Low Power, Technology

Power reduction hacks for better PPA

In ASIC Physical Design, mitigating IR drop (voltage drop due to resistance in power delivery networks) is critical for power integrity. Here are key power reduction methods used for achieving a better PPA and to help close on the IR drop limits, along with practical implementations:

1. Power Grid Optimization

A. Mesh Density Enhancement

Technique: Increase width/spacing of power rails and straps.
Impact: Reduces resistance ((R = \rho \cdot L/A)) in high-current regions.
Tool Command (Cadence Innovus):

  set_power_ring_strategy -nets {VDD VSS} -width 2um -spacing 1um

B. Decoupling Capacitors (Decaps)

Technique: Place decaps near high-switching logic (e.g., clock buffers, ALUs).
Impact: Local charge reservoirs suppress transient IR drop.
Implementation:

  add_decaps -cell DECAP_1UM -density 5% -region {x1 y1 x2 y2}

2. Clock Domain Optimization

A. Clock Gating

Technique: Disable clock trees for idle logic blocks.
Impact: Reduces dynamic power (switching activity) → lowers peak current.
Example:

  insert_clock_gating -gate_type ICG -threshold 10

B. Multi-Bit Flip-Flop (MBFF) Merging

Technique: Combine single-bit FFs into multi-bit FFs.
Impact: Fewer clock buffers → lower clock network power.
Tool Command:

  set_optimize_registers -merge_flops true

3. Logic & Placement Strategies

A. Voltage Scaling

Technique: Use Multi-Vt libraries:
LVT (Low-Vt) for critical paths.
HVT (High-Vt) for non-critical paths to reduce leakage.
Impact: Balances performance and IR drop.
Command:

  set_leakage_optimization true -power_driven true

B. Dynamic Voltage Frequency Scaling (DVFS)

Technique: Adjust voltage/frequency based on workload.
Impact: Lowers average current demand.
Implementation: Requires power switches and adaptive clocking.

4. Routing & Signal Integrity

A. Shielding High-Speed Nets

Technique: Route critical nets (clocks) between VDD/VSS.
Impact: Reduces crosstalk-induced current spikes.
Command:

  shield_net -net CLK -shield_net VSS -width 0.5um

B. Staggered Buffer Insertion

Technique: Insert buffers in staggered phases.
Impact: Smoothes di/dt (current slew rate) to avoid IR spikes.
Example:

  set_buffer_opt_strategy -stagger_insertion true

5. Advanced Techniques

A. Power Gating

Technique: Use header/footer switches to shut off power to idle blocks.
Impact: Eliminates leakage but requires wake-up time.
Implementation:

  insert_power_switch -name PSW -control_signal SLEEP -header

B. TSV Redundancy (3D ICs)

Technique: Add redundant Through-Silicon Vias (TSVs).
Impact: Reduces resistance in vertical power delivery.
Analysis:

  analyze_3dic_power -tsv_resistance 0.01 -tier 2

6. IR-Aware Timing Closure

A. Voltage-Aware STA

Technique: Use PrimeTime-SI with voltage derating.
Impact: Accounts for IR drop in timing signoff.
Command:

  set_voltage_derate -drop 0.9V -corner worst

B. ECO Fixes

Technique: Resize cells or add buffers in IR-drop hotspots.
Example:

  eco_add_buffer -cell BUF_X2 -location {x y} -net VDD

7. Toolflow for IR Closure

A. RedHawk/Voltus Analysis

# Ansys RedHawk
redhawk_analyze -ir_drop -dynamic -scenario worst_case

B. Power-Aware Place & Route

# Cadence Innovus
set_power_opt_mode -place true -route true

Key Takeaways

Preventive Measures: Optimize power grid and decap early.
Dynamic Reduction: Clock gating, MBFF merging.
Signoff Correlation: Voltage-aware STA and EM checks.

Example Interview Q&A:
Q: “How would you fix IR drop in a CPU block?”
A:

“First, I’d analyze RedHawk reports to identify hotspots. For localized drop, I’d add decaps and widen power straps. For global drop, I’d implement clock gating and use HVT cells in non-critical paths. Finally, I’d validate with voltage-aware STA.”