Xilinx Constraints Guide

The PERIOD constraint syntax for this example is:
NET “ClockName” TNM_NET = “TNM_NET_Name”;
Chapter 3: Timing Constraint Strategies
TIMESPEC “TS_name” = PERIOD “TNM_NET_Name” PeriodValue HIGH HighValue%;
For PERIOD, the PeriodValue defines the duration of the clock period. In this case, the
input clock to the DCM has a period of 5 ns. The HighValue of the PERIOD constraint
defines the percent of the clock waveform that is HIGH. In this example, the waveform
has a 50/50 duty cycle resulting in a HighValue of 50%. The syntax for this example is:
NET “ClkIn” TNM_NET = “ClkIn”;
TIMESPEC “TS_ClkIn” = PERIOD “ClkIn” 5 ns HIGH 50%;
Based on the input clock PERIOD constraint given above, the DCM automatically
creates two output clock constraints for the DCM outputs, and automatically performs
analysis between the two domains.
Manually Related Clock Domains
In some cases, the relationship between synchronous clock domains cannot be
automatically determined by the tools. One example occurs when related clocks enter
the FPGA device on separate pins. In this scenario, the recommended constraint
methodology is to create separate PERIOD constraints for both input clocks and define
a manual relationship between the clocks. Once the manual relationship is defined,
all paths between the two synchronous domains are automatically analyzed with all
frequency, phase, and uncertainty information automatically taken into account.
The Xilinx constraints system allows for complex manual relationships to be defined
between clock domains using PERIOD. This manual relationship can include clock
frequency and phase transformations. The methodology for this process is:
1. Define PERIOD for the primary clock
2. Define the PERIOD constraint for the related clock using the first PERIOD constraint
as a reference
Example
In this example two related clocks enter the FPGA device through separate external
pins. The first clock, CLK1X, is the primary clock, and the second clock, CLK2X180 is
the related clock. The circuit for this example is shown in the following figure:
The PERIOD syntax for this example is:
NET “PrimaryClock” TNM_NET = “TNM_Primary”;
NET “RelatedClock” TNM_NET = “TNM_Related”;
TIMESPEC “TS_primary” = PERIOD “TNM_Primary” PeriodValue HIGH HighValue%;
TIMESPEC “TS_related” = PERIOD “TNM_Related” TS_Primary_relation PHASE value;
Constraints Guide
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