Intel® Quartus® Prime Timing Analyzer Cookbook

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ID 683081
Date 11/12/2018
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Document Table of Contents
Document Table of Contents x
Intel® Quartus® Prime Timing Analyzer Cookbook
Intel® Quartus® Prime Timing Analyzer Cookbook x
Clocks and Generated Clocks I/O Constraints Exceptions Miscellaneous Intel® Quartus® Prime Timing Analyzer Cookbook Document Revision History
Clocks and Generated Clocks x
Basic Non-50/50 Duty Cycle Clock Offset Clocks Basic Clock Divider Using -divide_by Toggle Register Generated Clock PLL Clocks Multi-Frequency Analysis
Multi-Frequency Analysis x
Clock Multiplexing Externally Switched Clock PLL Clock Switchover
I/O Constraints x
Input and Output Delays with Virtual Clocks Tri-State Outputs System Synchronous Input System Synchronous Output I/O Timing Requirements (tSU, tH, and tCO)
Exceptions x
Multicycle Exceptions False Paths False Path Clock-to-Clock False Path Register-to-Register
Miscellaneous x
JTAG Signals Input and Output Delays with Multiple Clocks Clock Enable Multicycle
Intel® Quartus® Prime Timing Analyzer Cookbook

False Paths

You do not need to analyze timing on all paths. Synchronization of non-critical paths can be removed or cut from timing analysis. When you declare non-critical paths, the Intel® Quartus® Prime Fitter can focus on the optimization of critical paths and can reduce overall compilation time.
Figure 16.  Register-to-Register Cut Path

False paths can be applied either to clock-to-clock transfers or to individual registers. Applying false paths to clock-to-clock transfers cuts all paths between the target clocks.

In the False Path Clock-to-Clock example, the path is cut and not analyzed by the Timing Analyzer for any register-to-register path where the source clock is clkA and the destination clock is clkB. This does not affect register-to-register paths where the source register is clocked by clkB and the destination register is clocked by clkA.

Note: The set_false_path command cuts paths from clock clkA to clkB. The command does not cut paths from clkB to clkA. To cut paths from clkB to clkA, you must apply an additional set_false_path command (for example, set_false_path -from clkB -to clkA). Alternatively, you can use set_clock_groups to cut paths from clkA to clkB and from clkB to clkA with one command.

Applying false paths to individual registers cuts only the path specified.

False Path Clock-to-Clock 

create_clock -period 12 [get_ports clkA]
create_clock -period 5 [get_ports clkB]
set_false_path -from [get_clocks {clkA}] -to [get_clocks {clkB}]

In the False Path Register-to-Register example, the register-to-register path from register reg1 to register reg2 is cut. All other paths remain unaffected.

False Path Register-to-Register

create_clock -period 10 [get_ports clkA]
create_clock -period 5 [get_ports clkB]
set_false_path -from [get_pins {reg1|q}] -to [get_pins {reg2|d}]
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