Signal-Integrity Interview Questions

Crosstalk delay affects timing. This occurs when both the aggressor net and the victim net are switching simultaneously or within a close timing window.

• Opposite Direction Switching: If the aggressor switches in the opposite direction to the victim (e.g., aggressor rises 0->1 while victim falls 1->0), the effect is increased delay on the victim net.
• The aggressor’s transition effectively increases the amount of charge the victim driver needs to supply/remove through the coupling capacitance (Miller effect). This makes the victim’s transition slower.
• Slower transition hurts setup timing but helps hold timing.
• Same Direction Switching: If the aggressor switches in the same direction as the victim (e.g., both rise 0->1), the effect is decreased delay on the victim net.
• The aggressor’s transition helps charge/discharge the coupling capacitance in the same direction the victim driver is trying to go, so speeding up the victim’s transition.
• Faster transition helps setup timing but hurts hold timing. This is often the worst-case scenario for hold analysis.

oPhysical Design Database (DEF): Contains the placed locations of all cells and the layout of the power grid

Parasitic Resistance (RC Extraction or SPEF): Accurate resistance values for all segments of the PDN (metal wires and vias). This usually comes from an RC extraction tool run on the power grid layout (e.g., from Quantus/StarRC). Sometimes derived from LEF/tech files for early estimates.

Library Power Information (.lib**):** Specifies the average leakage power (or current) consumed by each standard cell and macro.

o Vector-based Analysis: Used VCD (Value Change Dump) or FSDB files generated from gate-level simulations of specific, high-activity scenarios (e.g., boot-up sequence, high-performance benchmark execution, specific test modes). These vectors provide accurate, cycle-by-cycle switching activity for those specific scenarios, allowing us to identify peak IR drop and EM stress under known critical operating conditions

• Vectorless Analysis: To ensure broader coverage and identify potential worst-case scenarios not easily captured by specific VCDs, used vectorless dynamic analysis.
• Usually Vectorless is often used earlier in the flow for faster feedback, while vector-based analysis with critical scenarios mandatory for final signoff
• Given Toggle Rate? à Yes, toggle rates were used, primarily for:
• Static IR Drop/Power Analysis: Average toggle rates (often derived from synthesis estimates, statistical propagation, or averaged from simulations) used along with leakage data to calculate the average current for static analysis.
• Vectorless Dynamic Analysis (Seeding): Some vectorless techniques might use initial toggle rate information as a starting point for activity propagation or statistical analysis.
• Given TWF file? Yes, TWF (Timing Window File) files were used as input for dynamic analysis, especially for vectorless methods
• A TWF file contains information about the possible switching time windows for signals in the design.
• For each net or pin, it specifies the earliest and latest possible time (relative to the clock edge) that a signal transition (rise or fall) can occur

oStatic IR Drop Analysis: Calculates the average voltage drop across the PDN based on average current consumption (leakage + average switching).

Dynamic IR Drop Analysis (Voltage Droop): Simulates transient voltage drops based on switching activity.

Power/Ground EM Analysis: Checks for electromigration violations (average, RMS, peak current density) on the VDD and VSS network wires and vias based on calculated currents.

Signal EM Analysis: Checks for EM violations on signal interconnects, which is increasingly important at advanced nodes.

We have dynamic IR issue where specific location; cells are already low drive strength and no space to add decaps. We can still reduce IR by following ways:

• Improve PDN: add extra stripes or vias in that region if possible.
• Change in switching window: Analyze the switching activity. If many cells (even low-drive ones) in the area switch simultaneously causing the droop, try to introduce small timing delays on non-critical paths feeding into this region to slightly spread out the switching events over time. This reduces the peak current demand at any single instant.
• VT Swapping (Higher Vt): if there are some LVT cells contributing to the switching current in the region, swapping them to higher Vt (SVT/HVT)
• Re-evaluate Floorplan/Placement:

·Inputs for Crosstalk/Noise Analysis:

Timing Libraries (.lib**,** .db**):**

Parasitics (SPEF): Requires detailed parasitic extraction data, including accurate coupling capacitance (Cc​) values between nets, along with resistance (R) and ground capacitance (Cg​). This is crucial as Cc​ is the mechanism for crosstalk.

Netlist:

Constraints (SDC): Standard timing constraints (clocks, exceptions, etc.) are needed to determine timing windows. STA tools use timing windows to determine if aggressor and victim nets can switch simultaneously, making crosstalk delay/noise relevant.