non-default rules (ndr)

**Non-Default Rules (NDR)** are **custom design rules** applied to specific critical nets that require **more stringent routing specifications** than the standard default rules used for general signal routing — providing enhanced signal integrity, timing control, and reliability for the most important nets on the chip. **Why NDR Is Needed** - Default routing rules (minimum width, minimum spacing) are optimized for **maximum density** — packing as many wires as possible into available routing space. - Some nets need better quality than maximum-density routing provides: - **Clock Networks**: Must have low skew, low jitter, low coupling. - **High-Speed I/O**: Need controlled impedance and minimal crosstalk. - **Reset/Enable Signals**: Must be immune to noise-induced glitches. - **Analog References**: Voltage references need shielding from digital noise. - **Critical Timing Paths**: Worst-case setup paths need reduced capacitance and coupling. **Common NDR Specifications** - **Wider Wire Width**: Increase wire width by 2× or more — reduces resistance and increases electromigration margin. Example: default 40 nm → NDR 80 nm. - **Wider Spacing**: Increase spacing to adjacent wires by 2× or more — reduces capacitive coupling and crosstalk. Example: default 40 nm → NDR 80 nm or 120 nm. - **Double Via**: Require via redundancy on all connections for the NDR net. - **Shielding**: Route the net with grounded shield wires on both sides — maximum crosstalk protection. - **Layer Restriction**: Restrict the net to specific metal layers (e.g., thick upper metals for lower resistance). - **No Jogs**: Require straight-line routing without direction changes. **NDR Application in Practice** - **Clock Trees**: The most common NDR application. Clock wires are routed with wider width and spacing (often called "clock NDR" or "CTS NDR"). - Wider spacing reduces clock-to-signal crosstalk → less jitter. - Wider width reduces clock wire resistance → less voltage drop, faster edge rates. - **Power/Ground**: Critical power connections use NDR for wider width and via redundancy. - **High-Speed Differential Pairs**: Use NDR for controlled impedance, matched spacing, and matched length. **NDR in the Design Flow** - NDR rules are defined in the constraint file (SDC, physical constraints). - The router reads NDR definitions and applies them to specified nets. - NDR nets consume more routing resources — they may increase routing congestion and require additional metal layers. - **Trade-off**: Better signal quality for NDR nets vs. increased area and congestion for the overall design. Non-default rules are the **key mechanism** for differentiating routing quality between critical and non-critical nets — they ensure that the most important signals on the chip receive the best possible interconnect quality.