Relaxing Setup While Maintaining Hold - 2023.2 English

Vivado Design Suite User Guide: Using Constraints (UG903)

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2023.2 English

The following figure shows a path between two flip-flops that are enabled every two cycles. It is safe to define a Multicycle Path constraint on this path to indicate that the first edge of the destination clock is not active, and only the second edge of the destination clock captures a new data.

Figure 1. Registers Enabled Every Two Cycles

The following constraint establishes a new setup relationship:

set_multicycle_path 2 -setup -from [get_pins data0_reg/C] -to [get_pins data1_reg/D]

Section "Performing Timing Analysis' in the Vivado Design Suite User Guide: Design Analysis and Closure Techniques (UG906) describes how the hold relationships are derived from the setup relationships. When modifying the setup relationship, the hold relationships are also modified to follow the changes in the setup launch and capture edges.

Important: If the new hold requirements become too aggressive, it likely results in difficult timing closure. It is your responsibility to relax the hold requirement assuming it is safe for the design.

In the same example as Figure 1, after moving the setup check to the second capture edge, the hold check is automatically moved to the first capture edge (that is, one clock period before the setup check).

The following figure shows how both the setup and hold relationships have changed when only the setup path multiplier has been defined with the Multicycle Path constraint.

Figure 2. Multicycle Path: Relaxing Setup Only

Holding the data in the data0_reg for one cycle is not needed for this path to be functional due to the clock enable. In this case, AMD recommends changing the hold relationship back to the original, which is between the same launch and capture edges. To do so, you must add a second Multicycle Path constraint that modifies the hold check only:

set_multicycle_path 1 -hold -end -from [get_pins data0_reg/C] \
-to [get_pins data1_reg/D]

The -end option is used with set_multicycle -hold command because the edges of the capture clock must be moved backward.

Note: Because the launch and capture clocks have the same waveforms, the -end option is optional. Moving the capture edges backward result in the same hold relationship as moving the launch edges forward. To simplify the expressions, the -end option has been removed from the next two examples.

The following figure shows the updated setup and hold relationships after applying both Multicycle Path constraints.

Figure 3. Multicycle Path: Relaxing Setup and Hold

To summarize this example, the following constraints are necessary to properly define a multicycle path of two (2) between data0_reg/C and data1_reg/D:

set_multicycle_path 2 -setup -from [get_pins data0_reg/C] -to [get_pins data1_reg/D]
set_multicycle_path 1 -hold -from [get_pins data0_reg/C] -to [get_pins data1_reg/D]

For a multicycle with a setup multiplier of four (4), the constraints are:

set_multicycle_path 4 -setup -from [get_pins data0_reg/C] -to [get_pins data1_reg/D] 
set_multicycle_path 3 -hold -from [get_pins data0_reg/C] -to [get_pins data1_reg/D]
Figure 4. Multicycle Path with Setup Multiplier of Four (4)