Power, performance, area, and efficiency

A physical design is not judged by one number. Power, performance, area, routability, and correctness interact, and improving one can make another worse.

Overview

The common shorthand is:

PPA

meaning:

Power
Performance
Area

Power

Digital power is often discussed as:

dynamic power
static/leakage power

A common intuition for switching power is:

P_dynamic ∝ α C V² f

where:

• α is switching activity
• C is capacitance
• V is supply voltage
• f is switching frequency

This is an intuition, not a complete chip-power equation.

Performance

For this synchronous block, performance is constrained by timing.

A smaller achievable clock period means a higher possible clock frequency:

f = 1 / T

Area

ASIC area may refer to different things:

standard-cell area
core area
die/block area

Do not mix them.

Efficiency is not one universal metric

Examples include:

area efficiency
energy per operation
performance per area
power density

Choose a metric that matches the design goal.

Prerequisites

• post-route timing

Steps

1. Open final metrics

RUN=$(ls -dt runs/* | head -1)

cat "$RUN/final/metrics.csv"

For machine-readable inspection:

python3 -m json.tool "$RUN/final/metrics.json" | less

2. Search useful metrics

grep -iE \
'area|util|power|slack|tns|wns|wire|cell|drc|lvs|antenna|ir' \
"$RUN/final/metrics.csv"

Metric names can change as the flow evolves.

Use the semantic meaning, not a memorized line number.

3. Record physical area

Record separately:

standard-cell area
core area
die/block area

For our explicit floorplan:

die = 150 µm × 150 µm

but your actual reports are the authority.

4. Calculate area utilization

A useful conceptual metric is:

cell utilization =
standard-cell area / core area

Do not confuse this with placement target density.

5. Record performance

Use post-route timing.

If your clock period is:

10 ns

the requested frequency is:

100 MHz

But if setup timing fails, the design has not demonstrated operation at that target.

A rough period estimate based only on worst setup slack can help intuition:

approximate required period
≈ constrained period - worst setup slack

when WNS is negative under the same assumptions.

Treat this as an approximation, not a complete signoff calculation.

6. Inspect power reports

Find power reports:

find "$RUN" -type f -iname '*power*' -o -iname '*irdrop*'

Search:

grep -RniE "total|internal|switch|leak|power" \
  "$RUN"/*sta* "$RUN"/*power* 2>/dev/null | head -120

Record the categories that the current flow actually reports.

7. Interpret power carefully

Power accuracy depends on switching-activity assumptions.

If the flow does not have realistic activity information, treat power as an estimate suitable for learning and relative comparison.

Do not present a vectorless estimate as measured silicon power.

8. Define one project efficiency metric

For example:

performance density =
frequency target / core area

or:

cell packing efficiency =
standard-cell area / core area

State exactly what you calculated and its units.

Do not invent a vague single "chip efficiency percentage."

9. Think in tradeoffs

Examples:

smaller core
→ less area
→ possibly more congestion
→ potentially worse timing

larger drive cells
→ potentially better timing
→ more area
→ more load/power

more buffers
→ may fix slew/timing
→ adds area and power

lower clock frequency
→ easier setup timing
→ lower throughput

10. Save your PPA table

Add to the project README:

## ASIC PPA

| metric | value |
|--------|------:|
| Standard-cell area | |
| Core area | |
| Die/block area | |
| Cell utilization | |
| Clock period | |
| Requested frequency | |
| Worst setup slack | |
| Worst hold slack | |
| Total power estimate | |
| Dynamic/switching power | |
| Leakage/static power | |
| Worst IR drop | |
| Total routed wirelength | |

Results

You should be able to explain why this statement is wrong:

Design A is better because it has fewer cells.

A complete comparison needs the design objective and physical results.

Checklist

• Recorded standard-cell area
• Recorded core and die area
• Calculated utilization
• Recorded timing
• Recorded power estimate
• Recorded IR drop
• Recorded wirelength
• Defined any efficiency metric explicitly
• Continued to page 22

Questions? Ask in the network Discord.