Xilinx Constraints Guide

Falling Edge Constraints
Chapter 4: Xilinx Constraints
The falling edge OFFSET IN constraint defines the time that the data becomes valid
prior to falling clock edge used to capture the data. In this example, the data becomes
valid 250 ps after the falling clock edge. This results in an OFFSET IN BEFORE value
of -250 ps with the value negative because it begins after the clock edge. Once the data
begins, it remains valid for 2 ns. This results in a VALID value of 2 ns. The FALLING
keyword is used with this constraint to indicate that the constraint applies to only the
falling edge synchronous elements, and that the OFFSET IN BEFORE value is specified
to the falling clock edge.
For more information about the RISING and FALLING keywords, see the Timing
Constraints User Guide.
UCF Syntax
The complete UCF syntax of the clock PERIOD and OFFSET IN constraint for the
example is shown below.
NET “clock” TNM_NET = CLK;
TIMESPEC TS_CLK = PERIOD CLK 5.0 ns HIGH 50%;
OFFSET = IN -250 ps VALID 2 ns BEFORE clock RISING;
OFFSET = IN -250 ps VALID 2 ns BEFORE clock FALLING
UCF Source Synchronous DDR Center Aligned Example
The Source Synchronous Dual Data Rate (DDR) Center aligned case consists of an
interface where the clock is sent from the transmitting device aligned with the center of
the data. In a dual data rate interface, data is captured with both the rising and falling
clock edges. In the DDR case, separate OFFSET IN constraints must be defined for
the rising and falling clock edge registers capturing the data. Using the RISING and
FALLING keywords with the OFFSET IN constraint simplifies this task.
Example Waveform
In this example a dual data rate interface is shown with a clock period of 5 ns and 50/50
duty cycle. The rising and falling data is valid for 2 ns and is centered over the high and
low clock edges. This results in a 250 ps margin before and after data valid window.
Rising Edge Constraints
The rising edge OFFSET IN constraint defines the time that the data becomes valid prior
to rising clock edge used to capture the data. In this example, the data becomes valid 1 ns
before the rising clock edge. This results in an OFFSET IN BEFORE value of 1 ns with the
value positive because it begins before the clock edge. Once the data begins, it remains
valid for 2 ns. This results in a VALID value of 2 ns. The RISING keyword is used with
this constraint to indicate that the constraint applies to only the rising edge synchronous
elements, and that the OFFSET IN BEFORE value is specified to the rising clock edge.
Falling Edge Constraints
The falling edge OFFSET IN constraint defines the time that the data becomes valid
prior to falling clock edge used to capture the data. In this example, the data becomes
valid 1 ns before the falling clock edge. This results in an OFFSET IN BEFORE value
of 1 ns with the value positive because it begins before the clock edge. Once the data
begins, it remains valid for 2 ns. This results in a VALID value of 2 ns. The FALLING
keyword is used with this constraint to indicate that the constraint applies to only the
falling edge synchronous elements, and that the OFFSET IN BEFORE value is specified
to the falling clock edge.
Constraints Guide
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