support both side gate contacts for sky130 mos layout (#51)

* support both side gate contacts for sky130 mos layout

* consistent punctuation

pdks/sky130_common_pdk/src/mos.rs

@@ -129,8 +129,6 @@ impl Sky130Pdk {
         let gate_ct = ctx.instantiate::<Via>(&gate_ctp)?;
         let gate_bbox = gate_ct.layer_bbox(poly);
 
-        let mut gate_pins = Vec::with_capacity(nf as usize);
-
         let xpoly = x0 - POLY_DIFF_EXTENSION;
         let mut ypoly = y0 + DIFF_EDGE_TO_GATE;
         let wpoly = cx - xpoly + POLY_DIFF_EXTENSION;
@@ -141,9 +139,13 @@ impl Sky130Pdk {
         let poly_fudge_x = 60;
         let mut poly_rects = Vec::with_capacity(nf as usize);
         for _ in 0..nf {
+            let mut p1x = xpoly + wpoly;
+            if params.contact_strategy == GateContactStrategy::BothSides {
+                p1x += poly_fudge_x;
+            };
             let rect = Rect {
                 p0: Point::new(xpoly - poly_fudge_x, ypoly),
-                p1: Point::new(xpoly + wpoly, ypoly + params.length()),
+                p1: Point::new(p1x, ypoly + params.length()),
             };
             poly_rects.push(rect);
             ctx.draw(Element {
@@ -152,17 +154,16 @@ impl Sky130Pdk {
                 inner: Shape::Rect(rect),
             })?;
 
-            ypoly += params.length();
-            ypoly += FINGER_SPACE;
+            ypoly += params.length() + FINGER_SPACE;
         }
 
         let gate_span = Span::new(poly_rects[0].p0.y, poly_rects.last().unwrap().p1.y);
         // Place gate contacts and create gate ports
         match params.contact_strategy {
-            GateContactStrategy::SingleSide => {
+            GateContactStrategy::SingleSide | GateContactStrategy::BothSides => {
                 assert!(
                     nf <= 3,
-                    "can only contact nf=3 transistors on a single side"
+                    "can only contact nf>=3 transistors on a single side"
                 );
                 let line = gate_bbox.height();
                 let space = POLY_SPACE;
@@ -174,6 +175,7 @@ impl Sky130Pdk {
                 );
 
                 let mut npc_boxes = Vec::new();
+                let mut npc_boxes_x = Vec::new();
 
                 for i in 0..nf as i64 {
                     let empty_rect = Rect::new(Point::zero(), Point::zero());
@@ -195,16 +197,33 @@ impl Sky130Pdk {
                     let mut port = CellPort::new(format!("gate_{i}"));
                     port.add(gate_metal, Shape::Rect(ct_box));
                     ctx.add_port(port).unwrap();
-                    gate_pins.push(ct_box);
+
+                    if params.contact_strategy == GateContactStrategy::BothSides {
+                        let mut gate_ct = ctx.instantiate::<Via>(&gate_ctp)?;
+                        let ofsx = rect.p1.x - gate_bbox.p0.x;
+                        let ct_ofs = Point::new(ofsx, ofsy);
+                        gate_ct.translate(ct_ofs);
+                        let ct_box = gate_ct.layer_bbox(gate_metal).into_rect();
+                        let mut port = CellPort::new(format!("gate_{i}_x"));
+                        port.add(gate_metal, Shape::Rect(ct_box));
+                        ctx.add_port(port).unwrap();
+                        let npc_bbox = gate_ct.layer_bbox(npc).into_rect();
+                        npc_boxes_x.push(npc_bbox);
+                        ctx.draw(gate_ct)?;
+                    }
 
                     let npc_bbox = gate_ct.layer_bbox(npc).into_rect();
                     npc_boxes.push(npc_bbox);
-
                     ctx.draw(gate_ct)?;
+                }
 
-                    let top_npc = npc_boxes.last().unwrap();
+                for boxes in [npc_boxes, npc_boxes_x] {
+                    if boxes.is_empty() {
+                        continue;
+                    }
+                    let top_npc = boxes.last().unwrap();
                     let npc_merge_rect = Rect::new(
-                        Point::new(npc_boxes[0].p0.x, npc_boxes[0].p0.y),
+                        Point::new(boxes[0].p0.x, boxes[0].p0.y),
                         Point::new(top_npc.p1.x, top_npc.p1.y),
                     );
                     ctx.draw(Element {

substrate/src/pdk/mos/mod.rs

@@ -33,15 +33,17 @@ pub struct LayoutMosParams {
 /// Specifies the geometric arrangement of contacts for transistor gates.
 #[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
 pub enum GateContactStrategy {
-    /// Attempt to place all contacts on one side (usually the left)
+    /// Attempt to place all contacts on one side (usually the left).
     SingleSide,
-    /// Alternate contact placement
+    /// Attempt to place contacts on both sides of the transistor.
+    BothSides,
+    /// Alternate contact placement.
     Alternate,
-    /// ABBA placement
+    /// ABBA placement.
     Abba,
-    /// Merge all contacts on one side (usually the left)
+    /// Merge all contacts on one side (usually the left).
     Merge,
-    /// Other; effect depends on layout generator
+    /// Other; effect depends on layout generator.
     Other(String),
 }
 

substrate/tests/mos.rs

@@ -12,20 +12,20 @@ fn test_sky130_mos_nand2() {
         &LayoutMosParams {
             skip_sd_metal: vec![vec![]; 2],
             deep_nwell: false,
-            contact_strategy: substrate::pdk::mos::GateContactStrategy::Merge,
+            contact_strategy: substrate::pdk::mos::GateContactStrategy::BothSides,
             devices: vec![
                 MosParams {
-                    w: 200_000,
+                    w: 2000,
                     l: 150,
                     m: 1,
-                    nf: 5,
+                    nf: 2,
                     id: MosId::new(0),
                 },
                 MosParams {
-                    w: 100_000,
+                    w: 4000,
                     l: 150,
                     m: 1,
-                    nf: 5,
+                    nf: 2,
                     id: MosId::new(1),
                 },
             ],