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

5.2.4. Loops

A loop is a feedback path in a circuit. When a circuit is heavily pipelined, loops are often a limiting reason to increasing design fMAX through register retiming. A loop may be very short, containing only a single register or much longer, containing dozens of registers and combinational logic clouds. A register in a divide-by-two configuration is a short loop.

Figure 115. Simple Loop

When the critical chain is a feedback loop, register retiming cannot change the number of registers in a loop without changing functionality. Registers can retime around a loop without changing functionality, but adding registers to the loop changes functionality. To explore performance gains, the Fast Forward Compile process adds registers at particular boundaries of the circuit, such as clock domain boundaries.

Figure 116. FIFO Flow Control LoopIn a FIFO flow control loop, upstream processing stops when the FIFO is full, and downstream processing stops when the FIFO is empty.
Figure 117. Counter and Accumulator LoopIn a counter and accumulator loop, a register's new value depends on the old value. This includes variants like LFSRs (linear feedback shift register) and gray code counters.
Figure 118. State Machine LoopIn a state machine loop, the next state depends on the current state of the circuit.
Figure 119. Reset Circuit LoopReset circuit loops include monitoring logic to reset on an error condition.

Use loops to save area through hardware re-use. Components that you re-use over several cycles typically involve loops. Such components include CRC calculations, filters, floating point dividers, and word aligners. Closed loop feedback designs, such as IIR filters and automatic gain control for transmitter power in remote radiohead designs, also use loops.


Section Content
Example of Loops Limiting the Critical Chain

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