Part 9 of 10

Power Analysis

IR drop, EM analysis, and power grid verification

By Praveen Kumar Vagala

1,098 views

Why Power Analysis?

Power issues can cause chip failure even if timing is clean.

IR Drop - Voltage doesn't reach cells at full level
Electromigration - High current destroys wires over time
Power consumption - Heat, battery life concerns

IR Drop

IR Drop = Current × Resistance

VDD Pad = 1.0V │ R = 10mΩ │ ▼ ┌────┴────┐ │ 0.99V │ └────┬────┘ R = 20mΩ │ ▼ ┌────┴────┐ │ 0.97V │ ← Cells see lower voltage! └────┬────┘ R = 30mΩ │ ▼ ┌────┴────┐ │ 0.94V │ ← 6% drop - may cause failures! └─────────┘

Static IR Drop

Average voltage drop based on average current draw.

Dynamic IR Drop

Instantaneous drop during switching (much worse than static).

Static Dynamic Voltage: ─────────────── ─────╲ ╱───── Constant drop ╲╱ Sudden dip during switching!

IR Drop Analysis

# Setup power analysis (Voltus/Innovus)
read_activity_file -format VCD design.vcd
# or
read_activity_file -format SAIF design.saif

# Set power pads
set_power_pads -net VDD -pad {PAD_VDD*}
set_power_pads -net VSS -pad {PAD_VSS*}

# Run analysis
analyze_power_grid -net VDD
analyze_power_grid -net VSS

# Check IR drop
report_power_rail_results

IR Drop Guidelines

Type	Limit
Static IR	< 5% of VDD
Dynamic IR	< 10% of VDD
Hot spots	Identify and fix

Fixing IR Drop

IR Drop Fixes: 1. Add more power straps 2. Widen power straps ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ (more straps) (wider straps) 3. Add more power pads 4. Reduce switching ▓VDD▓ ▓VDD▓ (clock gating, lower freq) (more pads at edge) 5. Move high-current blocks closer to power pads

Electromigration (EM)

High current density causes metal atoms to move, eventually breaking wires.

Current flow → ═══════════════════════════ ↓ ↓ ↓ Metal atoms move ↓ ↓ ↓ ═══════╳═══════════════════ │ Void forms → Wire breaks!

EM Rules

Parameter	Description
Javg	Average current density limit
Jpeak	Peak current density limit
Jrms	RMS current density limit

# EM analysis
analyze_rail -type em -net VDD

# Check EM violations
report_em_violations

# Typical limits (example):
# Javg: 1 mA/um for M1
# Javg: 2 mA/um for upper metals

Fixing EM Violations

Widen power straps/rails
Add parallel straps
Use thicker metal layers
Reduce current (lower frequency, clock gating)

Power Consumption Analysis

Power Components

Total Power = Dynamic Power + Static Power

Dynamic = α × C × V² × f
  α = switching activity
  C = capacitance
  V = voltage
  f = frequency

Static = I_leak × V
  I_leak = leakage current

Power Report

# Generate power report
report_power -hierarchy

Example output:
Module          Switching   Internal   Leakage    Total
─────────────────────────────────────────────────────────
top             125.3mW     85.2mW     5.1mW      215.6mW
  cpu_core       80.1mW     52.3mW     3.2mW      135.6mW
  cache          35.2mW     25.1mW     1.5mW       61.8mW
  peripherals    10.0mW      7.8mW     0.4mW       18.2mW

Power Optimization

Technique	Reduces	Method
Clock gating	Dynamic	Turn off clock to idle blocks
Multi-Vt	Leakage	Use HVT on non-critical paths
Power gating	Both	Turn off power to idle blocks
DVFS	Both	Lower V and f when not needed
Operand isolation	Dynamic	Hold inputs stable

Power Grid Sign-off Flow

┌─────────────────────────────────────────────────────────────┐ │ 1. Extract power grid resistance │ │ • RC extraction of power nets │ ├─────────────────────────────────────────────────────────────┤ │ 2. Get switching activity │ │ • VCD/SAIF from simulation │ │ • or vectorless (statistical) │ ├─────────────────────────────────────────────────────────────┤ │ 3. Static IR drop analysis │ │ • Check average drop │ ├─────────────────────────────────────────────────────────────┤ │ 4. Dynamic IR drop analysis │ │ • Check peak drop during switching │ ├─────────────────────────────────────────────────────────────┤ │ 5. EM analysis │ │ • Check current density limits │ ├─────────────────────────────────────────────────────────────┤ │ 6. Fix violations and iterate │ │ • Strengthen power grid where needed │ └─────────────────────────────────────────────────────────────┘

Power Analysis Tools

Vendor	Tool	Purpose
Cadence	Voltus	Power/IR/EM analysis
Synopsys	PrimeTime PX	Power analysis
Synopsys	RedHawk	IR/EM analysis
ANSYS	RedHawk-SC	IR/EM analysis

Summary

Check	Limit
Static IR drop	< 5% VDD
Dynamic IR drop	< 10% VDD
EM (Javg)	Per layer limit
Power consumption	Meet power budget

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