Power optimization

Power Optimization

A measurement-driven playbook for analyzing and reducing FPGA power. Power is a physical, post-implementation property: it depends on placement, routing, clock enables, switching activity, and I/O drive — not just RTL. Every claim of power reduction MUST be backed by a fresh report_power from an implemented design. Synthesis-stage estimates are directional only.

When to use

• User asks to "reduce power", "optimize power", "lower power consumption", "cut dynamic power", or "fix leakage / static power".
• User reports the device runs too hot: high junction temperature, thermal margin warnings, fan/heatsink concerns, or a battery/thermal budget.
• User wants a power report (report_power) explained, or a confidence-level / breakdown analysis (clocks vs logic vs signals vs BRAM vs DSP vs I/O vs static).
• User wants power compared before/after a change, or wants to know whether a strategy/constraint change moved the power number.

When NOT to use

• Timing is failing (negative WNS/TNS). Power optimization can lengthen paths and make timing worse. Close timing FIRST (use the timing-closure skill), then optimize power. Refuse to trade away met timing for power without explicit user sign-off.
• The design has not been implemented yet. Power on a pre-implementation design is a wild guess. If only synthesis exists, say so and run implementation first (or state the estimate is low-confidence).
• Pure functional bugs, CDC, lint, or bitstream-generation problems — use the matching skill. Power is downstream of a working, routed design.
• The user wants a custom XPE/spreadsheet pre-silicon power budget before any RTL exists. This skill operates on a real design in the project.

Prerequisites (verify first)

Run these BEFORE any analysis. Do not assume state.

1. test_connection — confirm the Tcl bridge to Vivado is live. If it fails, stop and tell the user to start Vivado + paste tcl_server.tcl.
2. get_project_info — confirm a project is open and learn the target part (power is part- and speed-grade-specific) and the active run.
3. get_run_status — confirm whether implementation is complete. report_power is only meaningful on an implemented (placed + routed) design.
   • If implementation is NOT done: run it via run_implementation_async, then poll get_run_status until complete. Do NOT block.
   • If only synthesis exists and the user wants a quick directional number, open_synthesized_design + report_power is allowed but you MUST label the result "synthesis estimate, low confidence — not signoff."
4. open_implemented_design — load the routed design into memory so power and utilization queries reflect real placement/routing.
5. Capture the baseline before touching anything:
   • report_power (total W, dynamic vs static split, per-category breakdown, junction temperature, thermal margin, and the confidence level).
   • report_utilization (LUT/FF/BRAM/DSP/IO/CLK counts — power scales with these and with clock count). Record these numbers. Every later iteration is compared against this baseline.

Power-report literacy (read before acting)

report_power is only as good as its switching-activity assumptions. Know which mode you are in:

• Vectorless estimation (default): Vivado guesses toggle rates from a default primary-clock activity and a default toggle rate (often ~12.5%). The report's Confidence level will read Low or Medium. Treat the absolute watts as approximate; treat the relative before/after delta and the category breakdown as the actionable signal.
• SAIF-driven (high confidence): real switching activity from a gate-level simulation SAIF is annotated onto the netlist. This is the only path to a High confidence number and to trustworthy dynamic power. SynthPilot cannot annotate a SAIF for you through these tools — if the user needs signoff-grade dynamic power, tell them to run a post-implementation timing simulation, dump a SAIF, and annotate it in Vivado directly; this skill then consumes the resulting report_power.

Always read and report the confidence level. Never present a Low/Medium vectorless number as a guaranteed result.

Methodology

Work in a strict measure → classify → smallest-safe-change → re-measure loop. One change-class per iteration so cause and effect are attributable.

1. Establish the baseline and the goal

• Read the baseline report_power. Identify total power, the dynamic/static split, and the dominant categories (sort the breakdown: Clocks, Logic, Signals, BRAM, DSP, I/O, GTs/MMCM/PLL, Static/Device).
• Read junction temperature and thermal margin. If thermal margin is negative or near zero, this is a thermal problem (cooling/ambient/derating), not just a logic-power problem — flag it explicitly.
• Confirm the user's target: a watt budget, a temperature limit, or "as low as reasonable." Without a target, optimize the largest movable category and stop when changes get risky.
• Static power is dominated by silicon (part, speed grade, junction temp) and is mostly fixed for a given device. The only real levers are: a lower-power/slower speed grade, a smaller device, lower Vccint (device/board decision), or reducing junction temperature (which lowers leakage). Surface these as decisions; do not pretend RTL changes will move static power much.

2. Classify the dominant power source

Use the breakdown to pick ONE target category, then consult the table below. Attack the biggest movable contributor first.

3. Apply the SMALLEST SAFE change first (constraints/strategy before RTL)

Ordered by safety:

1. Tool strategy — power-aware implementation strategy. Set set_implementation_strategy (e.g. Area_Explore to shrink logic/route, or Flow_RuntimeOptimized only for quick iteration). Vivado's power_opt_design (intelligent clock gating) is enabled inside power-oriented strategies; this is the safest single lever because it does not touch RTL. Re-run implementation and re-measure.
2. Constraints / I/O properties — for I/O-dominated power, the fix is in the XDC (slew, drive strength, on-chip termination, lower-power I/O standard via create_io_constraint / editing the constraint file). For unused clocks, removing or gating the source.
3. IP reconfiguration — for BRAM/DSP/MMCM/PLL-dominated power, reconfigure the offending IP (e.g. BRAM cascade/sleep, fewer MMCM/PLL outputs, lower clock frequency in configure_clocking_wizard / create_clocking_wizard).
4. RTL / architecture (recommend, do not silently apply) — clock-enable gating, reducing active clock domains, operand isolation, RAM enable gating, downclocking idle logic. Propose the edit, show the file/line via read_file / read_file_lines, and let the user approve before update_file / replace_in_file.

Apply exactly ONE change-class, then go to step 4. Never stack a strategy change, an I/O change, and an RTL change in one pass — you will not know what worked.

4. Re-implement and re-measure (the only proof)

• Re-run with run_implementation_async; poll get_run_status until done. (Never claim a power result from a stale run.)
• open_implemented_design, then report_power again. Compare against the previous iteration: total W, the targeted category, AND the confidence level.
• Guardrail check every iteration: report_timing_summary (WNS/TNS must not have gone negative) and report_drc (no new violations). A power win that breaks timing or DRC is not a win — revert it.
• Record the before/after delta. If the change did not help (or hurt timing), revert it and pick the next item from the table.

5. Loop or stop (see STOP conditions)

Repeat steps 2–4. Stop when a STOP condition is met, then produce the Output.

Classification table — symptom → likely cause → smallest safe fix

Dominant category in report_power	Likely root cause	Smallest safe fix first	Tools
Clocks high	Too many/high-freq clock nets always toggling; ungated clock trees; redundant MMCM/PLL outputs	Power-aware impl strategy (enables clock gating); reduce clock frequency where spec allows; remove unused generated clocks; consolidate clock domains	report_clock_networks, report_clock_utilization, set_implementation_strategy, configure_clocking_wizard, then re-impl + report_power
Logic / Signals high	High average toggle rate; large always-active combinational/sequential mass; high-fanout nets driving wide loads	Power-aware impl strategy + power_opt clock gating; then RECOMMEND clock-enable gating / operand isolation in RTL	report_high_fanout_nets, report_utilization_hierarchical (find the hot module), set_implementation_strategy, then RECOMMEND RTL via read_file_lines
BRAM high	Memories with enable always asserted; over-wide/over-deep RAMs; no sleep/cascade	Reconfigure memory IP (narrower/cascade/sleep); RECOMMEND RAM enable gating so unused banks idle	report_utilization, get_memory_config, configure_memory, then re-impl + report_power
DSP high	DSP slices clocked/active when idle; no operand isolation	Strategy that reduces/area-optimizes DSP (Flow_AreaMultThresholdDSP synth); RECOMMEND operand isolation / DSP enable gating	set_synthesis_strategy, report_utilization, then RECOMMEND RTL
I/O high	Excess slew rate, high drive strength, power-hungry I/O standard, unnecessary on-chip termination, high-toggle outputs	Edit XDC: lower SLEW (SLOW), reduce DRIVE, choose lower-power IOSTANDARD, remove needless termination	report_io, create_io_constraint / edit constraint file via read_file+replace_in_file, then re-impl + report_power
MMCM / PLL / GT high	More clock outputs or transceiver lanes than used; high VCO; unused GT channels powered	Reduce MMCM/PLL outputs & VCO; disable unused GT lanes (IP reconfigure)	report_clock_utilization, configure_clocking_wizard, IP reconfigure, then re-impl + report_power
Static / Device high (large share)	Silicon leakage: high junction temp, hot speed grade, oversized device	Mostly a device/thermal DECISION: lower junction temp (cooling/ambient), lower-power speed grade, smaller part. Surface as options — do NOT expect RTL to move it	report_power (junction temp, thermal margin), then present options + STOP
Junction temp / thermal margin critical	Inadequate cooling, high ambient, or genuinely high total power	Reduce dynamic power (rows above) AND flag cooling/ambient/derating; recompute margin after	report_power, then iterate dynamic-power rows + surface thermal trade-off

Loop and STOP conditions

Iterate the measure→fix→re-measure loop. STOP and hand back to the human when any of:

• Goal met. Fresh post-implementation report_power meets the user's watt or temperature target AND report_timing_summary shows WNS ≥ 0 (or unchanged from baseline) AND report_drc is clean. Report the confidence level with the number.
• Diminishing returns. Two consecutive iterations each yield < ~2% total power improvement. Further squeezing isn't worth the risk; present remaining options.
• Safe options exhausted. Strategy, constraints, and IP-config levers are used up and only RTL/architectural rewrites or a device/speed-grade/board change remain. Present those options + their costs (timing risk, redesign effort, BOM change) and STOP. Do not start large RTL surgery autonomously.
• A trade-off threshold is hit. A power win would require degrading timing below the user's margin, changing the device/part, or waiving a real DRC. Stop, state the trade-off, ask the user.
• Static-power wall. The remaining gap is dominated by static/leakage power that only a device/thermal decision can move. Present device/cooling options and STOP.
• Low-confidence ceiling. All numbers are vectorless Low/Medium confidence and the user needs signoff-grade dynamic power. Stop and request a SAIF from post-implementation simulation; do not present the estimate as final.

Never thrash: if a change doesn't help, revert it and move to the next table row rather than re-trying variants of the same lever.

Safety rails (do not violate)

• Evidence before claims. "Power reduced from X W to Y W" REQUIRES a fresh report_power from a completed implementation run — never inferred from synthesis, never from a stale run, never extrapolated. Always state the confidence level of the number you quote.
• Timing and DRC are gates, not afterthoughts. Every iteration MUST verify report_timing_summary (no new negative slack) and report_drc (no new violations). A lower power number on a now-failing design is a regression.
• No fake-pass. Do NOT make the power number "look good" by switching the report to an optimistic activity assumption, lowering the default toggle rate, or quoting a Low-confidence vectorless estimate as final. Do NOT silence a real DRC/methodology violation to clear the path. If you must assume a switching activity, state the assumption explicitly.
• Smallest safe change first. Strategy/constraints/IP-config before RTL. Recommend RTL and architectural edits (clock gating, operand isolation, enable gating) — show the file and lines, and get user approval before update_file / replace_in_file. Never silently rewrite RTL for power.
• Never trade away met timing or correctness for power without explicit user sign-off. Lowering frequency, gating clocks, or shrinking memories can change functional behavior or break timing — surface the consequence first.
• Static power honesty. Do not claim RTL or constraint changes will meaningfully reduce static/leakage power. State plainly that static power is a device/speed-grade/junction-temperature property and route those to a human decision.
• One change-class per iteration so the before/after delta is attributable.
• Real tools only. Use only the SynthPilot tool names referenced here. If a capability is missing (e.g. automated SAIF annotation), say so rather than inventing a tool.

Output

Produce a concise power report, not a narrative:

1. Baseline: total power (W), dynamic/static split, top 3 categories with their watts/percent, junction temp + thermal margin, target part, and the report confidence level.
2. Findings: the dominant movable category and its root cause (per the classification table).
3. Change log (per iteration): what change-class was applied (strategy / constraint / IP / recommended-RTL), and the before→after numbers for total power, the targeted category, WNS, and DRC status. Show your work.
4. Result: final total power and dynamic/static split from the LAST implemented report_power, with confidence level, plus confirmation that report_timing_summary and report_drc are still clean. Quantify the reduction vs baseline (W and %).
5. Remaining options (if stopped before goal): the exhausted-safe-options list — RTL/architectural edits, device/speed-grade/board changes, cooling changes, or a SAIF-driven re-measure for signoff — each with its cost and risk. Hand back to the human; do not proceed unilaterally.
6. Caveat: if numbers are vectorless Low/Medium confidence, state that dynamic-power figures are estimates and signoff requires a SAIF-annotated re-measure.