{"id":26248,"date":"2025-05-19T00:05:33","date_gmt":"2025-05-19T00:05:33","guid":{"rendered":"https:\/\/wattsemi.com\/?p=26248"},"modified":"2025-06-04T04:25:22","modified_gmt":"2025-06-04T04:25:22","slug":"rtl-to-gdsii-congestion-reduction-strategies","status":"publish","type":"post","link":"https:\/\/wattsemi.com\/?p=26248","title":{"rendered":"RTL to GDSii: Congestion Reduction strategies"},"content":{"rendered":"\t\t
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In ASIC physical design<\/strong>, congestion<\/strong> occurs when there are more routing demands than available resources (tracks) in a region, leading to routing failures, increased delays, and DRC violations<\/strong>. Managing congestion is crucial for achieving a routable design<\/strong> with optimal performance.<\/p>\n

<\/p>\n

\u00a0<\/div>\n

<\/p>\n

\n

<\/p>\n

\"\"<\/figure>\n

<\/p>\n

Methods to Reduce Congestion<\/strong><\/h3>\n

<\/p>\n

\"\"<\/figure>\n

<\/p>\n

1. Floorplanning Techniques<\/strong><\/h4>\n

<\/p>\n

Macro Placement Optimization<\/strong><\/p>\n

<\/p>\n

    \n
  • \n
      <\/ul>\n<\/li>\n<\/ul>\n
        \n
      • \n
          \n
        • Place large macros (RAMs, CPUs) at the periphery to free up routing channels.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

          <\/p>\n

            \n
          • \n
              \n
            • Avoid placing macros in high-density standard cell regions.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

              <\/p>\n

                \n
              • \n
                  \n
                • Use channel spacing between macros for smoother routing.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                  <\/p>\n

                  <\/p>\n

                  Power Grid Planning<\/strong><\/p>\n

                  <\/p>\n

                    \n
                  • \n
                      <\/ul>\n<\/li>\n<\/ul>\n
                        \n
                      • \n
                          \n
                        • Avoid overly dense power stripes that block routing tracks.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                          <\/p>\n

                            \n
                          • \n
                              \n
                            • Use staggered power stripes instead of continuous ones.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                              <\/p>\n

                              <\/p>\n

                              Hierarchical Floorplanning (Block-Level)<\/strong><\/p>\n

                              <\/p>\n

                                \n
                              • \n
                                  <\/ul>\n<\/li>\n<\/ul>\n
                                    \n
                                  • \n
                                      \n
                                    • Divide the design into sub-blocks with controlled interconnect density.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                      <\/p>\n

                                        \n
                                      • \n
                                          \n
                                        • Use fence regions to restrict cell placement near congested areas.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                          <\/p>\n

                                          <\/p>\n

                                          2. Placement Optimization<\/strong><\/h4>\n

                                          <\/p>\n

                                            \n
                                          • \n
                                              <\/ul>\n<\/li>\n<\/ul>\n

                                              Cell Spreading<\/strong><\/p>\n

                                              <\/p>\n

                                                \n
                                              • \n
                                                  \n
                                                • Increase cell spacing<\/strong> in congested regions\u00a0(e.g., setPlaceMode -place_global_cell_spacing<\/code> in Cadence Innovus).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                  <\/p>\n

                                                    \n
                                                  • \n
                                                      \n
                                                    • Use density screens<\/strong> (-uniformDensity<\/code>) to prevent hot spots.

                                                      <\/li>\n<\/ul>\n<\/li>\n
                                                    • Utilization Control

                                                      <\/strong>Keep standard cell utilization<\/strong> \u2264 70-80% in congested areas.<\/li>\n<\/ul>\n

                                                      <\/p>\n

                                                        \n
                                                      • \n
                                                          \n
                                                        • Use partial placement blockage<\/strong> to restrict cell density.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n


                                                          Congestion-Driven Placement<\/strong><\/p>\n

                                                          <\/p>\n

                                                            \n
                                                          • \u00a0<\/li>\n<\/ul>\n

                                                            <\/p>\n

                                                              \n
                                                            • \n
                                                                \n
                                                              • Run global placement with congestion awareness<\/strong> (e.g., -timingDriven -congEffort high<\/code>).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                <\/p>\n

                                                                  \n
                                                                • \n
                                                                    \n
                                                                  • Use ECO placement<\/strong> to redistribute cells post-optimization.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                    <\/p>\n

                                                                    <\/p>\n

                                                                    3. Routing Strategies<\/strong><\/h4>\n

                                                                    <\/p>\n

                                                                      \n
                                                                    • \n
                                                                        <\/ul>\n<\/li>\n<\/ul>\n

                                                                        Layer Assignment<\/strong><\/p>\n

                                                                        <\/p>\n

                                                                          \n
                                                                        • \n
                                                                            \n
                                                                          • Route local nets<\/strong> on lower metals (M1-M3) and global nets<\/strong> on higher metals (M4+).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                            <\/p>\n

                                                                              \n
                                                                            • Avoid excessive via stacking<\/strong> in dense regions.

                                                                              Track Utilization Balancing<\/strong><\/li>\n<\/ul>\n

                                                                              <\/p>\n

                                                                                \n
                                                                              • \u00a0<\/li>\n<\/ul>\n

                                                                                <\/p>\n

                                                                                  \n
                                                                                • \n
                                                                                    \n
                                                                                  • Use alternate routing directions<\/strong> (e.g., M1: Horizontal, M2: Vertical).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                    <\/p>\n

                                                                                      \n
                                                                                    • \n
                                                                                        \n
                                                                                      • Avoid routing bottlenecks<\/strong> by spreading signals across layers.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                        Non-Default Rules (NDR)<\/strong><\/p>\n

                                                                                        <\/p>\n

                                                                                          \n
                                                                                        • \u00a0<\/li>\n<\/ul>\n

                                                                                          <\/p>\n

                                                                                            \n
                                                                                          • \n
                                                                                              \n
                                                                                            • Use double-width spacing<\/strong> for critical nets to reduce conflicts.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                              <\/p>\n

                                                                                                \n
                                                                                              • \n
                                                                                                  \n
                                                                                                • Apply shielding<\/strong> for noise-sensitive nets to prevent detours.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                  <\/p>\n

                                                                                                  <\/p>\n

                                                                                                  4. Logic & Netlist Optimizations<\/strong><\/h4>\n

                                                                                                  <\/p>\n

                                                                                                    \n
                                                                                                  • \n
                                                                                                      <\/ul>\n<\/li>\n<\/ul>\n

                                                                                                      Buffer & Inserter (Buf\/Inv) Tree Balancing<\/strong><\/p>\n

                                                                                                      <\/p>\n

                                                                                                        \n
                                                                                                      • \n
                                                                                                          \n
                                                                                                        • Reduce fanout congestion<\/strong> by inserting buffers in high-fanout nets.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                          <\/p>\n

                                                                                                            \n
                                                                                                          • \n
                                                                                                              \n
                                                                                                            • Use clock tree synthesis (CTS)<\/strong> optimizations to minimize skew-induced congestion.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                              <\/p>\n

                                                                                                              Logic Restructuring<\/strong><\/p>\n

                                                                                                              <\/p>\n

                                                                                                                \n
                                                                                                              • \n
                                                                                                                  \n
                                                                                                                • Replicate high-fanout cells<\/strong> to distribute load.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                  <\/p>\n

                                                                                                                    \n
                                                                                                                  • \n
                                                                                                                      \n
                                                                                                                    • Split long wires<\/strong> with repeaters to reduce routing pressure.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                      <\/p>\n

                                                                                                                      Net Weighting<\/strong><\/p>\n

                                                                                                                      <\/p>\n

                                                                                                                        \n
                                                                                                                      • \n
                                                                                                                          \n
                                                                                                                        • Assign higher priority<\/strong> to timing-critical nets (setNetWeight<\/code>).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                          <\/p>\n

                                                                                                                          <\/p>\n

                                                                                                                          5. Physical Synthesis & ECO Fixes<\/strong><\/h4>\n

                                                                                                                          <\/p>\n

                                                                                                                            \n
                                                                                                                          • \n
                                                                                                                              <\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                              Congestion-Aware Synthesis<\/strong><\/p>\n

                                                                                                                              <\/p>\n

                                                                                                                                \n
                                                                                                                              • \n
                                                                                                                                  \n
                                                                                                                                • Use physical synthesis tools<\/strong> (e.g., Fusion Compiler, Genus iSpatial) to optimize for routability.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                  <\/p>\n

                                                                                                                                    \n
                                                                                                                                  • \n
                                                                                                                                      \n
                                                                                                                                    • Apply gate sizing<\/strong> to reduce cell density.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                      <\/p>\n

                                                                                                                                        \n
                                                                                                                                      • \n
                                                                                                                                          \n
                                                                                                                                        • Post-Route ECOs<\/strong><\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                          <\/p>\n

                                                                                                                                            \n
                                                                                                                                          • \n
                                                                                                                                              \n
                                                                                                                                            • Perform minor placement legalization<\/strong> to relieve congestion.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                              <\/p>\n

                                                                                                                                                \n
                                                                                                                                              • \n
                                                                                                                                                  \n
                                                                                                                                                • Reroute<\/strong> problematic nets with manual guidance.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                  <\/p>\n

                                                                                                                                                  <\/p>\n

                                                                                                                                                  6. Advanced Techniques<\/strong><\/h4>\n

                                                                                                                                                  <\/p>\n

                                                                                                                                                    \n
                                                                                                                                                  • \n
                                                                                                                                                      <\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                      Pin Access Optimization<\/strong><\/p>\n

                                                                                                                                                      <\/p>\n

                                                                                                                                                        \n
                                                                                                                                                      • \n
                                                                                                                                                          \n
                                                                                                                                                        • Use offset pins<\/strong> or middle-of-line (MOL) routing<\/strong> to ease pin access.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                          <\/p>\n

                                                                                                                                                            \n
                                                                                                                                                          • \n
                                                                                                                                                              \n
                                                                                                                                                            • Avoid pin congestion<\/strong> by spreading pins in macros.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                              <\/p>\n

                                                                                                                                                              DFM (Design for Manufacturability) Adjustments<\/strong><\/p>\n

                                                                                                                                                              <\/p>\n

                                                                                                                                                                \n
                                                                                                                                                              • \n
                                                                                                                                                                  \n
                                                                                                                                                                • Follow metal density rules<\/strong> to avoid CMP (Chemical Mechanical Polishing) issues.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                  <\/p>\n

                                                                                                                                                                    \n
                                                                                                                                                                  • \n
                                                                                                                                                                      \n
                                                                                                                                                                    • Use fill cells<\/strong> to maintain uniform density.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                      <\/p>\n

                                                                                                                                                                      <\/p>\n

                                                                                                                                                                      \n

                                                                                                                                                                      <\/p>\n

                                                                                                                                                                      \u00a0<\/div>\n

                                                                                                                                                                      <\/p>\n

                                                                                                                                                                      Congestion Analysis & Debugging<\/strong><\/h3>\n

                                                                                                                                                                      <\/p>\n

                                                                                                                                                                      Early-Stage Checks<\/strong>:<\/p>\n

                                                                                                                                                                      <\/p>\n

                                                                                                                                                                        \n
                                                                                                                                                                      • \n
                                                                                                                                                                          <\/ul>\n<\/li>\n<\/ul>\n
                                                                                                                                                                            \n
                                                                                                                                                                          • \n
                                                                                                                                                                              \n
                                                                                                                                                                            • Use global routing congestion maps<\/strong> (e.g., in Innovus, ICC2).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                              <\/p>\n

                                                                                                                                                                                \n
                                                                                                                                                                              • \n
                                                                                                                                                                                  \n
                                                                                                                                                                                • Identify hotspots<\/strong> (red\/yellow zones) in congestion reports.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                                  <\/p>\n

                                                                                                                                                                                  <\/p>\n

                                                                                                                                                                                  Metrics to Monitor<\/strong>:<\/p>\n

                                                                                                                                                                                  <\/p>\n

                                                                                                                                                                                    \n
                                                                                                                                                                                  • \n
                                                                                                                                                                                      <\/ul>\n<\/li>\n<\/ul>\n
                                                                                                                                                                                        \n
                                                                                                                                                                                      • \n
                                                                                                                                                                                          \n
                                                                                                                                                                                        • Overflow (%)<\/strong>: Percentage of unroutable nets.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                                          <\/p>\n

                                                                                                                                                                                            \n
                                                                                                                                                                                          • \n
                                                                                                                                                                                              \n
                                                                                                                                                                                            • Cell Density<\/strong>: Should be < 80% in most regions.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                                              <\/p>\n

                                                                                                                                                                                                \n
                                                                                                                                                                                              • \n
                                                                                                                                                                                                  \n
                                                                                                                                                                                                • Track Utilization<\/strong>: Should not exceed 90%.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                                                  <\/p>\n

                                                                                                                                                                                                  <\/p>\n

                                                                                                                                                                                                  Common Fixes<\/strong>:<\/p>\n

                                                                                                                                                                                                  <\/p>\n

                                                                                                                                                                                                    \n
                                                                                                                                                                                                  • \n
                                                                                                                                                                                                      <\/ul>\n<\/li>\n<\/ul>\n
                                                                                                                                                                                                        \n
                                                                                                                                                                                                      • \n
                                                                                                                                                                                                          \n
                                                                                                                                                                                                        • If congestion is localized<\/strong>: Adjust placement, add routing blockages.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                                                          <\/p>\n

                                                                                                                                                                                                            \n
                                                                                                                                                                                                          • \n
                                                                                                                                                                                                              \n
                                                                                                                                                                                                            • If global<\/strong>: Reduce utilization, optimize netlist.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n

                                                                                                                                                                                                              <\/p>\n

                                                                                                                                                                                                              <\/p>\n

                                                                                                                                                                                                              \"\"<\/figure>\n

                                                                                                                                                                                                              <\/p>\n

                                                                                                                                                                                                              \n

                                                                                                                                                                                                              <\/p>\n

                                                                                                                                                                                                              Example Flow for Congestion Reduction<\/strong><\/h3>\n

                                                                                                                                                                                                              <\/p>\n

                                                                                                                                                                                                                \n
                                                                                                                                                                                                              1. \n
                                                                                                                                                                                                                  <\/ol>\n<\/li>\n<\/ol>\n
                                                                                                                                                                                                                    \n
                                                                                                                                                                                                                  1. \n
                                                                                                                                                                                                                      \n
                                                                                                                                                                                                                    1. Initial Floorplan<\/strong> \u2192 Identify congestion hotspots.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n

                                                                                                                                                                                                                      <\/p>\n

                                                                                                                                                                                                                        \n
                                                                                                                                                                                                                      1. \n
                                                                                                                                                                                                                          \n
                                                                                                                                                                                                                        1. Adjust Macro Placement<\/strong> \u2192 Free up routing channels.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n

                                                                                                                                                                                                                          <\/p>\n

                                                                                                                                                                                                                            \n
                                                                                                                                                                                                                          1. \n
                                                                                                                                                                                                                              \n
                                                                                                                                                                                                                            1. Run Congestion-Aware Placement<\/strong> \u2192 Spread cells uniformly.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n

                                                                                                                                                                                                                              <\/p>\n

                                                                                                                                                                                                                                \n
                                                                                                                                                                                                                              1. \n
                                                                                                                                                                                                                                  \n
                                                                                                                                                                                                                                1. Optimize Power Grid<\/strong> \u2192 Avoid blocking routing tracks.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n

                                                                                                                                                                                                                                  <\/p>\n

                                                                                                                                                                                                                                    \n
                                                                                                                                                                                                                                  1. \n
                                                                                                                                                                                                                                      \n
                                                                                                                                                                                                                                    1. Rerun Routing & Verify<\/strong> \u2192 Check overflow reduction.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n

                                                                                                                                                                                                                                      <\/p>\n

                                                                                                                                                                                                                                      <\/p>\n

                                                                                                                                                                                                                                      \n

                                                                                                                                                                                                                                      <\/p>\n

                                                                                                                                                                                                                                      Summary<\/strong><\/h3>\n

                                                                                                                                                                                                                                      <\/p>\n

                                                                                                                                                                                                                                      \n\n\n\n\n\n\n\n\n\n
                                                                                                                                                                                                                                      Method<\/strong><\/th>\nWhen to Apply<\/strong><\/th>\nImpact<\/strong><\/th>\n<\/tr>\n<\/thead>\n
                                                                                                                                                                                                                                      Macro Placement<\/td>\nEarly floorplanning<\/td>\nHigh (reduces routing blocks)<\/td>\n<\/tr>\n
                                                                                                                                                                                                                                      Cell Spreading<\/td>\nPost-placement<\/td>\nMedium (improves track usage)<\/td>\n<\/tr>\n
                                                                                                                                                                                                                                      Buffer Insertion<\/td>\nHigh-fanout nets<\/td>\nMedium (reduces net crowding)<\/td>\n<\/tr>\n
                                                                                                                                                                                                                                      Layer Optimization<\/td>\nDuring routing<\/td>\nHigh (better resource usage)<\/td>\n<\/tr>\n
                                                                                                                                                                                                                                      Net Weighting<\/td>\nCritical nets<\/td>\nLow (prioritizes key routes)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n

                                                                                                                                                                                                                                      <\/p>\n

                                                                                                                                                                                                                                      Final Tip:<\/strong> Use iterative refinement<\/strong>\u2014congestion fixes often require multiple placement\/routing trials.<\/p>\n

                                                                                                                                                                                                                                      <\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"

                                                                                                                                                                                                                                      In ASIC physical design, congestion occurs when there are more routing demands than available resources (tracks) in a region, leading to routing failures, increased delays, and DRC violations. Managing congestion is crucial for achieving a routable design with optimal performance. \u00a0 Methods to Reduce Congestion 1. Floorplanning Techniques Macro Placement Optimization Place large macros (RAMs, […]<\/p>\n","protected":false},"author":2,"featured_media":26250,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"postBodyCss":"","postBodyMargin":[],"postBodyPadding":[],"postBodyBackground":{"backgroundType":"classic","gradient":""},"footnotes":""},"categories":[83,40],"tags":[79,81,78,80,82],"class_list":["post-26248","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-rtl-to-gds2","category-technology","tag-pd","tag-placement","tag-routing-congestion","tag-rtl-to-gds2","tag-synthesis"],"_links":{"self":[{"href":"https:\/\/wattsemi.com\/index.php?rest_route=\/wp\/v2\/posts\/26248","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wattsemi.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wattsemi.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wattsemi.com\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/wattsemi.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=26248"}],"version-history":[{"count":12,"href":"https:\/\/wattsemi.com\/index.php?rest_route=\/wp\/v2\/posts\/26248\/revisions"}],"predecessor-version":[{"id":26349,"href":"https:\/\/wattsemi.com\/index.php?rest_route=\/wp\/v2\/posts\/26248\/revisions\/26349"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/wattsemi.com\/index.php?rest_route=\/wp\/v2\/media\/26250"}],"wp:attachment":[{"href":"https:\/\/wattsemi.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=26248"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wattsemi.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=26248"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wattsemi.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=26248"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}