Signal-Integrity Interview Questions

oWe can run IR aware full flow

• IR aware placement : Spread high power density hotspots to reduce IR drop. Tools settings using setPlaceMode. This is useful even during routing as during optimization tool adds buf/inv and updates placement increamentally.
• CCopt/SkewClock: skew the clock to minimize peak current
• Reinforce_pg: Auto RPG: Loacl PG stripe/via addition in hotspot area. (separate utility from cadence. Needs voltus IR setup as input.
• Fill: Maximize PG hookup at signoff stage.
• https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1O3w000009foP9EAI&pageName=ArticleContent

In general routers are now performs rudimentary current density estimation on wires and handles IR/RM.

oSAIF contains Activity over time window but it doesn’t have timestamp for each change in signal. VCD notes each timestamp for change.

• So Saif has overall toggle count over time but not exact waveform of change.
• Saif is relatively less in size wherein VCD contains more detailed info and hence large in size.

o Signal Integrity (SI): Quality of an electrical signal as it travels from a driver to a receiver through an interconnect. Major SI concerns include:

o Crosstalk: Unwanted coupling between adjacent signal nets.

o IR Drop: Voltage drop on the power/ground network affecting cell performance.

o Electromigration (EM): Reliability issue due to metal migration under high current density.

o Timing Jitter/Phase Noise: Variations in the timing of signal edges, especially critical for clocks.

1. Increase Spacing: Physically increase the distance between the victim and aggressor nets during routing. This directly reduces coupling capacitance (Cc​). (Most effective but consumes routing area).

Shielding: Insert a static net (tied to VDD or VSS) between the victim and aggressor. The shield net intercepts coupling capacitance, preventing interference. This can add coupling cap on signal net causing delay.

Layer Change: Route the victim or aggressor on different metal layers for a portion of their length to reduce parallel run length and coupling.

o Handling Static IR Drop: Static IR drop is primarily due to the resistance of the power grid (Vdrop​=Iavg​×Rgrid​). If you cannot improve Rgrid​ (by widening straps/adding vias):

• Reduce Average Current (Iavg​):

Downsize Non-Critical Cells: Replace cells contributing significantly to static leakage in the affected region with smaller drive-strength variants (if timing permits).

VT Swapping (High VT): Swap cells to higher Vt variants (LVT -> SVT -> HVT) in the affected area. Higher Vt cells have significantly lower leakage current. This requires available timing slack.

o Fixing IR drop involves 1) strengthening the Power Distribution Network (PDN) to reduce its resistance or 2) reducing the current drawn by the logic.

• During Placement/Floorplan (Preventative)
• Robust PDN Design: Plan a dense power grid using wide straps/rings on low-resistance metal layers with ample vias
• Macro Placement: Place high-power macros near power sources or ensure they have strong connections to the power grid.
• Cell Placement: Avoid clustering high-power or high-switching activity cells in one area. Use density controls.
• During Post-Route Optimization / ECO Stage:
• Strengthen PDN:
• Add/Widen Power Straps: Introduce more power/ground stripes in areas with high voltage drop or increase the width of existing straps.
• Add Power Vias: Increase the number of vias connecting different layers of the power grid, at connections to cell rails, to reduce vertical resistance.
• Add Decap Cells (Primarily for Dynamic IR):
• Reduce Current Draw:
• Cell Downsizing: Replace high-power cells in the violating region with smaller drive-strength equivalents, if no timing violations.
• VT Swapping (Higher Vt): Swap cells to higher-Vt to reduce leakage current (helps static IR) and slightly reduce peak dynamic current (helps dynamic IR), If no timing issues.
• Spread High-Activity Cells: If dynamic IR drop is caused by simultaneously switching cells clustered together, try spreading these cells apart during ECO placement