Xilinx V2.1 - Manual (Page 133)

Using the System Generator Constraints Files 133
System Generator Software Features
enable or clear port may result in large fanout signals, thus degrading system
performance.
Figure 4-3: Use Global Port selections if necessary
•
Use cross-probing between the Xilinx Timing Analyzer and Leonardo or
Synplify Pro to identify critical paths. Design hierarchy is preserved when
using the Leonardo or Synplify project files that System Generator creates, thus
making it easy to correlate between the Timing Analyzer report and the Simulink
model. For more information refer to Xilinx Application note 406 at
http://www.xilinx.com/xapp/xapp406.pdf
Using the System Generator Constraints Files
When System Generator transforms a design into HDL, it also writes a constraints file
(also known as a ucf file). Constraints tell downstream tools how to process the
design. With the assistance of constraints, downstream tools can produce a higher
quality implementation than otherwise could have been obtained, and can do so
using considerably less time. Constraints supply the following information:
•
The period to be used for the system clock.
•
The speed, with respect to the system clock, at which various portions of the
design must run.
•
The pin locations at which ports should be placed.
•
The speed at which ports must operate.
System Clock Period
The system clock period (i.e., the period of the fastest clock in the design) can be
specified in the System Generator block. System Generator writes this period to the
constraints file, and the downstream tools use the period as a goal when
implementing the design.
The example below shows the constraints that specify the system clock period.
Multicycle Path Constraints
Many designs consist of parts that run at different clock rates. For the fastest parts,
the system clock period is used, and for the remaining parts, the clock period is an
integer multiple of the system clock period. It is important that downstream tools
know what speed each part of the design must achieve. With this information,
efficiency and effectiveness of the tools are greatly increased, resulting in reduced
compilation times and improved hardware realizations.