SPICE netlist export

Reticle recognises transistors from layout geometry (device recognition: DeviceKind, terminal nets, and channel W/L in DBU) and writes that as a SPICE subcircuit, for exchange with external simulators and schematic-capture tools. The writer lives in reticle-extract (spice.rs), a sibling of device recognition, not a change to it: it consumes an already-built DeviceNetlist and adds no new extraction logic.

The exchange subset

The interchange subset is fixed by the committed contract fixture crates/reticle-extract/tests/fixtures/contracts/spice_exchange_inverter.spice (with its structural companion .json):

• * full-line comments and blank lines, skipped on read, optional on write.
• One .subckt NAME <ports...> / .ends wrapper.
• One X device-instance card per device: Xn <drain> <gate> <source> <bulk> <model> w=<W> l=<L>, n the device’s index in DeviceNetlist::devices.
• Decimal-micron w=/l= params, and .end.

The netlist lane’s writer (to_spice_subckt, format_spice, write_spice) emits this subset; the xschem lane reads it back. Both lanes build against the committed fixture before the other lane’s half exists (see ADR 0108), so neither blocks on the other.

What is written, and from where

• Ports are the nets any device terminal references, kept in the extracted netlist’s own stable (lowest-member-index) order – the writer invents no ordering of its own.
• W/L convert from the Device DBU fields to decimal microns by exact integer long division, never a float: 650 DBU at 1000 DBU/micron is the literal string "0.65", with no 0.6500000001-style formatting drift.
• The model name comes from SpiceTech, a small table the caller supplies, keyed only on DeviceKind (NMOS/PMOS). Device recognition reads no threshold-voltage flavour, body-bias variant, or other model-selection detail from geometry, so the table cannot honestly derive one either; see ADR 0108 for why this is caller-supplied rather than baked into a single “sky130” default.

What is honestly absent

• Area/perimeter parameters (ad/pd/as/ps): Device carries no diffusion-area data, so these are never invented.
• A guessed node name: a terminal extract_devices could not bind to a net (Option::None) is written as the documented placeholder node NC.
• A general SPICE importer: parse_spice reads back only the subset format_spice writes, enough to round-trip the writer’s own output and validate the committed contract fixture. It never panics on malformed input – every failure is a SpiceParseError – but it is not a hardened importer for arbitrary, untrusted decks, and has not been through a fuzz campaign the way Reticle’s binary-format readers (GDS, OASIS) have.

Cross-test

crates/reticle-extract/tests/spice_writer.rs extracts a hand-built SKY130 inverter (separate VPB/VNB body-tie straps, so the bulk terminals bind to their own nets rather than shorting into the power rails the way the smaller device-recognition fixture does), writes it, and checks the result three ways: the structured output matches the contract JSON’s kind/terminals/model/W/L; the emitted text parses back to the same structure; and the committed .spice fixture itself parses and agrees with the writer’s structural output.