sim-run

name	sim-run
description	Run circuit simulation (DC, tran, AC) on Virtuoso. Use when executing Spectre simulation, running analysis, or checking simulation results.
argument-hint	["analysis","e.g. \"tran 10us\" or \"ac 1Hz-1GHz\" or \"dc\""]
allowed-tools	Bash(virtuoso *)

Run Simulation

Execute Spectre simulation via virtuoso sim run.

Prerequisites

Simulation must be set up first (see /sim-setup):

• simulator('spectre) configured
• design(lib cell view) set
• modelFile(...) configured
• desVar(...) set for any parameterized variables
• resultsDir(...) set

Run analysis

# DC operating point
virtuoso sim run --analysis dc --param saveOppoint=t --timeout 120 --format json

# Transient
virtuoso sim run --analysis tran --stop 10u --timeout 300 --format json

# AC
virtuoso sim run --analysis ac --start 1 --stop 1e9 --dec 10 --timeout 300 --format json

Important: resultsDir — DO NOT CHANGE

⚠️ The ADE session binds run() to the resultsDir established when the GUI session was first created. Changing it to any other path silently breaks run() (returns nil).

# ✅ Find the canonical path from the runSimulation script
cat <netlist_dir>/runSimulation | grep "\-raw"
# → -raw ../../../../tmp/opt_5t_ota/psf  →  canonical = /tmp/opt_5t_ota

# ✅ If resultsDir is nil, restore to canonical path
virtuoso skill exec 'resultsDir("/tmp/opt_5t_ota")'

# ❌ Never do this before a run — it will break run()
virtuoso skill exec 'resultsDir("/tmp/some_new_path")'

sim run now returns a clear error if resultsDir is nil instead of auto-setting it to a temp path (which would break the ADE session binding).

Also: do not call sim setup again once a working ADE session exists — it resets the Ocean session state and can cause run() to return nil.

Verify success

After run(), check spectre.out for errors:

# Check the log
virtuoso skill exec 'resultsDir()' --format json
# Then read the spectre.out file at <resultsDir>/psf/spectre.out

# A successful run has PSF data files (dcOp.dc, tran.tran, etc.)
# A failed run only has artistLogFile, simRunData, variables_file

Key indicators

run() output	Meaning
Returns resultsDir path AND PSF data files exist	Simulation completed ✓
Returns resultsDir path BUT only artistLogFile/simRunData/spectre.out	Spectre failed — run() does NOT check spectre's exit code
Returns nil, takes <0.01s	Analysis not configured or session lost
Returns nil, takes >0.1s	Netlisting ran but spectre may have failed

⚠️ run() returning a non-nil path is not sufficient proof of success. Spectre can terminate with fatal errors (e.g., SFE-868) and run() still returns the resultsDir. Always verify PSF data files exist:

tail -2 <resultsDir>/psf/spectre.out
# ✓ "spectre completes with 0 errors"
# ✗ "spectre terminated prematurely due to fatal error"

Common errors in spectre.out

Error	Fix
SFE-868: ADE-generated path .../oa/lib/../models/...	Patch input.scs: replace with direct absolute model path + section=tt — see /spectre-netlist-gotchas §8
SFE-868: Cannot open input file (other)	Model path wrong — verify file exists
SFE-675: no valid section name	Empty section "" in modelFile for .lib — remove it
SFE-1997: parameter not assigned	Set desVar() for the missing parameter
OSSHNL-116: Cannot descend into views	Subcell missing spectre view — remove instance or add view

Spectre mode selection (measured)

Source: Adapted from virtuoso-bridge-lite, commit 2110dde (2026-05-31), MIT-licensed. Measured on an 11-bit sub-radix-2 SAR ADC tran (N=128 coherent FFT, ax baseline ≈ 220 s).

Precision ordering — the trade-off between solver strictness and ENOB:

arg	preset	speed	ENOB Δ vs aps	use for
"spectre"	(none)	slowest	reference	least license demand, basic direct
"aps"	+preset=aps	1.0× (gold)	0.000	sign-off accuracy reference
"cx"	+preset=cx	1.2×	−0.03	sign-off for designs with mixed-signal stiff loops (cmp metastability)
"ax"	+preset=ax	2.0×	−0.03 (within noise)	default for daily work
"mx"	+preset=mx	3.8×	−0.29	design exploration, corner sweeps where 0.3 ENOB is acceptable
"lx"	+preset=lx	5.9×	−2.8 (unusable for SAR)	small-signal AC / linear DC sweeps; not for circuits with cmp/regen
"vx"	+preset=vx	8.8×	−8.5 (totally fails)	verification-style connectivity / DC convergence only — never for transient signal fidelity

spectre_mode_args("ax")     # default for daily transient work
spectre_mode_args("aps")    # reference / sign-off
spectre_mode_args("mx")     # fast iteration if ENOB ≤ 0.3 loss is OK

Critical: SAR / latched-comparator circuits and any topology with metastable regeneration depend on tight reltol (1e-4 or better) to resolve LSB-scale differential inputs. lx relaxes reltol to ~1e-3 and drops ENOB by ~3 bits on such circuits; vx disables LTE bounding entirely and produces garbage. Reserve those two for non-signal-fidelity work (DC, connectivity, link-test).

If a Maestro config you inherit specifies +preset=lx or +preset=vx for a transient performance sim, that's almost always a bug.

When (and when not) to replace cells with Verilog-A for speedup

Source: Same commit 2110dde (2026-05-31), MIT-licensed.

Verilog-A behavioral replacement of cells is a tempting acceleration lever, but the speedup is non-monotonic in cell size — replacing big cells helps, replacing small cells hurts. Measured on an 11-bit SAR ADC tran (ax mode, N=64, baseline 132 s):

Cell replaced	Transistor count	Wall-time change	Result
Output capture DFFs (1-pin behavior, 12 instances × 1 D-FF each)	12 × ~10 MOS	0% (neutral)	✓ Easy, no gain — skip unless cleaning the netlist
Per-bit SAR latch with feedback (12 × ~12 MOS + 4 std cells)	~200 MOS total	−13% (slower)	✗ transition() event-queue overhead × 11 concurrent instances exceeds the BSIM equation savings
StrongARM comparator (47 MOS)	47 MOS, 1 instance	+9-17%	✓ Big cell, single instance — clear win

Rule of thumb: VA replacement helps when the cell is large (≥ 40 MOS) and instantiated once or twice. It hurts when the cell is small (< 20 MOS) and many instances share the same input event source — each @(cross()) adds to the spectre event queue; with N concurrent instances watching the same node, queue overhead grows ~N× while the BSIM savings stay linear in N.

The actually-effective SAR speed levers (measured, not from VA):

Lever	Mechanism	Typical speedup	ENOB cost
Cut FFT N (e.g., 128 → 64)	Tran stop time scales linearly	~40%	0 (within meas noise)
strobeoutput=strobeonly + lean save	Cuts download + parse overhead; file size 1000× smaller	~5-10% wall, 1500× disk	0
Replace 1-2 big cells (cmp / opamp) with VA	Skip BSIM equations for ~50+ MOS	~10-20%	depends on VA fidelity
Drop LPE std-cell models for schematic-spi	Remove per-cell wire parasitics	~20%	minor timing shift
Increase maxstep	Fewer solver iterations	~20% per 2×	depends on circuit, risky for cmp metastability
Spectre mode ax → mx	Looser solver tolerance	~50%	−0.3 ENOB on SAR

The first four stack without ENOB cost. The last two trade accuracy for speed.

Output size control: save list, strobing, format

Source: Same commit 2110dde (2026-05-31), MIT-licensed.

By default the .scs netlist's tran tran ... directive saves at every solver timestep for every signal — a clocked SAR-style transient at maxstep=5p over hundreds of ns produces 100+ MB of PSF ASCII per signal group. Three knobs:

1. saveOptions options save=<mode> + explicit save list

save CLKS RSTP I_SAR.VTOPP DOUT\<11\> ... DOUT\<0\>
saveOptions options save=selected

• save=allpub — every public node + every terminal current (huge default).
• save=selected — only the nodes/terminals in the explicit save line.
• save=lvlpub — pub down to a given hierarchy level.

For production runs of large mixed-signal designs, always use save=selected with a curated 10-20 signal list. save=allpub is the most common cause of runaway PSF size on lab-cluster sims.

2. strobeoutput=<mode> (gotcha: "all" is bigger, not smaller)

The tran tran ... directive accepts strobeperiod and strobeoutput:

tran tran stop=t_end maxstep=5p \
    strobeperiod=1/Fs strobeoutput=strobeonly ...

Mode	What gets saved	Use for
strobeoutput=all	Every solver timestep PLUS strobed samples (biggest file)	Debugging — need waveform shape between samples
strobeoutput=strobeonly	Only strobed samples (1 sample per strobeperiod)	ENOB / SNDR / corner sweeps where you only need per-cycle values

The name "all" misleads — it means "both continuous and strobed views," not "all signals." Switching to strobeonly typically cuts file size 500×-1500× on N=64..256 sims. For ENOB-only runs of a clocked ADC, strobeonly is the right default.

3. output_format — PSF ASCII vs binary

virtuoso sim run uses output_format="psfascii" by default (the in-tree PSF ASCII parser handles it). output_format="psfbin" is NOT supported by the in-tree parser — it produces a .raw directory the local side cannot read. If you need 10× smaller PSF files: ship a binary parser (e.g., wrap psf_utils). Until then, the size lever is save=selected + strobeoutput=strobeonly, not the format.

Transient noise (tranNoise=yes)

Source: Same commit 2110dde (2026-05-31), MIT-licensed.

tran tran is deterministic by default — no thermal / 1/f noise injected. Most BSIM models have noise params but they only fire during noise analysis or when tranNoise=yes is on the tran line:

tran tran stop=t_end maxstep=5p \
    tranNoise=yes noisefmax=50G noiseseed=1 noisetmin=1 binnum=16 noiseruns=1 \
    write="spectre.ic" writefinal="spectre.fc" annotate=status

Param	Meaning	Default-ish value
tranNoise=yes	Enable the noise injection at all	off
noisefmax=<f>	Max frequency for noise integration	5×Fclock or 1× signal BW (smaller = faster)
noiseseed=<n>	RNG seed for one run	1
noisetmin=<t>	Earliest time when noise becomes active	0 (or 1×Ts to skip startup)
binnum=<n>	Frequency-bin discretization (Wiener model)	16
noiseruns=<n>	Stochastic Monte Carlo runs — N seeds, ensemble output	1 (Maestro defaults to 100, which is 100× compute)

Gotcha: When inheriting a tran line from Maestro, noiseruns=100 is common. That makes spectre repeat the full transient 100 times with different noise seeds for ensemble statistics — fine for jitter histograms / phase noise analyses, but lethal for ENOB measurement (which only needs one realization). Override to noiseruns=1 unless you genuinely want ensemble.

ENOB cost of enabling noise on a 11-bit SAR: roughly −0.3 to −0.5 bit (strongarm cmp noise is the dominant source). Compute cost of tranNoise=yes noiseruns=1 is ~1.5-2× a noiseless tran.

Gotchas: Spectre 21.1 + IC618 lab cluster

Source: Adapted from virtuoso-bridge-lite, commit 7786645 (2026-05-31), MIT-licensed. Silent or near-silent foot-guns from real lab runs.

• -param X=Y CLI flag is BROKEN. Spectre 21.1 parses the value as a second input netlist → SPECTRE-132: input file has been re-specified as 'X=Y'. Workaround: bake parameters into the netlist per sweep point (e.g., regenerate the master with txt.replace("parameters X=0", f"parameters X={val}")).
• parameters X=Y re-declaration after include "header.scs" does not update DEPENDENT expressions. E.g., header has parameters N=64 t_end=((N+N_extra)/Fs), then later parameters N=256 — N updates but t_end stays at 276 ns (eagerly bound from the first declaration). Symptom: tran stops far too early. Fix: copy header locally and edit the parameters line in place.
• Default timeout=600 s is too short for noised long-tran. With tranNoise=yes + N≥256 or 6+-way parallel contention, a single run can exceed 600 s wall while spectre is still progressing — bridge reports "Remote command timed out" but spectre.out actually shows clean completion. Fix: bump to 3600 s in sim run calls.
• PSF parser keeps \<> escape chars in signal names. Saved signal DOUT\<0\> parses as dict key r"DOUT\<0\>", not "DOUT<0>". Symptom: KeyError: 'DOUT<0>' even though save list looks right.
• strobeoutput=all in psfascii outputs only the continuous tran. Despite the docs implying "both continuous + strobed", Spectre 21.1's psfascii emitter writes just the continuous stream into tran.tran.tran. You'll get ~140k samples per signal instead of N strobed values. Fix: either Python-strobe yourself with np.searchsorted(t, k/Fs + offset), or use strobeoutput=strobeonly (which DOES work and shrinks the PSF ~1500×).