PT_S13W1_02.ppt

Agenda: Day One Clock Constraints 1 Timing Exceptions 2 Timing Analysis Techniques 3 Scripting: Collections and Procedures 4 Unit Objectives Define what are timing exceptions State the legal start and end points for timing exceptions State the command which reports ignored timing exceptions Given a multicycle path and specifications, apply a multicycle path constraint After completing this unit, you should be able to: Using Timing Exceptions What Are Timing Exceptions? Timing exceptions are used to override the default single-cycle constraints described by create_clock, set_input_delay, and set_output_delay Related Commands set_false_path set_multicycle_path set_max_delay set_min_delay Removes timing constraints from that timing path Allows more than one clock cycle for a timing path Specifies max and min delays Verifies any timing exceptions placed on a design Restores the default timing constraints on specified paths report_exceptions reset_path Legal Start and End Points for Exceptions Timing exceptions must be placed from and to existing timing points Legal Timing Start End Points Start End End Start For intermediate points in the timing path use the -through option Reduce the Number of Timing Exceptions To reduce the time to verify correct timing exceptions and analysis time; minimize the total number of exceptions. The path from Q2_reg to Q3_reg is a false path; it is used for test purposes only. How many exceptions do each of the following represent? set_false_path -from Q2_reg[*]/CP -to Q3_reg[*]/D set_false_path -from Q2_reg[*]/CP set_false_path -from clk2 -to clk3 2 1 4 Using the -through Option The -through option requires more CPU for large designs Do not use -through unless it is necessary set_false_path -from U1/CP -through U2/A -through U3/B -through U4/A -through U5/C -to U6/D Sub-Optimal: Optimal: set_false_path -to U6/D To help PT’s run time, use -to option when possible Disabled Arcs Instead of False Paths Use set_disabl