wire load model

Wire Load Model (WLM) is a statistical model of interconnect wire length and RC parasitics based on net fanout — used during synthesis and pre-layout STA to estimate delay before actual routing completes.

Why Wire Load Models?

• During synthesis: No physical routing exists — cannot compute actual wire length/delay.
• Need parasitic estimate for timing closure decisions.
• WLM: Table of estimated wire length as a function of fanout, derived from similar designs.

WLM Structure

WIRE_LOAD "wlm_typical_10K" {
  RESISTANCE  0.00010 ;
  CAPACITANCE 0.000110 ;
  AREA        0.003 ;
  SLOPE       0.040 ;
  FANOUT_LENGTH  1  0.050 ;
  FANOUT_LENGTH  2  0.100 ;
  FANOUT_LENGTH  4  0.200 ;
  FANOUT_LENGTH  8  0.400 ;
  FANOUT_LENGTH 16  0.800 ;
}

• `FANOUT_LENGTH`: Estimated wire length (μm) for given fanout.
• R and C per unit length from technology LEF or Liberty file.
• Net delay: $R_{wire} \times C_{wire}$ added to cell output delay.

WLM Limitations

• Accuracy: ±50% of actual post-route delay (statistical average).
• High-fanout nets: WLM underestimates — clock buffers, reset trees.
• Hierarchical blocks: Different WLM for each hierarchy level.
• Modern flows: Many designs bypass WLM entirely, using prototype routing for better estimates.

Zero Wire Load

• Special case: All wire delays = 0.
• Used for: Technology exploration, behavioral synthesis, first-pass area estimation.
• Not used for final timing sign-off.

Post-Route vs. WLM

• WLM-based synthesis: Close timing at ±50% accuracy.
• Post-route STA: Refine closure with actual extracted parasitics.
• Gap between WLM and actual: 10–30% timing difference common.

Virtual Flat WLM

• Most conservative: Assumes net can be routed anywhere in the die.
• Most accurate pre-layout for flat designs.
• Less suitable for hierarchical block-level synthesis.

Wire load models are the timing estimation bridge between synthesis and physical implementation — while they lack precision, they prevent synthesis from optimizing away critical-path cells that will be needed once routing reveals actual wire lengths.