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Chapter 7: Implementation > 7.5. Implementation Outputs - Pg. 166

166 Chapter 7 complete package development tool, then the tool automatically creates the correct format and feeds it into the implementation process. The user-defined constraint files contain such information as timing, pin assignments, and internal placement for logic. User-defined constraints put restrictions on the implementation tool and should be used with caution. Constraints make the tool work harder, because it must consider the restrictions that it must follow and still do its job. Implementing a design that utilizes most of the device's resources can greatly increase implementation time and may even cause the process to fail. I am not saying that constraints are bad and should not be used, because they can be necessary. I am just saying to consider all the factors when determining when and what should be constrained. If at all possible, try to keep the device utilization below a reasonable percent. As stated before, high resource utilization increases implementation time and makes it difficult if not impossible for the design to be placed and routed. What is reasonable? I leave it to you to determine a reasonable percent, but consider the room needed for potential growth and spare pins. Some companies predefine how to derate the resources. I like 50%, but that is not always possible, so this number changes from design to design. Over the years, preassigning pins to an FPGA has been my most used constraint. Assigning