The input clock is sdram_clk and the output clock to the sdram is sdram0_clk The problem is I'm not sure whether I need to specify any extra constraints in the UCF file for sdram0_clk. Is there anything else to take note of? Are sdram_clk and sdram0_clk considered as different clock domains? If so how do I get the data back to the sdram_clk domain?

Thanks in advance, Ben.

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Once you've assigned the I/O standard of the pin, there's not much else you can do with constraints. The clock to the DDR flip-flop should be on a global net, and there is nothing that the tools can do to change the clock to output timing. However adding a timing constraint for clock to output will cause the actual timing to be reported in the post p&r static timing report, which is useful.

Also I noticed you're using clock and ~clock rather than the clock and 180 degree phase-shifted clock. Remember that for DDR the duty cycle is very important so make sure your clock is duty-cycle corrected in a DCM.

Regards, Gabor

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I'm not sure if it's possible in your design, but by far the simplest solution would be to use a single external clock that drives both the SDRAM and all your FPGA logic (through a clock buffer if necessary). If you must derive the SDRAM clock from the FPGA then one thing you can do is drive out the derived clock from an I/O and feed it to both the SDRAM and loopback to an FPGA clock pin (again using a buffer if needed). Both of these approaches avoid having to worry about IOB delays in the clock output.

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David Spencer

I missed the "SDR" bit when I replied earlier. I agree that a radial clock distribution is easiest for single-data-rate parts. Also forget the bit about duty cycle, these aren't as picky as DDR parts, which use both edges.

I've made source-synchronous SDR SDRAM systems using a global clock pin as an I/O (not available in all FPGA series) and pin feedback for the internal clock. Otherwise you may need to use a wire for feedback. I wouldn't worry about length matching, especially if your part has DCM's for phase-shifting the internal clock (and Spartan 3 does). But this means that the clock you supply to the DDR output flop is not the same one used internally for the rest of your logic.

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