This tutorial is comprised of two labs that demonstrate aspects of constraining a design in the Vivado® Design Suite. The constraints format supported by the Vivado® Design Suite is called Xilinx® Design Constraints (XDC), which is a combination of the industry standard Synopsys® Design Constraints and proprietary Xilinx® constraints. For more information on Timing Closure, see the UltraFast Design Methodology Timing Closure Quick Reference Guide (UG1292).
XDCs are not just simple strings; they are Tcl commands that the Vivado Tcl interpreter sequentially reads and parses. You
can enter design constraints in several ways at different points in the design flow. You
can store XDCs in one or more files that can be added to a constraint set in Vivado Project Mode, or read the same files directly into
memory using the
read_xdc command in Non-Project mode.
For more information on Project and Non-Project modes, refer to the
Vivado Design Suite User Guide: Design
Flows Overview (UG892). With a design open in Vivado tools, you can also type constraints as commands
directly in the Tcl Console when working in the Vivado IDE or at the Tcl command prompt when working outside of the IDE. This is
particularly powerful for defining, validating, and debugging new constraints
interactively in the design.
The Vivado Design Suite synthesis and implementation tools are timing driven. Having accurate and correct timing constraints is vital for meeting design goals and ensuring correct operation. Because the Vivado tools are timing driven, it is important to fully constrain a design, but not over-constrain, or under-constrain it. Over-constraining a design can lead to long compile times and sub-optimal results because the tool can struggle with unrealistic design objectives. Under-constraining a design can cause the Vivado tools to perform unnecessary optimizations, such as examining paths with multicycle delays or false paths, and prevent focus on the real critical paths.
This tutorial discusses different methods for defining and applying design constraints.