SPICE Schematic Preview

Draws SPICE netlists as circuit schematics in VS Code's built-in Markdown preview. It draws; it never simulates.

Write a netlist in a fence tagged spice:

```spice
* RC low-pass driving an emitter follower
V1 in 0 AC 1
R1 in mid 10k
C1 mid 0 100n
Q1 vcc mid out 2N3904
R2 out 0 1k
VCC vcc 0 5
.model 2N3904 NPN
```

Only spice fences are claimed — cir, ngspice and other fences are left to other renderers. Placement and wiring are automatic; the netlist says what is connected, not where it goes.

What is drawn

Every connection to ground (0 or gnd) gets its own ground symbol. Node names are case-insensitive, as in SPICE. + continuation lines and *, ;, $ and // comments work as in ngspice. Analysis directives such as .tran are skipped, as are .subckt bodies and .control blocks; nothing after .end is read.

The first line of a SPICE file is its title; in a fence, start it with * to make it a comment.

A few things are noted in the SPICE Schematic Preview output channel rather than shown: a transistor whose model is not defined (drawn as NPN or NMOS), a subcircuit that is not defined (pins numbered), a bipolar substrate node, and K coupling.

Examples

Each of these is a complete fence; paste one into a Markdown file and open the preview. Values, models and analysis lines are written as for ngspice.

An LED with its series resistor — the smallest useful netlist:

```spice
V1 in 0 5
R1 in led 330
D1 led 0 RED
.model RED D
```

A second-order RLC low-pass; .ac is skipped, only the parts are drawn:

```spice
* RLC low-pass, f0 ≈ 16 kHz
V1 in 0 AC 1
L1 in out 1m
C1 out 0 100n
R1 out 0 1k
.ac dec 20 100 1meg
```

A common-emitter amplifier with its bias network, coupling and bypass capacitors:

```spice
VCC vcc 0 12
VIN in 0 SIN(0 10m 1k)
C1 in b 10u
R1 vcc b 47k
R2 b 0 10k
Q1 c b e BC547
RC vcc c 4.7k
RE e 0 1k
CE e 0 100u
C2 c out 10u
RL out 0 100k
.model BC547 NPN
```

A full-wave bridge rectifier with a smoothing capacitor; the AC source floats, so no ground symbol is attached to it:

```spice
VAC ac1 ac2 SIN(0 12 50)
D1 ac1 plus 1N4007
D2 ac2 plus 1N4007
D3 0 ac1 1N4007
D4 0 ac2 1N4007
C1 plus 0 1000u
RL plus 0 1k
.model 1N4007 D
```

A CMOS NAND gate: the .model lines decide which transistors are PMOS, and the fourth node of each MOSFET is drawn as a body pin only when it differs from the source:

```spice {caption="NAND2 in a 180 nm process" align="center"}
VDD vdd 0 1.8
VA a 0 PULSE(0 1.8 0 10p 10p 1n 2n)
VB b 0 PULSE(0 1.8 0 10p 10p 2n 4n)
M1 out a vdd vdd pch W=2u L=180n
M2 out b vdd vdd pch W=2u L=180n
M3 out a n1 0 nch W=1u L=180n
M4 n1 b 0 0 nch W=1u L=180n
CL out 0 5f
.model nch NMOS
.model pch PMOS
```

A subcircuit used twice — a buffer feeding a non-inverting stage. The .subckt line names the pins of both boxes; what is inside the subcircuit is not drawn:

```spice
V1 in 0 SIN(0 1 1k)
X1 in mid vcc vee mid opamp
R1 mid inv 10k
R2 inv out 47k
X2 mid inv vcc vee out opamp
RL out 0 10k
VCC vcc 0 15
VEE 0 vee 15
.subckt opamp inp inn vp vn out
E1 out 0 inp inn 100k
.ends
```

Continuation lines and every comment style ngspice accepts:

```spice
* Voltage divider with a long source line
V1 in 0 PULSE(0 5
+ 0 1n 1n
+ 1u 2u)          ; the pulse spans three lines
R1 in out 10k     $ upper leg
R2 out 0 10k      // lower leg
```

Included files

.include, .inc and .lib read files, as SPICE does, so a model library can decide which transistors are PNP and name a subcircuit's pins, and a fence can draw a circuit kept in its own file:

```spice
.include "models/opamps.lib"
.lib corners.lib tt
X1 inp inn vcc vee out LM358
```

.lib file section reads one section; .lib file alone reads the whole file, as in LTspice. Includes may nest. Elements in included files are drawn; an error inside one is shown at the fence's include, naming the file and line.

Files are read only when all of these hold; otherwise the schematic draws without them and the output channel says why:

• the workspace is trusted;
• the Markdown document is saved inside a local workspace folder;
• the path is relative to the Markdown document and stays inside that folder, through no symlink.

A file may be 8 MB; a fence may include 32 files and 16 MB in all. Saved changes to an included file redraw the schematics that use it, and SPICE: Refresh Included Files rereads everything. Only text is read, and only names and elements are taken from it; nothing is run.

Fence attributes

An optional attribute block after the tag adjusts how one schematic is presented:

```spice {alt="A CMOS inverter" caption="Figure 1: CMOS inverter" align="center"}
M1 out in vdd vdd pch
M2 out in 0 0 nch
.model nch NMOS
.model pch PMOS
```

Values are quoted. A mistyped or malformed attribute never costs you the schematic: it is ignored, and a note is written to the output channel.

Errors and limits

A netlist error is shown in place of the schematic, quoting the line with a caret under the column. A netlist may have up to 400 elements and 64 KB. Layout is limited by the spice.layoutTimeout setting (seconds, 0.5 to 60, default 3); a circuit that takes longer shows a timeout message until it changes, and a change to the setting retries it. Densely connected circuits of a few hundred parts can need more than the default.

Schematics are drawn on a white card in every theme. In Restricted Mode and virtual workspaces everything works except reading included files.

Development

Requires Node 22 or newer (see .nvmrc).

npm ci
npm run lint            # tsc --noEmit
npm test                # node --test, after building
npm run package         # vsce package
npm run test:package    # the VSIX inside a real VS Code
npm run screenshots     # regenerate media/screenshots from examples/demo.md

Symbols live in src/skin/symbols.svg; test/schematic.test.ts traces every wire of a set of reference circuits and fails if a drawing connects the wrong pins.

The netlist parsers in vendor/parsers/ are generated and committed; rebuilding them needs flex, Bison and Emscripten and is described in dev.md.

License

MIT for this extension. Bundled elkjs is EPL-2.0; symbols and layout code are adapted from netlistsvg (MIT) — see THIRD_PARTY_NOTICES.md.