Shifted and Widened Window

To shift and widen the capture window you constrain the hold time as well as the setup time. A wider window allows multiple clock cycles to capture data. In the following example, the capture window is two clock cycles.

Figure 1. Setup and Hold Constraints Shift and Widen the Capture Window

The constraints that represent this example are:

set_multicycle_path -setup -from a -to b 2
set_multicycle_path -hold -from a -to b 1
If n is equal to m, then the constraint would simply be:
set_multicycle_path -setup -from a -to b n
set_multicycle_path -hold -from a -to b n-1

To shift the window by n clock cycles with a window m cycles wide, use the equations:

  • setup = n
  • hold = m - 1

For example:

  • n = 4, m = 3
  • setup = 4
  • hold = 3 - 1 = 2

These values give you a window that is shifted by 4 clock cycles and is 3 clock cycles wide. 

set_multicycle_path -setup -from a -to b 4
set_multicycle_path -hold -from a -to b 2