Intel® Quartus® Prime Pro Edition User Guide: Timing Analyzer

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ID 683243
Date 3/28/2022
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Document Table of Contents
Document Table of Contents x
1. Answers to Top FAQs 2. Timing Analysis Introduction 3. Using the Intel® Quartus® Prime Timing Analyzer A. Intel® Quartus® Prime Pro Edition User Guides
2. Timing Analysis Introduction x
2.1. What's New In This Version 2.2. Timing Analysis Basic Concepts 2.3. Timing Analysis Overview Document Revision History
2.2. Timing Analysis Basic Concepts x
2.2.1. Timing Path and Clock Analysis 2.2.2. Clock Setup Analysis 2.2.3. Clock Hold Analysis 2.2.4. Recovery and Removal Analysis 2.2.5. Multicycle Path Analysis 2.2.6. Metastability Analysis 2.2.7. Timing Pessimism 2.2.8. Clock-As-Data Analysis 2.2.9. Multicorner Timing Analysis 2.2.10. Time Borrowing
2.2.1. Timing Path and Clock Analysis x
2.2.1.1. The Timing Netlist 2.2.1.2. Timing Paths 2.2.1.3. Data and Clock Arrival Times 2.2.1.4. Launch and Latch Edges
2.2.5. Multicycle Path Analysis x
2.2.5.1. Multicycle Clock Hold 2.2.5.2. Multicycle Clock Setup
2.2.10. Time Borrowing x
2.2.10.1. Time Borrowing Limitations 2.2.10.2. Time Borrowing with Latches
3. Using the Intel® Quartus® Prime Timing Analyzer x
3.1. Timing Analysis Flow 3.2. Step 1: Specify Timing Analyzer Settings 3.3. Step 2: Specify Timing Constraints 3.4. Step 3: Run the Timing Analyzer 3.5. Step 4: Analyze Timing Reports 3.6. Applying Timing Constraints 3.7. Timing Analyzer Tcl Commands 3.8. Timing Analysis of Imported Compilation Results 3.9. Using the Intel® Quartus® Prime Timing Analyzer Document Revision History 3.10. Intel® Quartus® Prime Pro Edition User Guide: Timing Analyzer Archive
3.4. Step 3: Run the Timing Analyzer x
3.4.1. Setting the Operating Conditions 3.4.2. Enabling Time Borrowing Optimization
3.5. Step 4: Analyze Timing Reports x
3.5.1. Generating Timing Reports 3.5.2. Cross-Probing with Design Assistant 3.5.3. Launching Design Assistant from Timing Analyzer 3.5.4. Locating Timing Paths in Other Tools 3.5.5. Correlating Constraints to the Timing Report
3.5.1. Generating Timing Reports x
3.5.1.1. Report Fmax Summary 3.5.1.2. Report Timing 3.5.1.3. Report Timing By Source Files 3.5.1.4. Report Data Delay 3.5.1.5. Report Net Delay 3.5.1.6. Report Clocks and Clock Network 3.5.1.7. Report Clock Transfers 3.5.1.8. Report Metastability 3.5.1.9. Report CDC Viewer 3.5.1.10. Report Asynchronous CDC 3.5.1.11. Report Logic Depth 3.5.1.12. Report Neighbor Paths 3.5.1.13. Report Register Spread 3.5.1.14. Report Route Net of Interest 3.5.1.15. Report Retiming Restrictions 3.5.1.16. Report Reset Statistics 3.5.1.17. Report Pipelining Information 3.5.1.18. Report Time Borrowing Data 3.5.1.19. Report Exceptions and Exceptions Reachability 3.5.1.20. Report Bottlenecks
3.5.1.13. Report Register Spread x
3.5.1.13.1. Registers with High Timing Path Endpoint Tension 3.5.1.13.2. Registers with High Immediate Fan-Out Tension
3.5.1.20. Report Bottlenecks x
3.5.1.20.1. Specifying Custom Bottleneck Criteria
3.5.2. Cross-Probing with Design Assistant x
3.5.2.1. Cross-Probing from Design Assistant to Timing Analyzer
3.6. Applying Timing Constraints x
3.6.1. Recommended Initial SDC Constraints 3.6.2. SDC File Precedence 3.6.3. Modifying Iterative Constraints 3.6.4. Using Entity-bound SDC Files 3.6.5. Creating Clocks and Clock Constraints 3.6.6. Creating I/O Constraints 3.6.7. Creating Delay and Skew Constraints 3.6.8. Creating Timing Exceptions 3.6.9. Using Fitter Overconstraints 3.6.10. Example Circuit and SDC File
3.6.1. Recommended Initial SDC Constraints x
3.6.1.1. Create Clock (create_clock) 3.6.1.2. Derive PLL Clocks (derive_pll_clocks) 3.6.1.3. Derive Clock Uncertainty (derive_clock_uncertainty) 3.6.1.4. Set Clock Groups (set_clock_groups)
3.6.4. Using Entity-bound SDC Files x
3.6.4.1. Entity-bound Constraint Scope 3.6.4.2. Entity-bound Constraint Examples
3.6.5. Creating Clocks and Clock Constraints x
3.6.5.1. Creating Base Clocks 3.6.5.2. Creating Virtual Clocks 3.6.5.3. Creating Generated Clocks (create_generated_clock) 3.6.5.4. Deriving PLL Clocks 3.6.5.5. Creating Clock Groups (set_clock_groups) 3.6.5.6. Accounting for Clock Effect Characteristics 3.6.5.7. Constraining CDC Paths
3.6.5.1. Creating Base Clocks x
3.6.5.1.1. Automatic Clock Detection and Constraint Creation
3.6.5.2. Creating Virtual Clocks x
3.6.5.2.1. Specifying I/O Interface Uncertainty 3.6.5.2.2. I/O Interface Clock Uncertainty Example
3.6.5.3. Creating Generated Clocks (create_generated_clock) x
3.6.5.3.1. Clock Divider Example (-divide_by) 3.6.5.3.2. Clock Multiplexer Example
3.6.5.5. Creating Clock Groups (set_clock_groups) x
3.6.5.5.1. Exclusive Clock Groups (-logically_exclusive or -physically_exclusive) 3.6.5.5.2. Asynchronous Clock Groups (-asynchronous) 3.6.5.5.3. set_clock_groups Constraint Tips
3.6.5.6. Accounting for Clock Effect Characteristics x
3.6.5.6.1. Set Clock Latency (set_clock_latency) 3.6.5.6.2. Clock Uncertainty
3.6.6. Creating I/O Constraints x
3.6.6.1. Input Constraints (set_input_delay) 3.6.6.2. Output Constraints (set_output_delay)
3.6.7. Creating Delay and Skew Constraints x
3.6.7.1. Advanced I/O Timing and Board Trace Model Delay 3.6.7.2. Maximum Skew (set_max_skew) 3.6.7.3. Net Delay (set_net_delay)
3.6.8. Creating Timing Exceptions x
3.6.8.1. Timing Exception Precedence 3.6.8.2. False Paths (set_false_path) 3.6.8.3. Minimum and Maximum Delays 3.6.8.4. Multicycle Paths 3.6.8.5. Multicycle Exception Examples 3.6.8.6. Delay Annotation 3.6.8.7. Constraining Design Partition Ports
3.6.8.4. Multicycle Paths x
3.6.8.4.1. Common Multicycle Applications 3.6.8.4.2. Relaxing Setup with Multicycle (set_multicyle_path) 3.6.8.4.3. Accounting for a Phase Shift (-phase)
3.6.8.5. Multicycle Exception Examples x
3.6.8.5.1. Default Multicycle Analysis 3.6.8.5.2. End Multicycle Setup = 2 and End Multicycle Hold = 0 3.6.8.5.3. End Multicycle Setup = 2 and End Multicycle Hold = 1 3.6.8.5.4. Same Frequency Clocks with Destination Clock Offset 3.6.8.5.5. Destination Clock Frequency is a Multiple of the Source Clock Frequency 3.6.8.5.6. Destination Clock Frequency is a Multiple of the Source Clock Frequency with an Offset 3.6.8.5.7. Source Clock Frequency is a Multiple of the Destination Clock Frequency 3.6.8.5.8. Source Clock Frequency is a Multiple of the Destination Clock Frequency with an Offset
3.7. Timing Analyzer Tcl Commands x
3.7.1. The quartus_sta Executable 3.7.2. Collection Commands
3.7.2. Collection Commands x
3.7.2.1. Wildcard Characters 3.7.2.2. Adding and Removing Collection Items 3.7.2.3. Query of Collections 3.7.2.4. Using the get_pins Command
1. Answers to Top FAQs
2. Timing Analysis Introduction
3. Using the Intel® Quartus® Prime Timing Analyzer
A. Intel® Quartus® Prime Pro Edition User Guides

3.6.1.4. Set Clock Groups (set_clock_groups)

The Set Clock Groups (set_clock_groups) constraint allows you to specify which clocks in the design are unrelated. By default, the Timing Analyzer assumes that all clocks with a common base or parent clock are related, and that all transfers between those clock domains are valid for timing analysis. You can exclude transfers between specific clock domains from timing analysis by cutting clock groups.

Conversely, clocks without a common parent or base clock are always unrelated, but timing analysis includes the transfers between such clocks, unless those clocks are in different clock groups (or if all of their paths are cut with false path constraints).

You define the clock signals to include in each Group (-group), then specify the relationship between different groups, and then specify whether the groups are Logically exclusive (-logically_exclusive), Physically exclusive (-physically_exclusive, or Asynchronous ( -asynchronous) from one another. 

set_clock_groups -asynchronous -group {<clock1>...<clockn>} ... \
    -group {<clocka>...<clockn>}
  • -logically_exclusive—defines clocks that are logically exclusive and not active at the same time, such as multiplexed clocks
  • -physically_exclusive—defines clocks that that cannot be physically on the device at the same time.
  • -asynchronous—defines completely unrelated clocks that have different ideal clock sources. This flag means the clocks are both switching, but not in a way that can synchronously pass data.

For example, if there are paths between an 8ns clock and 10ns clock, even if the clocks are completely asynchronous, the Timing Analyzer attempts to meet a 2ns setup relationship between these clocks, unless you specify that they are not related.

The following shows example constraints for a clock mux with two inputs, with multi-rate clocks, and the appropriate combinations of logical and physical exclusivity: 

# First profile
create_clock -name clk_a1 -period 10 [get_ports clk_a]
create_clock -name clk_b1 -period 20 [get_ports clk_b]

# Second profile
create_clock -name clk_a2 -period 100 [get_ports clk_a] -add
create_clock -name clk_b2 -period 200 [get_ports clk_b] -add

# Mark base clocks as asynchronous to each other
set_clock_groups -asynchronous -group {clk_a?} -group {clk_b?}

# Define muxed clocks for each profile
set muxout [get_pins -compatibility_mode {mux*|combout}]
foreach profile {1 2} {
     set mux_clk_a "mux_clk_a${profile}"
     set mux_clk_b "mux_clk_b${profile}"
     
     create_generated_clock -name $mux_clk_a -source [get_ports clk_a] \
          -master_clock "clk_a${profile}" $muxout -add
     create_generated_clock -name $mux_clk_b -source [get_ports clk_b] \
          -master_clock "clk_b${profile}" $muxout -add
     
     # Mark each muxed clock as logically exclusive to each other
     set_clock_groups -logically_exclusive -group $mux_clk_a \
          -group $mux_clk_b
}

# Mark profile clocks as physically exclusive to each other
# (Do this after defining the derived clocks so they get cut too)
set_clock_groups -physically_exclusive -group {*clk_?1} \
     -group {*clk_?2}
Figure 91. Example Constraints Design Topology

Although the Set Clock Groups dialog box only permits two clock groups, you can specify an unlimited number of -group {<group of clocks>} options in the .sdc file. If you omit an unrelated clock from the assignment, the Timing Analyzer acts conservatively and analyzes that clock in context with all other domains to which the clock connects.

The Timing Analyzer does not currently analyze crosstalk explicitly. Instead, the timing models use extra guard bands to account for any potential crosstalk-induced delays. The Timing Analyzer treats the -asynchronous and -exclusive options the same for crosstalk-related analysis, but treats asynchronous and exclusive clock groups differently for things like max skew reporting and synchronizer detection.

A clock cannot be within multiple groups (-group) in a single assignment; however, you can have multiple set_clock_groups assignments.

Another way to cut timing between clocks is to use set_false_path. To cut timing between sys_clk and dsp_clk, you can use: 

set_false_path -from [get_clocks sys_clk] -to [get_clocks dsp_clk]
set_false_path -from [get_clocks dsp_clk] -to [sys_clk]

This technique is effective if there are only a few clocks, but can become unmanageable with a large number of constraints. In a simple design with three PLLs that have multiple outputs, the set_clock_groups command can cut timing between clocks with less than ten lines, but the set_false_path command may require more than 50 lines.

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