Setup/Recovery Relationship - 2021.2 English

Vivado Design Suite User Guide: Design Analysis and Closure Techniques (UG906)

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2021.2 English

The setup check is performed only on the most pessimistic setup relationship between two clocks. By default, this corresponds to the smallest positive delta between the launch and capture edges. For example, consider a path between two flip-flops that are sensitive to the rising edge of their respective clock. The launch and capture edges of this path are the clock rising edges only.

The clocks are defined as follows:

  • clk0 has a period of 6 ns with first rising @ 0 ns and falling edge @ 3 ns.
  • clk1 has a period of 4 ns with first rising @ 0 ns and falling edge @ 2 ns.

As the following figure shows, there are two unique setup relationships: Setup(1) and Setup(2).

Figure 1. Setup Relationships

The smallest positive delta from clk0 to clk1 is 2 ;ns, which corresponds to Setup(2). The Common Period is 12 ;ns, which corresponds to the time between two simultaneous alignments of the two clocks.

Tip: The relationships are established when considering the ideal clock waveforms, that is, before applying the insertion delay from the clock root to the flip-flop clock pin.
Important: If the common period cannot be found over 1000 cycles of both clocks, the worst setup relationship over these 1000 cycles is used for timing analysis. For such case, the two clocks are called unexpandable, or clocks with no common period. The analysis will likely not correspond to the most pessimistic scenario. You must review the paths between these clocks to assess their validity and determine if they can be treated as asynchronous paths instead.

Once the path requirement is known, the path delays, the clocks uncertainty and the setup time are introduced to compute the slack. The typical slack equation is:

Data Required Time (setup) =

capture edge time

+ destination clock path delay

- clock uncertainty

- setup time

Data Arrival Time (setup) =

launch edge time

+ source clock path delay

+ datapath delay

Slack (setup) = Data Required Time - Data Arrival Time

As the equation shows, a positive setup slack occurs when the data arrives before the required time.

The recovery check is similar to the setup check, except that it applies to asynchronous pins such as preset or clear. The relationships are established the same way as for setup, and the slack equation is the same except that the recovery time is used instead of the setup time.