03-06-2009 07:08 PM
In Backplane.twx, for these constraints,
TIMEGRP "DSI_CLK_GRP" OFFSET = OUT 6 ns AFTER "XGMII5_RXCLK" REFERENCE_PIN "DSI_CLK1" RISING;
TIMEGRP "DSI_CLK_GRP" OFFSET = OUT 6 ns AFTER "XGMII5_RXCLK" REFERENCE_PIN "DSI_CLK1" FALLING;
the .twx shows that the Mininum allowable offset is 5.671 ns, so I set the offset out value is 6ns (global clk period is 6.4ns), by this way, I get the offset values of this group datas, Am I in the right direction?
sometimes, the Mininum allowable offset value is bigger than one period, so that mean the offset out value will be more than one period, that is opposite with the offset constrainst's rules, So I am confused... thanks more much
03-09-2009 10:13 AM
The OFFSET OUT constraint is used to ensure that you meet the set-up time, with respect to your clock, of an external device. The assumption here is that the external device clock is the same as the FPGA's clock for the signal in question.
So if you have an external device that requires a 3 ns set-up time and you have a 10 ns period clock, you could set the following OFFSET OUT constraint:
NET "foo" OFFSET = OUT 7 ns AFTER "CLK" RISING;
10 ns period minus 3 ns set-up requirement is 7 ns.
03-31-2009 01:48 AM