Hyperflex® Architecture High-Performance Design Handbook

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ID 683353
Date 12/06/2024
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Document Table of Contents
Document Table of Contents x
Answers to Top FAQs 1. Hyperflex® FPGA Architecture Introduction 2. Hyperflex® Architecture RTL Design Guidelines 3. Compiling Hyperflex® Architecture Designs 4. Design Example Walk-Through 5. Retiming Restrictions and Workarounds 6. Optimization Example 7. Hyperflex® Architecture Porting Guidelines 8. Appendices 9. Hyperflex® Architecture High-Performance Design Handbook Archive 10. Hyperflex® Architecture High-Performance Design Handbook Revision History
1. Hyperflex® FPGA Architecture Introduction x
1.1. Hyperflex® Architecture Design Concepts
2. Hyperflex® Architecture RTL Design Guidelines x
2.1. High-Speed Design Methodology 2.2. Hyper-Retiming (Facilitate Register Movement) 2.3. Hyper-Pipelining (Add Pipeline Registers) 2.4. Hyper-Optimization (Optimize RTL)
2.1. High-Speed Design Methodology x
2.1.1. Set a High-Speed Target 2.1.2. Experiment and Iterate 2.1.3. Compile Components Independently 2.1.4. Optimize Sub-Modules 2.1.5. Avoid Broadcast Signals
2.1.1. Set a High-Speed Target x
2.1.1.1. Speed and Timing Closure 2.1.1.2. Speed and Latency
2.2. Hyper-Retiming (Facilitate Register Movement) x
2.2.1. Reset Strategies 2.2.2. Clock Enable Strategies 2.2.3. Preserving Registers During Synthesis 2.2.4. Timing Constraint Considerations 2.2.5. Clock Synchronization Strategies 2.2.6. Metastability Synchronizers 2.2.7. Initial Power-Up Conditions 2.2.8. Retiming through RAMs and DSPs
2.2.1. Reset Strategies x
2.2.1.1. Removing Asynchronous Resets 2.2.1.2. Synchronous Resets on Global Clock Trees 2.2.1.3. Synchronous Resets on I/O Ports 2.2.1.4. Duplicate and Pipeline Synchronous Resets
2.2.2. Clock Enable Strategies x
2.2.2.1. Localized Clock Enable 2.2.2.2. High Fan-Out Clock Enable 2.2.2.3. Clock Enable with Timing Exceptions
2.2.4. Timing Constraint Considerations x
2.2.4.1. Optimize Multicycle Paths 2.2.4.2. Overconstraints
2.2.5. Clock Synchronization Strategies x
2.2.5.1. Clock Domain Crossing Constraint Guidelines
2.2.7. Initial Power-Up Conditions x
2.2.7.1. Specifying Initial Memory Conditions 2.2.7.2. Initial Conditions and Retiming 2.2.7.3. Initial Conditions and Hyper-Registers 2.2.7.4. Retiming Reset Sequences
2.2.7.3. Initial Conditions and Hyper-Registers x
2.2.7.3.1. Implementing Clock Gating 2.2.7.3.2. Quartus® Prime Settings for Initial Conditions
2.3. Hyper-Pipelining (Add Pipeline Registers) x
2.3.1. Conventional Versus Hyper-Pipelining 2.3.2. Pipelining and Latency 2.3.3. Use Registers Instead of Multicycle Exceptions
2.3.2. Pipelining and Latency x
2.3.2.1. Pipelining at Variable Latency Locations 2.3.2.2. Automatic Pipeline Insertion
2.3.2.1. Pipelining at Variable Latency Locations x
2.3.2.1.1. Specifying a Latency-Insensitive False Path
2.3.2.2. Automatic Pipeline Insertion x
2.3.2.2.1. Step 1: Create the Variable Latency Module 2.3.2.2.2. Step 2: Instantiate the Variable Latency Module 2.3.2.2.3. Step 3: Verify Automatic Pipeline Insertion Option 2.3.2.2.4. (Optional) Auto-Pipeline Insertion without a Variable Latency Module
2.4. Hyper-Optimization (Optimize RTL) x
2.4.1. General Optimization Techniques 2.4.2. Optimizing Specific Design Structures
2.4.1. General Optimization Techniques x
2.4.1.1. Shannon’s Decomposition 2.4.1.2. Time Domain Multiplexing 2.4.1.3. Loop Unrolling 2.4.1.4. Loop Pipelining 2.4.1.5. Precomputation
2.4.1.1. Shannon’s Decomposition x
2.4.1.1.1. Shannon’s Decomposition Example 2.4.1.1.2. Identifying Circuits for Shannon’s Decomposition
2.4.1.4. Loop Pipelining x
2.4.1.4.1. Loop Pipelining Theory 2.4.1.4.2. Loop Pipelining Demonstration 2.4.1.4.3. Loop Pipelining and Synthesis Optimization
2.4.2. Optimizing Specific Design Structures x
2.4.2.1. High-Speed Clock Domains 2.4.2.2. Restructuring Loops 2.4.2.3. Control Signal Backpressure 2.4.2.4. Flow Control with FIFO Status Signals 2.4.2.5. Flow Control with Skid Buffers 2.4.2.6. Read-Modify-Write Memory 2.4.2.7. Counters and Accumulators 2.4.2.8. State Machines 2.4.2.9. Memory 2.4.2.10. DSP Blocks 2.4.2.11. General Logic 2.4.2.12. Modulus and Division 2.4.2.13. Resets 2.4.2.14. Hardware Re-use 2.4.2.15. Algorithmic Requirements 2.4.2.16. FIFOs 2.4.2.17. Ternary Adders
2.4.2.1. High-Speed Clock Domains x
2.4.2.1.1. Visualizing Clock Networks 2.4.2.1.2. Viewing Clock Networks in the Fitter Report 2.4.2.1.3. Viewing Clocks in the Timing Analyzer
2.4.2.9. Memory x
2.4.2.9.1. Hyperflex® Architecture True Dual-Port Memory 2.4.2.9.2. Use Simple Dual-Port Memories 2.4.2.9.3. Hyperflex® Architecture Simple Dual-Port Memory Example 2.4.2.9.4. Memory Mixed Port Width Ratio Limits 2.4.2.9.5. Unregistered RAM Outputs
3. Compiling Hyperflex® Architecture Designs x
3.1. Compiling Submodules Independently 3.2. Design Assistant Design Rule Checking
3.2. Design Assistant Design Rule Checking x
3.2.1. Running Design Assistant During Compilation 3.2.2. Running Design Assistant in Analysis Mode
3.2.2. Running Design Assistant in Analysis Mode x
3.2.2.1. Cross-Probing from Design Assistant to Visualization Tools 3.2.2.2. Launching Design Assistant from Chip Planner 3.2.2.3. Launching Design Assistant from Timing Analyzer
4. Design Example Walk-Through x
4.1. Median Filter Design Example
4.1. Median Filter Design Example x
4.1.1. Step 1: Compile the Base Design 4.1.2. Step 2: Add Pipeline Stages and Remove Asynchronous Resets 4.1.3. Step 3: Add More Pipeline Stages and Remove All Asynchronous Resets 4.1.4. Step 4: Optimize Short Path and Long Path Conditions
5. Retiming Restrictions and Workarounds x
5.1. Setting the dont_merge Synthesis Attribute 5.2. Interpreting Critical Chain Reports
5.2. Interpreting Critical Chain Reports x
5.2.1. Insufficient Registers 5.2.2. Short Path/Long Path 5.2.3. Fast Forward Limit 5.2.4. Loops 5.2.5. One Critical Chain per Clock Domain 5.2.6. Critical Chains in Related Clock Groups 5.2.7. Complex Critical Chains 5.2.8. Extend to locatable node 5.2.9. Domain Boundary Entry and Domain Boundary Exit 5.2.10. Critical Chains with Dual Clock Memories 5.2.11. Critical Chain Bits and Buses 5.2.12. Delay Lines
5.2.1. Insufficient Registers x
5.2.1.1. Insufficient Registers Example 5.2.1.2. Optimizing Insufficient Registers 5.2.1.3. Critical Chains with Dual Clock Memories
5.2.2. Short Path/Long Path x
5.2.2.1. Hyper-Register Locations Not Available 5.2.2.2. Example for Hold Optimization 5.2.2.3. Optimizing Short Path/Long Path 5.2.2.4. Add Registers 5.2.2.5. Duplicate Common Nodes 5.2.2.6. Data and Control Plane
5.2.3. Fast Forward Limit x
5.2.3.1. Optimizing Path Limit
5.2.4. Loops x
5.2.4.1. Example of Loops Limiting the Critical Chain
6. Optimization Example x
6.1. Round Robin Scheduler
7. Hyperflex® Architecture Porting Guidelines x
7.1. Design Migration and Performance Exploration 7.2. Top-Level Design Considerations
7.1. Design Migration and Performance Exploration x
7.1.1. Black-boxing Verilog HDL Modules 7.1.2. Black-boxing VHDL Modules 7.1.3. Clock Management 7.1.4. Pin Assignments 7.1.5. Transceiver Control Logic 7.1.6. Upgrade Outdated IP Cores
8. Appendices x
8.1. Appendix A: Parameterizable Pipeline Modules 8.2. Appendix B: Clock Enables and Resets
8.2. Appendix B: Clock Enables and Resets x
8.2.1. Synchronous Resets and Limitations 8.2.2. Retiming with Clock Enables 8.2.3. Resolving Short Paths
8.2.1. Synchronous Resets and Limitations x
8.2.1.1. Synchronous Resets Summary
8.2.2. Retiming with Clock Enables x
8.2.2.1. Example for Broadcast Control Signals
Answers to Top FAQs
1. Hyperflex® FPGA Architecture Introduction
2. Hyperflex® Architecture RTL Design Guidelines
3. Compiling Hyperflex® Architecture Designs
4. Design Example Walk-Through
5. Retiming Restrictions and Workarounds
6. Optimization Example
7. Hyperflex® Architecture Porting Guidelines
8. Appendices
9. Hyperflex® Architecture High-Performance Design Handbook Archive
10. Hyperflex® Architecture High-Performance Design Handbook Revision History

5.2.9. Domain Boundary Entry and Domain Boundary Exit

The Path Info column lists the Domain Boundary Entry or Domain Boundary Exit for a critical chain. Domain boundary entry and domain boundary exit refer to paths that are unconstrained, paths between asynchronous clock domains, or between a clock domain and top-level device input-outputs. Domain boundary entry and exit can also be indicated for some false paths as well.

A domain boundary entry refers to a point in the design topology, at a clock domain boundary, where Hyper-Retiming can insert register stages (where latency can enter the clock domain) if Hyper-Pipelining is enabled. The concept of a domain boundary entry is independent of the dataflow direction. Hyper-Retiming can insert register stages at the input of a module, and perform forward retiming pushes. Hyper-Retiming can also insert register stages at the output of a module, and perform backward retiming pushes. These insertions occur at domain boundary entry points.

A domain boundary exit refers to a point in the design topology, at a clock domain boundary, where Hyper-Retiming can remove register stages and the latency can exit the clock domain, if Hyper-Pipelining is enabled. Removing a register seems counter intuitive. However, this method is often necessary to retain functional correctness, depending on other optimizations that Hyper-Retiming performs.

Sometimes a critical chain indicates a domain boundary entry or exit when there is an unregistered I/O feeding combinational logic on a register-to-register path as shown in the following figure.

Figure 125. Domain Boundary with Unregistered Input/Output

The register-to-register path might be shown as a critical chain segment with a domain boundary entry or a domain boundary exit, depending on how the path restricts Hyper-Retiming. The unregistered input prevents the Hyper-Retiming from inserting register stages at the domain boundary, because the input is unregistered. Likewise, the unregistered input can also prevent Hyper-Retiming from removing register stages at the domain boundary.

Critical chains with a domain boundary exit do not provide complete information for you to determine what prevents retiming a register out of the clock domain. To determine why a register cannot retime, review the design to identify the signals that connect to the other side of a register associated with a domain boundary exit.

Domain boundary entry and domain boundary exit can appear independently in critical chains. They can also appear in combination such as, a domain boundary exit without a domain boundary entry, or a domain boundary entry at the beginning and end of a critical chain.

The following critical chain begins and ends with domain boundary entry. The domain boundary entries are the input and output registers connecting to top-level device I/Os. The input register is round_robin_requests_r and the output register is round_robin_next.

Figure 126. Critical Chain Schematic with Domain Boundary

The limiting reason for the base compile is Insufficient Registers.

Figure 127. Retiming Limit Summary with Insufficient Registers

The following parts of the critical chain report show that the endpoints are labeled with Domain Boundary Entry.

Figure 128. Critical Chain with Domain Boundary Entry

Both the input and output registers are indicated as Domain Boundary Entry because the Fast Forward Compile often inserts register stages at these boundaries if Hyper-Pipelining were enabled.

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