Pushing the Limits of FMAX: Timing Closure Strategies

In this AMD ECS 2026 series, we will highlight selected non-confidential technical sessions and share their key learning points.

Pushing the Limits of FMAX: Timing Closure Strategies

The following are the key points discussed by the presenter during this session.

1.Vertical H Tree Clock Distribution for Enhanced Deskew

2.MBUFG (Multi-clock buffer) to maximize performance across clock domains

3.Deskew mechanisms

4.Timing closure methodologies

5.Key steps for closing timing

• Initial Design Check
• Timing baselining
• Timing violation resolution

A few approach to consider for timing closure:

1. Setup violation resolution

• Reduce logic delay
• Reduce net delay
• Improve clock skew
• Improve clock uncertainty

2. Hold violation resolution

• Improve clock skew
• Avoid positive hold requirements
• Reduce WHS and THS before routing
• Improve clock uncertainty

AMD Versal™ Timing Closure: One Page Summary

1. Evaluate critical warnings

• Critical warnings detected early can save significant time instead of solving problems later in the flow.

2. Ensure all clocks and clock domain crossings are constrained

• Ensure the design is not under-constrained due to missing clocks.
• Ensure safe clock domain crossings to avoid implementation tool wasting runtime and effort on false paths.

3. Evaluate clocking architecture

• Need to ensure proper use of clock buffers and optimal MMCM/PLL settings for best QoR.

4. Evaluate methodology report during initial design checks

• Identify and resolve common design issues before running full implementation.

5. Perform timing baselining on post-synthesis design

• Use timing report and verify the WNS and WHS values.

6. Analyze and resolve the setup and hold timing violations step-by-step.

MBUFG: Maximize Performance Across Clock Domains

1. Problem with traditional multi-clock generation

• Long clock insertion delays can cause larger clock skew, particularly for paths originating from the bottom or sides of the device.
• Using multiple BUFGs for different output clocks introduces clock delay variation.

2. MBUFG (Multi-Clock Buffer)

• A new buffer architecture that generates multiple output clocks: /1, /2, /4, and /8.
• Provides a common clock-generation node closer to the destination logic.
• Uses a single clocking resource for multiple output clocks.

3. Key benefits

• Reduced clock skew: Common clock source helps minimize clock delay variation.
• Improved CDC performance: Optimizes synchronous clock-domain crossing paths.
• Resource efficiency: Reduces the need for multiple global clock buffers (BUFGs).
• Better clock routing: Preserves clock routing quality while bringing the common node closer to the destination logic.

4. Implementation

MBUFG can be instantiated using:

• Clocking Wizard IP with the buffer_type setting.
• Automatic inference using the Vivado optimization directive:

opt_design -mbufg_opt 

Things to consider for clean Synthesis Run

1. Run Synthesis on Clean Design

• Initial design check: utilization, logic levels, etc.
• Identify HDL issues for recoding – leverage AMD templates.

2. Constrain Clocks

• Constrain all primary and generated clocks.
• Define clock groups.

3. No Unsafe Clock Domain Crossings (CDC)

• Identify asynchronous and over-constrained CDC paths.
• Define false paths and multi-cycle paths, if needed.

4. If synthesis timing is not met – revisit steps 2–3

• Breadth of approaches to resolve timing issues.
• Leverage preconfigured ("canned") strategies.

Kudos to the AMD presenter for sharing such valuable insights, and to our team for capturing these important takeaways!

After going through the three topics — Fmax, Timing Closure, and Clean Synthesis Run — we believe there are some key lessons we can apply to our FPGA design and optimization activities.

Ourkey takeaways:

• Fmax Optimization: Achieving higher operating frequency starts with good RTL architecture, optimized logic, effective pipelining, and efficient resource utilization. Fmax should be considered from the initial design stage rather than as a final implementation target.
• Timing Closure: Proper clock constraints, safe clock-domain crossings, and an optimized clocking architecture are fundamental. We should evaluate critical warnings early, establish post-synthesis timing baselines, and systematically resolve setup and hold violations.
• Clean Synthesis Run: Starting with a clean design helps establish a reliable baseline. We should progressively introduce clock constraints, verify CDC paths, review methodology reports, and revisit constraints whenever synthesis timing is not satisfactory.

The bigger picture: These three topics are closely connected. Good RTL design, correct timing constraints, robust clock architecture, and disciplined implementation practices collectively determine the achievable Fmax and overall Quality of Results (QoR).

Ourbiggest takeaway is that timing closure should be a design consideration from day one, not a firefighting exercise at the end of the FPGA implementation flow.

A very insightful session from AMD, with practical lessons worth incorporating into our future Versal and FPGA design projects!