MMCM/PLL Phase Shift Modes - 2021.2 English

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

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2021.2 English

During timing analysis, clock phase-shift can be modeled in two different ways by setting the MMCM/PLL PHASESHIFT_MODE property, as described in the following table.

PHASESHIFT_MODE Property Phase-Shift Modeling Comment
WAVEFORM Clock Waveform Modification set_multicycle_path –setup constraints are usually needed to adjust the timing path requirement on clock-domain-crossing paths from or to the phase-shifted clock.
LATENCY MMCM/PLL Insertion Delay No additional multicycle path constraint is needed.

The default MMCM/PLL clock phase-shift mode varies across Xilinx FPGA families. However, the default mode can be overridden by the user on a per PLL/MMCM basis.

Table 2. Default MMCM/PLL Clock Phase Shift Handling
Technology Default MMCM/PLL Clock Phase Shift Handling
7 Series Clock Waveform Modification (WAVEFORM)
UltraScale Clock Waveform Modification (WAVEFORM)
UltraScale+ MMCM/PLL Insertion Delay (LATENCY)
Important: The MMCM/PLL PHASESHIFT_MODE property does not affect the device configuration.
Important: When a pin phase-shift is defined on any of the CLKOUTx pins and multiple clocks reach the input pins of the MMCM/PLL, the mode PHASESHIFT_MODE=LATENCY is invalid and triggers a Warning Timing 38-437. In such a scenario, the MMCM/PLL should be configured to use the mode PHASESHIFT_MODE=WAVEFORM.

The use of PHASESHIFT_MODE=LATENCY is particularly convenient when introducing skew between two clocks in order to meet timing. No additional multicyle path constraint is needed for adjusting the timing path requirement when setting the clock phase-shift to negative, null, or positive.

For legacy designs migrated from 7 series or UltraScale to UltraScale+, when the property PHASESHIFT_MODE is not set on the MMCM/PLL, the default behavior applies and the MMCM/PLL clock phase-shift is modeled as delay latency instead of clock edge shift. In this case, all multicycle path constraints that were specified in the legacy designs to account for a clock phase-shift need to be reviewed and usually removed. Such constraints can easily be identified by running the Methodology Checks (report_methodology). TIMING-31 flags multicycle paths between clocks where one of the clocks is phase-shifted and is generated by a MMCM/PLL with PHASESHIFT_MODE set to LATENCY.

The Clocking wizard and High Speed SelectIO™ Wizard both provide options to force the clock phase-shift modeling on each MMCM/PLL. The property PHASESHIFT_MODE is automatically saved inside the IP XDC.