Example Three: Duty Cycle Change and Phase Shift with -edges and -edge_shift Options - 2022.1 English

Vivado Design Suite User Guide: Using Constraints (UG903)

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2022.1 English

Each edge of the generated clock waveform can also be individually shifted by a positive or negative value by using the -edge_shift option. Use this option only if a phase shift is needed.

The -edge_shift option cannot be used at the same time as any of the following:

  • -divide_by
  • -multiply_by
  • -invert

Consider the master clock clkin with a 10 ns period and a 50% duty cycle. It reaches the cell mmcm0 which generates a clock with a 25% duty cycle, shifted by 90 degrees. The generated clock definition refers to the master clock edges 1, 2, and 3. These edges respectively occur at 0 ns, 5 ns, and 10 ns. To obtain the desired waveform, shift the first and the third edges by 2.5 ns.

create_clock -name clkin -period 10 [get_ports clkin]
create_generated_clock -name clkshift -source [get_pins mmcm0/CLKIN] -edges {1 2 3} \
-edge_shift {2.5 0 2.5} [get_pins mmcm0/CLKOUT] # First rising edge:	0ns + 2.5ns = 2.5ns
# Falling edge:	5ns + 0ns	= 5ns
# Second rising edge: 10ns + 2.5ns = 12.5ns
Note: The -edge_shift values can be positive or negative.
Figure 1. Generated Clock Example Three