Xilinx Constraints Guide

Chapter 3: Timing Constraint Strategies
In the related PERIOD definition, the PERIOD value is defined as a time unit (period)
relationship to the primary clock. The relationship is expressed in terms of the primary
clock TIMESPEC. In this example CLK2X180 operates at twice the frequency of CLK1X
which results in a PERIOD relationship of ½. In the related PERIOD definition, the
PHASE value defines the difference in time between the rising clock edge of the source
clock and the related clock. In this example, the CLK2X180 clock is 180 degrees shifted,
so the rising edge begins 1.25 ns after the rising edge of the primary clock. The syntax
for this example is:
NET “Clk1X" TNM_NET = “Clk1X";
NET “Clk2X180" TNM_NET = “Clk2X180";
TIMESPEC "TS_Clk1X" = PERIOD "Clk1X" 5 ns;
TIMESPEC "TS_Clk2X180" = PERIOD "Clk2X180“ TS_Clk1X/2 PHASE +
1.25 ns ;
Asynchronous Clock Domains
Asynchronous clock domains are defined as those in which the transmit and capture
clocks bear no frequency or phase relationship. Because the clocks are not related, it is
not possible to determine the final relationship for setup and hold time analysis. For this
reason, it is recommended that proper asynchronous design techniques be employed
to ensure the successful capture of data. However, while not required, in some cases
designers wish to constrain the maximum data path delay in isolation without regard to
clock path frequency or phase relationship.
The Xilinx constraints system allows for the constraining of the maximum data path
delay without regard to source and destination clock frequency and phase relationship.
This requirement is specified using FROM-TO with the DATAPATHONLY keyword.
The methodology for this process is:
1. Define a time group for the source registers
2. Define a time group for the destination registers
3. Define the maximum delay of the net using FROM-TO between the two time groups
with DATAPATHONLY keyword.
For more information on using FROM-TO with the DATAPATHONLY keyword, see
FROM-TO.
Constraints Guide
58 www.xilinx.com UG625 (v. 13.2) July 6, 2011