Xilinx Constraints Guide

Chapter 4: Xilinx Constraints
UCF and NCF Syntax
Following are examples of UCF and NCF syntax.
• TIMESPEC PERIOD Method, Recommended
• NET PERIOD Method, Not Recommended
TIMESPEC PERIOD Method, Recommended
This is the primary, recommended method.
TIMESPEC “TSidentifier”=PERIOD “TNM_reference period” [units] [{HIGH | LOW}
[high_or_low_time [hi_lo_units]]] INPUT_JITTER value [units];
where
• identifier is a reference identifier that has a unique name
• TNM_reference is the identifier name that is attached to a clock net (or a net in the
clock path) using the TNM or TNM_NET constraint
When a TNM_NET constraint is traced into the CLKIN input of a DLL, DCM,
PLL, or MMCM component, new PERIOD specifications may be created at the
DLL/DCM/PLL/MMCM outputs. If new PERIOD specifications are created, new
TNM_NET groups to use in those specifications are also created.
Each new TNM_NET group is named the same as the corresponding
DLL/DCM/PLL/MMCM output net (outputnetname). The new PERIOD specification
becomes "TS_outputnetname=PERIOD outputnetname value units."
The new TNM_NET groups are then traced forward from the DLL/DCM/PLL/MMCM
output net to tag all synchronous elements controlled by that clock signal. The new
groups and specifications are shown in the timing analysis reports.
The following rules apply:
• period is the required clock period.
• units is an optional field to indicate the units for a clock period. The default is
nanoseconds (ns), but the timing number can be followed by ps, ms, micro, or %
to indicate the intended units.
• HIGH or LOW indicates whether the first pulse is to be High or Low.
• high_or_low_time is the optional High or Low time, depending on the preceding
keyword. If an actual time is specified, it must be less than the period. If no
high_or_low_time is specified, the default duty cycle is 50 percent.
• hi_lo_units is an optional field to indicate the units for the duty cycle. The default is
nanoseconds (ns), but the high_or_low_time number can be followed by ps, micro,
ms, or % if the High or Low time is an actual time measurement.
The following statement assigns a clock period of 40 ns to the net named CLOCK, with
the first pulse being High and having a duration of 25 nanoseconds.
NET “CLOCK” PERIOD=40 HIGH 25;
NET PERIOD Method, Not Recommended
Caution! This is a secondary method, and is not recommended.
NET “net_name” PERIOD=period [units] [{HIGH|LOW} [high_or_low_time [hi_lo_units]]];
Constraints Guide
212 www.xilinx.com UG625 (v. 13.2) July 6, 2011