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.