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how to reduce and check wire delays in vivado in details method
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In Vivado, reducing and checking wire delays is crucial for achieving timing closure in FPGA designs. Wire delays arise from signal propagation through the routing resources, and they can affect the overall performance, especially in high-speed designs. Below is a detailed method for reducing and checking wire delays:
1. Understanding Wire Delays in Vivado:
Wire delays occur due to the physical routing between logic blocks in an FPGA. These delays include:
Combinational path delays: Delays due to data signals traveling through logic gates and interconnects.
Clock path delays: Delays from the clock source to sequential elements (flip-flops).
Setup and hold violations: When data takes too long or arrives too early relative to the clock, setup and hold violations occur.
2. Steps to Reduce Wire Delays in Vivado:
a) Floorplanning:
Proper floorplanning can help reduce the physical distance between related logic elements, minimizing wire delays.
Group related logic: Use Pblocks to constrain related logic (such as state machines, datapaths, or clocking logic) to the same region of the FPGA.
Optimize placement: Ensure critical paths have shorter routing by placing the logic closer together.
Run I/O planning: Proper placement of I/O pins relative to the logic they connect to can reduce I/O-related delays.
b) Timing Constraints:
Defining accurate timing constraints helps the Vivado place-and-route tool optimize paths more effectively.
Apply proper timing constraints: Use set_clock_uncertainty, create_clock, and set_input/output_delay to ensure accurate timing analysis.
Minimize path lengths: Use the set_max_delay constraint to limit the maximum delay on critical paths.
Multi-cycle paths: For paths that don’t need to complete in one clock cycle, mark them as multi-cycle paths using set_multicycle_path.
c) Clock Management:
Reduce clock skew: Use dedicated clocking resources like clock buffers (BUFG/BUFGCTRL), MMCMs, or PLLs to reduce clock skew and minimize delays.
Clock domain crossing: Ensure correct synchronization between different clock domains using proper techniques (e.g., using FIFOs, dual-flop synchronizers).
d) Optimize Logic:
RTL optimization: Write efficient RTL code to reduce the number of logic levels. Deep combinational logic leads to longer delays.
Register retiming: Vivado can move flip-flops across combinational logic to balance the path delays. Enable retiming in the synthesis settings.
Pipelining: Inserting pipeline registers in long combinational paths reduces delays by dividing the logic into smaller stages.
Use appropriate synthesis options: For example, use high-effort optimization settings in Vivado synthesis to improve logic placement and timing.
e) Use Vivado Optimization Features:
PhysOpt (Physical Optimization): Vivado offers a physical optimization step after placement to improve timing. Enable this option to reduce routing delays.
Placement directives: You can use different placement strategies (like "Explore" or "High Effort") to explore alternative placements and reduce delay.
Use incremental compile: This technique allows you to make small changes without re-optimizing the entire design, keeping the optimized paths intact.
3. Checking Wire Delays in Vivado:
Once the design is implemented, you can check wire delays using different reports and tools in Vivado.
a) Timing Summary Report:
After implementation, generate a Timing Summary Report to check the delays on different paths.
Open the report by navigating to Reports -> Timing -> Report Timing Summary.
This report will give you information on setup, hold, clock-to-out, and input-to-clock delays.
b) Report Worst Slack:
Run Report Worst Slack to find the critical paths with the largest timing violations.
Analyze the slack values for setup and hold violations. If wire delays are high, they will appear as contributing factors in the timing analysis.
c) Report Timing Path Details:
For more detailed analysis, generate a Detailed Timing Report for the critical paths. This can be done using the report_timing command in the Tcl console or through the GUI.
This report shows the exact breakdown of delays along the path, including the wire delays and logic delays for each segment.
d) Use Vivado’s Delay Viewer:
Net Delay Analysis: Use the Net Delay feature in the Vivado IDE to view the actual delay of nets. This helps in identifying long wires that may cause critical delays.
Navigate to Implementation -> Net Delay to view and analyze the delays associated with specific nets.
e) Physical Resource View:
Use the Device View in the Vivado IDE to see the physical placement and routing of the design. This can help you visually inspect long wires and analyze the routing of critical paths.
Paths with excessive wire delays often appear with long, complex routing, which can be mitigated by optimizing the placement.
4. Analyzing and Addressing High Wire Delays:
Critical Path Analysis: Look for paths where the majority of the delay comes from routing rather than logic. High routing delays are typically due to long distances between connected logic blocks.
Re-route Paths: If a specific path has a high delay, Vivado allows you to manually assign or influence the routing to achieve shorter wire lengths. Use the Routing Constraints Editor to manage this.
Reduce fanout: High fanout nets can increase routing delays. Use buffering or register replication to reduce the fanout.
5. Additional Tips:
Use Low-Skew Global Buffers (BUFG): Ensure clocks are routed using low-skew clock buffers.
Optimize I/O Timing: For I/O-heavy designs, optimize input and output timing to reduce I/O delays, which contribute to overall timing.
Run post-implementation simulations: Running simulations after place and route (with back-annotated delays) can help you confirm that wire delays are within acceptable limits.
첫댓글 PhysOpt (Physical Optimization) 인에이블
IO planning 새롭게 다시 하기 => slack이 개선됨