Xilinx Constraints Guide

Chapter 4: Xilinx Constraints
Schematic Syntax
PERIOD = period {HIGH|LOW} [ high_or_low_time] INPUT_JITTER value;
UCF Syntax
NET “net_name” PERIOD = period {HIGH|LOW} [ high_or_low_time]
INPUT_JITTER value;
• period is the required clock period. The default units are nanoseconds, but the
timing number can be followed by ps, ns, micro, or ms. The period can also be
specified as a frequency value, using units of MHz, GHz, or kHZ.
• Units may be entered with or without a leading space.
• Units are case-insensitive.
• The HIGH|LOW keyword indicates whether the first pulse in the period is
high or low, and the optional high_or_low_time is the duty cycle of the first pulse.
HIGH is the default logic level if the logic level is not specified.
• 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%.
• The default unit for high_or_low_time is ns, but the number can be followed by
% or by ps, ns, micro or ms to specify an actual time measurement.
The PERIOD constraint is forward traced in exactly the same way a TNM would
be and attaches itself to all of the synchronous elements that the forward tracing
reaches. If a more complex form of tracing behavior is required (for example, where
gated clocks are used in the design), you must place the PERIOD constraint on a
particular net or use the preferred method described in the next section.
Specifying Derived Clocks
The preferred method of defining a clock period uses an identifier, allowing another
clock period specification to reference it. Xilinx® recommends using the same HIGH/LOW
keyword on the derived PERIOD constraints as the master PERIOD constraint. If the
master PERIOD constraint has the HIGH keyword or is the default, Xilinx recommends
using the same HIGH keyword on the derived PERIOD constraints. To define the
relationship in the case of a derived clock, use the following syntax:
UCF Syntax
TIMESPEC "TSidentifier"=PERIOD "timegroup_name" "TSidentifier" [* or /] factor
PHASE [+ |-] phase_value [units];
where
• identifier is a reference identifier that has a unique name
• factor is a floating point number
Note You can omit the [* or /] factor if the specification being defined has the
same value as the one being referenced (that is, they differ only in phase); this is
the same as using "* 1".
• phase_value is a floating point number
• units are ps, ms, micro, or ns (default)
The following rules apply:
• 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%.
• The default units for high_or_low_time is ns, but the number can be followed by
% or by ps, ns, micro, or ms to specify an actual time measurement.
Constraints Guide
210 www.xilinx.com UG625 (v. 13.2) July 6, 2011