fold large mos devices for sky130 commercial pdk (#43)

Author: rahulk29

pdks/sky130_commercial_pdk/src/mos.rs

@@ -34,19 +34,37 @@ impl Sky130CommercialPdk {
     }
 
     pub(crate) fn mos_schematic(ctx: &mut SchematicCtx, params: &MosParams) -> Result<()> {
+        use std::fmt::Write;
         let mos_db = ctx.mos_db();
         let name = &mos_db.get_spec(params.id)?.name;
         // TODO correctly handle nf and m.
         // The sky130 pdk uses w and l in microns.
         // So we must divide by 1_000 to convert nanometers to microns.
-        ctx.set_spice(format!(
-            "M0 d g s b {} w={:.3} l={:.3} nf={} mult={}",
-            name,
-            params.w as f64 / 1_000.0,
-            params.l as f64 / 1_000.0,
-            params.nf,
-            params.m,
-        ));
+        // The max allowed width of a single transistor is 100um, so we fold
+        // larger transistors into several 100um segments plus one smaller segment containing
+        // the leftover width.
+        let mut spice = String::new();
+        let n_extra = params.w / 100_000;
+        let l = params.l as f64 / 1_000.0;
+        let nf = params.nf;
+        let m = params.m;
+
+        for i in 1..=n_extra {
+            writeln!(
+                &mut spice,
+                "M{i} d g s b {name} w=100.0 l={l:.3} nf={nf} mult={m}"
+            )
+            .expect("failed to write to string");
+        }
+
+        let w = (params.w % 100_000) as f64 / 1_000.0;
+        writeln!(
+            &mut spice,
+            "M0 d g s b {name} w={w:.3} l={l:.3} nf={nf} mult={m}"
+        )
+        .expect("failed to write to string");
+
+        ctx.set_spice(spice);
         Ok(())
     }
 }