Different ways to Fix Setup or Hold VIOLATIONS in a design during PnR

How do I fix setup or hold violation during placement?

Setup and hold violations represent crucial timing constraints that can arise during the placement phase of digital circuit design, especially within ASIC and FPGA implementations. The following outlines a comprehensive methodology for identifying and rectifying these violations.

Understanding Setup and Hold Violations

Setup Violation: A setup time violation occurs when the data input to a flip-flop is not stable prior to the arrival of the clock edge, potentially resulting in the incorrect latching of data.

Hold Violation: Data input instability, occurring immediately following the clock edge, may result in data corruption due to the flip-flop's inability to properly latch the input.

Steps to Fix Setup Violations

Analyze Timing Reports: Use timing analysis tools to identify the specific paths causing setup violations. Look for critical paths and their delays.

Optimize Logic:

- Restructure Logic: Simplify or reorganize the logic to reduce the combinational path delays.

- Use Faster Logic Elements: If possible, replace slower gates with faster alternatives.

Increase Clock Period: If feasible, consider increasing the clock period to allow more time for signals to propagate.

Reposition Components: During placement, try to reduce the distance between flip-flops and their driving logic to minimize routing delay.

Add Buffers: Inserting buffers in critical paths can help drive the load and reduce delay.

Adjust Constraints: Review and adjust constraints in your design tools, such as clock definitions and false paths, to ensure accurate timing analysis.

Use Multi-Cycle Paths: If certain paths do not need to meet the setup time in every clock cycle, you can define them as multi-cycle paths.

Steps to Fix Hold Violations

Analyze Timing Reports: Similar to setup violations, use timing analysis tools to identify paths with hold violations.

Add Delay: Insert delay elements (like buffers) to increase the signal delay on paths that are too fast.

Reposition Components: Move the flip-flops or their driving logic closer to each other to minimize the routing delay.

Reduce Load: If possible, redesign the logic to decrease the capacitance seen by the flip-flops.

Use Faster Cells: Replace slower cells with faster ones that can provide the required timing margins.

Clock Skew: If applicable, consider adjusting clock skew to provide additional hold time for critical paths.