evas-sim 0.6.0

Event-driven Verilog-A Simulator with a Python compatibility engine and optional evas-rust backend

EVAS — Event-driven Verilog-A Simulator

A lightweight behavioral simulator for digital/mixed-signal Verilog-A models. EVAS ships with a Python compatibility engine by default and an optional evas-rust backend for supported event-driven designs. No ngspice, no KCL/KVL solver.

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Docs: evas.tokenzhang.com

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What EVAS does

EVAS simulates voltage-mode, event-driven Verilog-A behavioral models. You provide:

1. A .va file — your behavioral model (comparator, DAC, SAR logic, DWA controller, …)
2. A .scs testbench netlist — voltage sources, ahdl_include, and a tran statement
3. Run evas simulate — get tran.csv waveforms and optional plots

The main simulator is transient-first. Compiled models also expose lightweight behavioral helpers for analysis("ac"), ac_stim(), and Verilog-A noise source functions so source-style models can run AC/noise sweeps from Python without claiming SPICE-style linearized circuit analysis.

The bundled examples are a compact smoke-test set. For your own design, copy the closest example directory, swap in your .va, adjust the stimulus sources and save list, and run. The larger verification example library belongs with the agent workflow in veriloga-skills/evas-sim.

Installation

pip install evas-sim
evas list        # verify install — prints bundled example groups

If evas is not on PATH, use python -m evas.

The packaged default uses the Python engine so examples and user netlists run from PyPI or a fresh source checkout. Compatible Linux wheels also include the evas-rust shared library. To request evas-rust explicitly, pass --engine evas-rust, set EVAS_ENGINE=evas-rust, or add simulatorOptions options evas_engine=evas-rust to the testbench. If your platform installed the pure Python wheel, build the Rust backend from source first. The legacy evas2 and rust2 selectors remain accepted as compatibility aliases.

Simulating your own design

evas simulate path/to/tb.scs -o output/mydesign
evas simulate path/to/tb.scs -o output/mydesign --engine evas-rust

Output in -o dir: tran.csv (waveforms), strobe.txt (log messages), .png plots.

Before a full simulation, you can run a Spectre/AHDL-style static lint pass:

evas lint path/to/model.va
evas lint path/to/tb.scs --format json

evas lint follows ahdl_include statements in .scs files and reports two classes of issues: compat-error for EVAS/Spectre subset problems that should block a candidate, and warning diagnostics for AHDL-style modeling risks such as discrete signals directly driving analog contributions or suspicious transition usage. Lint warnings do not change simulation pass/fail status. Current warning coverage includes a Cadence AHDL-inspired subset for transition timing and simple continuous-input dataflow, conditional potential contributions, case defaults, exact branch equality tests, floor/ceil contribution discontinuities, gnd node portability, discrete function arguments, implicit integer casts, and simulator-stop tasks inside loops. Each lint diagnostic is backed by a small rule registry that records its code, severity, canonical rule name, phase, source category, and related Cadence/Spectre identifiers when known. Diagnostics produced from parsed Verilog-A nodes include source line and column coordinates when available, so repair tools can point users back to the triggering statement or expression. Compatibility diagnostics include Cadence/Spectre-aligned cases such as conditionally executed analog operators (transition, slew, idt) and discipline vector ranges that depend on runtime variables instead of numeric or parameter constant expressions. The lint regression suite also keeps a small set of public oracle fixtures under tests/fixtures/lint_oracle_cases. These cases record distilled expected EVAS diagnostic codes only; raw Cadence/Spectre logs and generated certification reports are not committed.

Minimal testbench template (.scs):

simulator lang=spectre
global 0

ahdl_include "my_module.va"

Vvdd (vdd 0) vsource type=dc dc=1.8
Vclk (clk 0) vsource type=pulse val0=0 val1=1.8 period=10n rise=0.1n fall=0.1n width=4.9n

IDUT (clk vdd out) my_module vdd=1.8

tran tran stop=200n maxstep=0.1n
save clk:2e out:6f

Testbench structure reference

The bundled examples contain five small groups and their testbenches. They are the in-package reference for common wiring patterns:

Pattern needed	Look at
Clocked digital logic	clk_div, digital_basics
Comparator with feedback	comparator/cmp_offset_search
ADC + sample-hold	adc_dac_ideal_4b
Noise / random stimulus	noise_gen
Multi-cycle edge timing	comparator/cmp_delay, edge_interval_timer

Supported Verilog-A

Support tiers

EVAS is strongest in the behavioral-event / waveform-oriented tier: voltage reads and drives, event-controlled state, timers, transitions, table/random/file helpers, and small mixed-signal logic/wreal subsets. This tier is implemented by a lightweight event/waveform engine and does not require a full conservative analog solver.

EVAS also accepts a limited behavioral-continuous-time tier. Operators such as ddt(), idt(), laplace_*, zi_*, and limexp() compile and run with explicit behavioral approximations for transient compatibility. They should not be read as Spectre-equivalent continuous-time transfer-function solving.

EVAS does not implement the conservative-current / KCL-MNA tier. Device-style models that rely on I(p,n) <+ ..., branch charge/current contributions, nonlinear device equations, or transistor-level AC/DC matrix solving remain outside the current simulator design.

Feature	Status
V(node) <+, V(a,b) differential	✅
@(cross(...)), @(above(...)), @(initial_step)	✅
cross(expr, dir, time_tol, expr_tol) event tolerances	✅ (behavioral approximation)
@(timer(period)), @(final_step)	✅
transition() with delay / rise / fall	✅
slew(x, maxrise, maxfall) transient limiter	✅ (behavioral approximation)
for, repeat, while, do while, if/else, case/endcase, begin/end	✅
arrays, including integer/real 1-D and 2-D state arrays	✅
branch (p,n) br; V(br) named branch voltage probes	✅
$analog_node_alias() and string OOMR voltage probes such as V(sigpath)	✅
$table_model() 1-D file tables and simple 2-D array-backed surfaces	✅
parameters and variables (real / integer / string)	✅
user-defined functions/tasks, including bounded recursive functions	✅
module, connectmodule, simple behavioral hierarchy	✅
`include, `define, `default_transition	✅
SI suffixes, math: sin cos exp ln log pow floor ceil …	✅
$temperature, $vt, $abstime	✅
$bound_step()	✅
$fopen(), $fclose(), $fscanf(), $fstrobe(), $fwrite(), $fdisplay()	✅
$display, $strobe, $sformat(), $swrite()	✅
$random, $dist_*(), $rdist_*() behavioral random helpers	✅
last_crossing(expr, dir, time_tol, expr_tol)	✅ (most-recent event-time approximation)
analysis("ac"), ac_stim()	✅ (behavioral Python sweep helper)
white_noise(), flicker_noise(), noise_table()	✅ (behavioral PSD / integrated-noise helper)
ddt(), idt(), laplace_*(), zi_*(), limexp()	✅ (behavioral transient approximation)
generate / genvar, specify / specparam, connectrules	not supported by design
custom nature / discipline semantics beyond the bundled VAMS stubs	not supported by design
analog primitive instances such as resistor / isource	not supported by design
I() <+, q() <+, branch charge/current contributions	not supported by design
SPICE-style AC/DC matrix solving, transistors	not supported by design
Spectre subckt hierarchy	not yet implemented

Accuracy Profiles

You can set simulatorOptions options evas_profile=<mode> in .scs:

• fast: lower refinement (refine_factor=8, refine_steps=4) for faster runtime
• balanced: default EVAS behavior (16, 8)
• precision: higher refinement (32, 16) for tighter event/cross timing

errpreset=conservative/liberal is still respected; evas_profile applies an explicit EVAS-side override when set. Practical correspondence: fast ≈ liberal, balanced ≈ moderate, precision ≈ conservative (guidance only, not solver-equivalence claim).

CSV output format

The save statement accepts per-signal format hints:

save vin:10e vout:6f clk:2e dout:d

Suffix	Example
:6e (default)	4.500000e-01
:Nf	fixed-point, N decimal places
:d	integer (for digital buses)

Bundled examples (reference only)

Five groups ship with the PyPI package for install verification, CLI sanity checks, and small starting templates. The full workflow-oriented example set is maintained outside this simulator package in veriloga-skills/evas-sim.

Group	Verilog-A modules	Notes
clk_div	clk_div
digital_basics	and_gate, or_gate, not_gate, dff_rst, inverter
noise_gen	noise_gen
adc_dac_ideal_4b	adc_ideal_4b, dac_ideal_4b, sh_ideal	3 stimuli: ramp / sine / 1000-pt sine
comparator	cmp_ideal, cmp_strongarm, cmp_offset_search, cmp_delay, edge_interval_timer	4 sub-examples

Contributing

git clone https://github.com/Arcadia-1/EVAS.git
cd EVAS
pip install -e ".[dev]"
pytest tests/ -v

License

MIT — see LICENSE.