Allowed values are the name of the top entity affected and a time value representing the desired period. The allclocknets constraint optimizes the period of the entire design. Timing constraints are propagated to the NGC as implementation constraints.Global constraints can be set from the Process Properties dialog box in the Project Navigator, from the command line options, by using the constraint file, or by setting attributes directly in your HDL file.Specific timing constraints can be set only by using the constraint file or by setting attributes directly in your HDL file.For global constraints, XST will continue optimization as long as there is improvement. For specific constraints, XST will stop optimizing the design when the value specified in the specific constraint is met.Specific timing constraints specify both a domain and a value.The main differences between a global and a specific constraint are as follows. MAX_DELAY identifies all paths defined by the following timing constraints: allclocknets, offset_in_before, offset_out_after, inpad_to_outpad. INPAD_TO_OUTPAD (inpad to outpad) sets a maximum combinational path constraint. OFFSET_OUT_AFTER (register to outpad) is similar to the previous constraint but sets the constraint from the sequential elements to all primary output ports.
OFFSET_IN_BEFORE (inpad to register) identifies all paths from all primary input ports to either all sequential elements or the sequential elements driven by the given clock signal name. PERIOD identifies all paths between all sequential element s controlled by the given clock signal name. To take into account inter clock domain delays, the command line switch -cross_clock_analysis must be set to yes. The possible domains are illustrated in the following schematic.ĪLLCLOCKNETS (register to register) identifies by default, all paths from register to register on the same clock for all clocks in a design. By default, they are not written to the NGC file. write_timing_constraints option is specified when using the command line. Note Timing constraints are only written to the NGC file when the Write Timing Constraints property is checked yes in the Process Properties dialog box in Project Navigator, or the Basically, global timing constraints are applied to an entire design (inpad_to_outpad or offset_in_before for instance), specific timing constraints are applied to a clock domain, and are associated with a value (expressed in ns or MHz). XST supports both global and specific timing constraints.
0 kommentar(er)
