DCM driving multiple OBUF's ... skew in between ...

Markus,

Keep the OBUFs to the right or left sides, and near the center of the parts (least H Clock Tree skew from IOB to IOB

You may also want to use the DDR output flops in the IOB to output the clock rather than running them straight through a simple output buffer. This keeps the clocks on dedicated clock lines and eliminates a bunch of skew. There are several app-notes that show this, look at some of the DDR memory ones for examples.

Here's code a used to drive a clock out to some synchronous SRAM. Note by swapping the D0 and D1 values you can invert the clock.

// infer a DDR type I/O cell for low clock skew FDDRRSE uddr_clk( .Q (sram_clk), .C0 (g_sr_clk), .C1 (~g_sr_clk), .CE (1'b1), .D0 (1'b1), .D1 (1'b0), .R (1'b0), .S (1'b0) );

John Providenza

John,

Youa re absolutely right! I failed to mention that you should always use the DDR FF for clock forwarding, as it has the best timing performance. Good catch.

Aust> You may also want to use the DDR output flops in the IOB to