indirect_assignment_examples

Indirect branch assignment examples (version3)

Self-contained correctness examples for OpenVAF/ngspice indirect branch assignment support (<lhs> : <rhs> == <expr>;), covering DC, AC, and transient analysis. Everything here uses the version3 toolchain:

• compiler : ../OpenVAF-master-20260610/target/opt/openvaf-r (or ../bin/macos/apple-silicon/openvaf-r)
• simulator: ../bin/macos/apple-silicon/ngspice

See ../Enhancement-2.md for the full implementation writeup.

The model: an ideal op-amp

opamp.va is the canonical LRM example (Accellera Std VAMS-2023, p. 114):

`include "disciplines.vams"

module opamp(out, pin, nin);
    inout out, pin, nin;
    electrical out, pin, nin;
    analog
        V(out):V(pin,nin) == 0;
endmodule

This declares an ideal, infinite-gain, zero-output-impedance op-amp: out is driven by a free unknown (an ideal voltage source), solved each time step so that V(pin,nin) == 0 — i.e. the two inputs are forced to the same voltage, with the output sourcing/sinking whatever is needed to make that true.

The test circuit: unity-gain buffer

All three .cir files instantiate the op-amp with negative feedback (nin tied directly to out) and a small load resistor:

.model amp opamp()
Namp1 out pin out amp
RL out 0 1k

With nin == out, the constraint V(pin,nin) == 0 becomes V(out) == V(pin) — a unity-gain voltage follower. This is the simplest circuit that exercises the feature end-to-end (the op-amp alone, without feedback, is under-determined).

Results

Analysis	Sweep	Expected	Observed
DC	V(pin) from −2V to 2V	V(out) == V(pin)	exact match across the sweep
AC	1 kHz – 1 GHz	0 dB gain, 0° phase (ideal, infinite bandwidth)	flat 0 dB / 0° across the full sweep
Transient	1 MHz sine on V(pin)	V(out) tracks V(pin) with no lag	overlays exactly

Layout

indirect_assignment_examples/
  opamp.va            ideal op-amp:  V(out):V(pin,nin) == 0
  opamp.osdi          compiled with version3 openvaf-r
  dc_sim.cir           unity-gain buffer, DC sweep of V(pin)
  ac_sim.cir           unity-gain buffer, AC sweep 1kHz-1GHz
  tran_sim.cir         unity-gain buffer, 1MHz sine transient
  run_examples.sh      runs all three with ngspice and writes dc/ac/tran.txt
  plot_results.py      plots dc/ac/tran.txt to dc/ac/tran.png
  dc.txt, ac.txt, tran.txt   raw wrdata output from the last run
  dc.png, ac.png, tran.png  plotted results (see above)

Reproduce

# (re)compile the model
../bin/macos/apple-silicon/openvaf-r opamp.va -o opamp.osdi

# run DC, AC, transient
bash run_examples.sh

# plot the results
python3 plot_results.py