Constraint Files - Defining Constraints
Constraints are normally defined in constraint files, using the constraint editor. To add a new constraint file to a project right-click on the project name, then from the Add New to Project submenu select Constraint File . Constraints can be defined by typing them in, or by adding them using the menu options available in the constraint file editor's Design menu.
Multiple constraint files can used, allowing you to separate constraints by their type. For example, you can add design-specific constraints (such as specifying a net to be a clock) in one constraint file, and implementation type constraints (such as pin allocations) in a second constraint file.
Constraint files are mapped to an FPGA design by defining a Configuration, this can be done by right-clicking on the project filename in the panel and selecting Configuration Manager . Multiple constraint files can be enabled in a configuration. When an FPGA design is implemented in a device you select a project/configuration combination, allowing the system to map the design to its implementation. By defining multiple configurations you to easily re-target a design from one device to another.
Certain constraints can also be defined on the source schematic files. Be aware that these can lock the design down, for example specifying the target device on the schematic would prevent you from implementing that design on another device until you edit that parameter. Constraints that would be appropriate to define on the schematic include circuit-specific constraints, such as defining a net to be a clock.