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Visitor zj635315280
Visitor
2,573 Views
Registered: ‎05-28-2014

Timing Analysis : about input delay

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What is the diffenrence between the two clock edge?

If the external device's Timing is Figure 2,what should i set about the two clock edge?

2.png

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2 Replies
Guide avrumw
Guide
2,564 Views
Registered: ‎01-23-2009

Re: Timing Analysis : about input delay

The first box specifies which edge of the clock the set_input_delay applies to. In your case, the interface is single data rate (SDR) and specified with respect to the rising edge, so it should be left at "rising". If the interface is DDR (your's isn't), then you need to run through this twice, one for the rising edge of clock and one for the falling edge of clock - on the second one the box marked "Add delay information to the existing delay" must be selected (but not on the first).

 

The second box specifies whether this delay applies to the rising input data (i.e. the input is transitioning from a 0->1) or the falling input data (i.e. the input is transitioning from a 1 -> 0). The SDC standard allows for asymmetrical data transitions. However, the FPGA itself is symmetrical, and very few devices specify asymmetrical transitions, so in general we do not select the check box next to "Delay values specifies {rising/falling} delay", so that set_input_delay applies to both 0->1 and 1->0 data transitions.

 

Avrum

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Voyager
Voyager
2,563 Views
Registered: ‎06-24-2013

Re: Timing Analysis : about input delay

Hey @zj635315280

 

What is the diffenrence between the two clock edge?

The first selection is whether your delay is relative to the rising or falling edge of the clock (default is rise), and the second one is whether this delay applies to the rising or falling edge of your input (default is both).

 

If the external device's Timing is Figure 2,what should i set about the two clock edge?

The timing is all relative to the rising edge of RXCLK and there is no differentiation between rising or falling transition on the data inputs. I.e. first option is rise and on the second option you simply remove the check-mark.

 

Hope this clarifies,

Herbert

-------------- Yes, I do this for fun!
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