Placing Macros and Sizing Channels

Where the interviewer goes next. Open a node to practise that follow-up.

WhyWhat IfProve ItCompareScale It

1. Placing Macros and Sizing Channels
2. WhyCounting Tracks in a ChannelL1 JuniorL2 Mid-Level

   How many vertical routing tracks does a 9.175 µm channel give you on met2 and met4 in sky130?

   Reveal the Model Answer

   Divide the width by each layer's pitch: met2 at 0.46 µm gives about 19 tracks and met4 at 0.92 µm about 9. Then subtract what is already used, such as power straps and pin access, and keep the router's reserve. That is far fewer than the nets two 39-pin macros want to exchange.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Divide the width by each layer's pitch: met2 at 0.46 µm gives about 19 tracks and met4 at 0.92 µm about 9. Key point 2 Then subtract what is already used, such as power straps and pin access, and keep the router's reserve. Key point 3 That is far fewer than the nets two 39-pin macros want to exchange.

   • Lesson L3.3Macros, Halos and Channels
   • Lesson L1.5Rows, Sites, Tracks and the Metal Stack
3. Prove ItSolve L3.3.1: Open a Route Past the MacrosSimulator-graded

   Show it, don't just say it: this Silicon Lab problem is graded by the simulator.

   Open Problem L3.3.1

4. CompareHalos vs Placement BlockagesL1 JuniorL2 Mid-Level

   Compare a halo around a macro with a placement blockage.

   Reveal the Model Answer

   A halo is a keep-out ring attached to the macro, so it moves with the macro. It keeps standard cells off the macro's edges and pin access. A blockage is a region you draw on the floorplan to keep cells, or routes, out of an area. It can be for any reason, such as a channel you want left for wires. Both cost placement area, so size them from the routing they protect.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 A halo is a keep-out ring attached to the macro, so it moves with the macro. Key point 2 It keeps standard cells off the macro's edges and pin access. Key point 3 A blockage is a region you draw on the floorplan to keep cells, or routes, out of an area. Key point 4 It can be for any reason, such as a channel you want left for wires. Key point 5 Both cost placement area, so size them from the routing they protect.

   • Lesson L3.3Macros, Halos and Channels
5. What IfTurning Macro Pins Instead of Widening the ChannelL2 Mid-LevelL3 Senior

   Asked if they widen the channel first

   What if you cannot afford a wider channel?

   Reveal the Model Answer

   Change where the nets go instead of where the channel is. In lesson L3.3, moving the macros' pins to the top edge sent the crossing nets out of the channel. Overflow fell to zero with a 4.6 µm halo, and wirelength dropped from 4981 to 3774 µm. Pin moves on a hard macro are an agreement with its owner, so raise them early.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Change where the nets go instead of where the channel is. Key point 2 In lesson L3.3, moving the macros' pins to the top edge sent the crossing nets out of the channel. Key point 3 Overflow fell to zero with a 4.6 µm halo, and wirelength dropped from 4981 to 3774 µm. Key point 4 Pin moves on a hard macro are an agreement with its owner, so raise them early.

   • Lesson L3.3Macros, Halos and Channels
6. Scale ItDiagnosing Routing CongestionL1 JuniorL2 Mid-Level

   The global router reports the overflow below for a block with two macros and a narrow channel between them. How do you find the cause and pick a fix?

   Two hardened MACs, glue logic between them (lesson L3.3, measured global route)
   channel 9.175 um, halo 2.0     overflow 522   usage 18.37 %   wirelength 14828 um
   channel 36.775 um              overflow 0                     wirelength  4981 um
   pins on top edge, halo 4.6     overflow 0                     wirelength  3774 um

   Reveal the Model Answer

   Overflow concentrated in one place, with low usage overall, points to a local cause. Here that cause is the channel, not a block that is too full. Look at where the demand comes from. It may be nets crossing the channel, pin access at the macro edges, or straps that took the tracks. The fixes act on that cause: a wider channel, pins turned away from it, or a different halo (keep-out ring). Each fix is measured by routed wirelength and timing, not by overflow alone.

   Read the diagram as text

   • Overflow in one channel: Usage low everywhere else
   • Nets crossing the channel
   • Pin access at the macro edges
   • Power straps on the tracks
   • Wider channel, or pins turned away
   • A different halo
   • Count the free tracks again

   • Overflow in one channel → Nets crossing the channel
   • Overflow in one channel → Pin access at the macro edges
   • Overflow in one channel → Power straps on the tracks
   • Nets crossing the channel → Wider channel, or pins turned away
   • Pin access at the macro edges → A different halo
   • Power straps on the tracks → Count the free tracks again

   Judge each fix by routed wirelength and timing, not by overflow alone.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Overflow concentrated in one place, with low usage overall, points to a local cause. Key point 2 Here that cause is the channel, not a block that is too full. Key point 3 Look at where the demand comes from. Key point 4 It may be nets crossing the channel, pin access at the macro edges, or straps that took the tracks. Key point 5 The fixes act on that cause: a wider channel, pins turned away from it, or a different halo (keep-out ring). Key point 6 Each fix is measured by routed wirelength and timing, not by overflow alone.

   What Interviewers Listen For (Junior)

   A good junior answer reads overflow and usage together. It suspects the channel rather than a block that is too full.

   Misconception

   "Overflow means the block is too full; lower the utilization."

   Total usage here is about 18 %. The overflow is in one channel, and widening it or turning the macro pins removes all of it without touching utilization.

   Source ASIC Point lesson L3.3 (measured global route)

   Half-Truth

   "Give the router more layers and the problem goes away."

   More layers do not help pins that cannot be reached, or a channel whose nets must all cross one gap. Enabling layers without rerouting, extracting and rechecking is not a fix.

   Source ASIC Point lessons L6.3 and L6.6

   • Lesson L3.3Macros, Halos and Channels
   • Lesson L6.1Global Routing: The Plan for Every Net
   • Lesson L6.3Congestion and Detours
   • Problem L3.3.1Open a Route Past the Macros
   • Problem L6.3.1Give the Crossbar Pins Room
7. WhyGlobal Routing vs Detailed RoutingL1 JuniorL2 Mid-Level

   Why does a clean global route not guarantee a clean detailed route?

   Reveal the Model Answer

   Global routing plans each net through coarse tiles and counts tracks per tile edge. That says nothing about how a wire reaches a pin or which vias fit. Detailed routing picks real tracks, vias and pin access under width, spacing, area and enclosure rules. It can run out of iterations and leave violations in a readable layout, so its own count matters too.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Global routing plans each net through coarse tiles and counts tracks per tile edge. Key point 2 That says nothing about how a wire reaches a pin or which vias fit. Key point 3 Detailed routing picks real tracks, vias and pin access under width, spacing, area and enclosure rules. Key point 4 It can run out of iterations and leave violations in a readable layout, so its own count matters too.

   • Lesson L6.1Global Routing: The Plan for Every Net
   • Lesson L6.2Detailed Routing: Tracks, Vias and Design Rules
8. What IfWhat a Clean Router Count CoversL2 Mid-Level

   What if the detailed router reports zero violations? Is the layout DRC clean?

   Reveal the Model Answer

   Not necessarily. The router checks the rules it knows from the technology LEF, a subset of the foundry deck. Full DRC on the merged layout, with cells, fill and every rule, is a separate run. Antenna checking is separate again. Sign-off quotes which checker ran with which deck.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Not necessarily. Key point 2 The router checks the rules it knows from the technology LEF, a subset of the foundry deck. Key point 3 Full DRC on the merged layout, with cells, fill and every rule, is a separate run. Key point 4 Antenna checking is separate again. Key point 5 Sign-off quotes which checker ran with which deck.

   • Lesson L6.2Detailed Routing: Tracks, Vias and Design Rules
   • Lesson L7.2DRC: Check the Final Shapes
9. CompareDensity Knobs vs Floorplan FixesL2 Mid-LevelL3 Senior

   Compare fixing congestion with placement density against fixing it in the floorplan.

   Reveal the Model Answer

   Density and routability-driven placement spread cells. That helps when demand comes from local crowding. It does nothing for nets that must cross a fixed gap. Floorplan fixes, such as macro position, channel width or pin edges, change where the nets go. Density changes distances but not the netlist's connectivity, so a connectivity hot spot needs the floorplan answer.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Density and routability-driven placement spread cells. Key point 2 That helps when demand comes from local crowding. Key point 3 It does nothing for nets that must cross a fixed gap. Key point 4 Floorplan fixes, such as macro position, channel width or pin edges, change where the nets go. Key point 5 Density changes distances but not the netlist's connectivity, so a connectivity hot spot needs the floorplan answer.

   • Lesson L6.3Congestion and Detours
   • Lesson L4.2Density and Congestion
   • Lesson L3.3Macros, Halos and Channels
10. Prove ItSolve L3.3.1: Open a Route Past the MacrosSimulator-graded

    Show it, don't just say it: this Silicon Lab problem is graded by the simulator.

    Open Problem L3.3.1

11. Scale ItRelieving Placement CongestionL2 Mid-LevelL3 Senior

    The global-route map shows overflow in one corner of the block. Which remedies do you try, in what order?

    Reveal the Model Answer

    Start cheap and local: lower the target density or turn on routability-driven placement, which inflates cells in crowded regions. Then look at the cause, such as a pin cluster, a wide multiplexer or a macro channel. Fix that cause with a pin plan or floorplan change. Lower utilization is the expensive last resort. Re-check timing after each step, because spreading costs wire.

    Self-Check

    Tick each key point you covered in your answer.

    Key point 1 Start cheap and local: lower the target density or turn on routability-driven placement, which inflates cells in crowded regions. Key point 2 Then look at the cause, such as a pin cluster, a wide multiplexer or a macro channel. Key point 3 Fix that cause with a pin plan or floorplan change. Key point 4 Lower utilization is the expensive last resort. Key point 5 Re-check timing after each step, because spreading costs wire.

    • Lesson L4.2Density and Congestion
    • Lesson L6.3Congestion and Detours
    • Problem L6.3.1Give the Crossbar Pins Room
12. Prove ItSolve L6.3.1: Give the Crossbar Pins RoomSimulator-graded

    Show it, don't just say it: this Silicon Lab problem is graded by the simulator.

    Open Problem L6.3.1

13. WhyWhy Local Congestion Fixes Come FirstL2 Mid-Level

    Why try routability-driven placement before lowering the utilization of the whole block?

    Reveal the Model Answer

    Routability-driven placement adds room only where the router predicts overflow. Lower utilization grows the whole block, so area and wire grow everywhere. The local fix keeps the dense, short-wired regions that had no problem. So it is the cheaper first step.

    Self-Check

    Tick each key point you covered in your answer.

    Key point 1 Routability-driven placement adds room only where the router predicts overflow. Key point 2 Lower utilization grows the whole block, so area and wire grow everywhere. Key point 3 The local fix keeps the dense, short-wired regions that had no problem. Key point 4 So it is the cheaper first step.

    Wrong Altitude

    "Lower utilization fixes congestion, so start there."

    It is the most expensive remedy, because it changes the block size and the parent floorplan. Try the local fixes first.

    • Lesson L4.2Density and Congestion