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7,779 Views
Registered: ‎08-30-2007

how to set offset value?

 

 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

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2 Replies
Historian
Historian
7,724 Views
Registered: ‎02-25-2008

Re: how to set offset value?

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.

 

-a

----------------------------Yes, I do this for a living.
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Xilinx Employee
Xilinx Employee
7,317 Views
Registered: ‎08-10-2008

Re: how to set offset value?

If the minimum allowable offset value is greater than 1 cycle, you need to make sure the clock path or the data path is longer than expected. You can reduce the relevant delay to make the data arrive the output port earlier than 1 cycle.
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