spectre

Spectre Skill

Run a .scs netlist locally or on a remote machine through SSH, then parse PSF results into Python dicts. Independent of VirtuosoClient — no GUI needed.

Before you start

1. virtuoso-bridge is a Python CLI — install it in a virtual environment with uv pip install -e virtuoso-bridge-lite.
2. virtuoso-bridge status — check connection, Spectre path, license
3. Check examples/02_spectre/ — use existing examples as a basis
4. spectre must be on PATH, or set VB_CADENCE_CSHRC (project or user .env) so the runner can source the Cadence environment. This applies to local and SSH execution.

Core pattern

from virtuoso_bridge.spectre.runner import SpectreSimulator, spectre_mode_args

sim = SpectreSimulator.from_env(
    spectre_args=spectre_mode_args("ax"),  # APS extended (recommended)
    work_dir="./output",
)
result = sim.run_simulation("my_netlist.scs", {})

if result.ok:
    vout = result.data["VOUT"]
else:
    print(result.errors)

With Verilog-A includes:

result = sim.run_simulation(
    "tb_adc.scs",
    {"include_files": ["adc.va", "dac.va"], "spectre_args": ["+aps"]},
)

include_files and per-run spectre_args have the same meaning in local and SSH mode. They stage include files and extend the command; they do not inject arbitrary circuit parameters. For local-only execution, use SpectreSimulator.local(...), or configure VB_REMOTE_HOST=localhost and use from_env().

Result object

Treat result.ok as the execution contract. A nonzero exit, explicit fatal Spectre output, netlist read-in error, or explicit convergence failure returns FAILURE/PARTIAL even if the raw directory contains incomplete files. Do not use a non-empty result.data as proof that the simulation succeeded.

Gotchas (Spectre 21.1 + IC618 lab cluster)

These are 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 (regenerate the master per sweep point 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: SpectreSimulator.from_env(timeout=3600, ...).
• 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×).

Parallel simulation

Submit simulations that run concurrently — each gets its own remote directory, no conflicts. For full API and multi-server setup, read references/parallel.md.

t1 = sim.submit(Path("tb_comp.scs"))    # returns Future immediately
t2 = sim.submit(Path("tb_dac.scs"))     # submit more anytime
result = t1.result()                     # block on one
results = SpectreSimulator.wait_all([t1, t2])  # or wait for batch

Simulation modes

Precision ordering (measured on an 11-bit sub-radix-2 SAR ADC tran, N=128 coherent FFT, ax baseline ≈ 220 s):

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

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 a 11-bit SAR ADC tran (ax mode, N=64, baseline 132s):

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.

Self-timed feedback loops are extra-fragile: replacing one element of an async chain (e.g., a SAR daisy-chain latch with feedback to CMPCK) requires matching not just the steady-state truth table but the propagation delay and edge timing to within a few ps. Standalone unit-test the VA before integrating into the chain; if the unit-test passes but the chain breaks, suspect transition() td interacting with multiple concurrent listeners.

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

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

Output size control: save list, strobing, format

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 ...

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

The bridge currently uses output_format="psfascii" by default, parsed via parse_spectre_psf_ascii. output_format="psfbin" is NOT supported by the in-tree parser (virtuoso_bridge/spectre/parsers.py has no parse_spectre_psf_bin). Passing it will produce a .raw directory the local side cannot read.

If you need 10× smaller PSF files: add a binary parser (e.g., wrap psf_utils — pure Python, pip install). Until then, the size lever is save=selected + strobeoutput=strobeonly, not the format.

Transient noise (tranNoise=yes)

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

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.

References

Load when needed — these contain detailed API docs:

• references/netlist_syntax.md — Spectre netlist format, analysis statements, parameterization
• references/parallel.md — Parallel simulation, multi-server, CLI job management, .env configuration

Examples

• examples/02_spectre/01_inverter_tran.py — inverter transient
• examples/02_spectre/01_veriloga_adc_dac.py — 4-bit ADC/DAC with Verilog-A
• examples/02_spectre/02_cap_dc_ac.py — capacitor DC + AC
• examples/02_spectre/04_strongarm_pss_pnoise.py — StrongArm PSS + Pnoise

Related skills

• virtuoso — GUI-based Virtuoso workflow (schematic/layout, ADE Maestro). Use when working inside Virtuoso GUI.