What is clock skew? What causes skew? How to balance skew?

o   Clock Skew: The difference in arrival time of the clock at capture FF and launch FF.

o    Local Skew: Skew between two specific, related flops (e.g., launch and capture flop of a timing path).

o    Global Skew: The difference between the maximum and minimum clock latency across all sinks in a domain.

o   Causes of Skew:

o    Different Path Lengths: The physical distance (wire length) from the clock root to different sinks varies due to their placement locations.

o    Different Path Delays: Even with equal lengths, paths can have different RC delays due to different routing layers, widths, or nearby coupling nets.

o    Buffer/Inverter Variations: Differences in buffer/inverter delays due to placement, drive strength variations, VT variations, or different loading conditions (fanout).

o    Uneven Loading: Branches of the clock tree driving significantly different capacitive loads can experience different delays.

o    On-Chip Variation (OCV): Process, voltage, and temperature variations across the die cause cells and wires in different paths to have slightly different delays.

o    Useful skew option used to help setup violations.

o   Resolving/Balancing Skew (CTS Process):

o    Buffer/Inverter Insertion & Sizing: The core CTS technique. The tool strategically inserts buffers/inverters and selects their drive strengths to equalize the delay down different branches of the tree. Delay might be added to faster paths to match slower paths.

o    Balanced Tree Topology: Algorithms aim to build a topologically balanced tree where branches have similar depths and loads.

o    Wire Length/Layer Adjustment: The tool tries to route clock nets to minimize length differences or uses specific layers where possible, although options are often limited by placement.

o    Dummy Load Insertion: Intentionally adding small capacitive loads (dummy cells) to faster paths to slow them down and match slower paths (less common now, buffer insertion is preferred).

o    Useful Skew (Post-CTS/Optimization): Intentionally introducing a controlled amount of skew to specific paths to help fix setup or hold violations