AR# 59893

Description

How can I set input delay when MMCM is used on the clock path? 

Solution

In general, the reference clock used for the -clock option of set_input_delay is the one created on the FPGA clock input pad.

If there is an MMCM on the clock path, the input path becomes a CDC (Clock Domain Crossing) path because the source clock (master clock) and destination clock (generated clock) are different. 

In this case, you may need to add set_multicycle_path to adjust the path requirement or use a virtual clock as the reference clock, or both.

In the following example,  the clk frequency is 100Mz, min input delay is 1ns and max input delay is 2ns.

Example 1

If the MMCM's phase shift is negative or zero and the MMCM output clock is 100Mz, you can constrain the interface with the below constraints.

# Create clock on the clock input pad and use it as reference clock in set_input_delay

create_clock -period 10.000 -name clk [get_ports clk]

set_input_delay -clock clk -max 2.000 [get_ports datain]

set_input_delay -clock clk -min 1.000 [get_ports datain]

Note: For a negative phase-shift, a Multi-cycle constraint is typically not needed to counter balance the effect of the phase-shift. 

An exception occurs if the phase-shift is so large that the clock launch or capture edges must be adjusted to keep realistic setup and hold requirements.

Example 2

If the MMCM's phase shift is positive and the output clock is 100Mz, you need to add a set_multicycle_path constraint based on the original constraints to adjust the clock edge to be checked and the path requirement.

set_multicycle_path 2 -from [get_ports datain]

Note: In this case, it is not necessary to modify the hold relationship with an additional set_multicycle_path constraint because it is already properly established relative to the setup relationship and all rising clock edges are active.

Example 3

If the MMCM has no phase shift and the output clock is 60Mz

You can make use of a virtual clock in this case.

# Create clock on the clock input pad

create_clock -period 10.000 -name clk [get_ports clk] 

# Create virtual clock and use it as the reference clock in set_input_delay

create_clock -name clk_port_vclk -period 16.67

set_input_delay -clock clk_port_vclk -max 2 [get_ports datain]

set_input_delay -clock clk_port_vclk -min 1 [get_ports datain]