Why not use top metal layers for PDN?

[oharboe]

Which layers that can be used in a macro for routing is limited to the top PDN layer.

Is there a reason not to move the PDN to higher metal layers in these files to allow using more routing layers?

https://github.com/The-OpenROAD-Project/OpenROAD-flow-scripts/blob/edcd265180fc6f2edc04b6743920fc027c56c797/flow/platforms/asap7/openRoad/pdn/BLOCK_grid_strategy.tcl#L32C1-L32C44

https://github.com/The-OpenROAD-Project/OpenROAD-flow-scripts/blob/edcd265180fc6f2edc04b6743920fc027c56c797/flow/platforms/asap7/openRoad/pdn/BLOCKS_grid_strategy.tcl#L28C1-L28C44

[maliberty]

You are trading routability at the block level vs at the top level. The right tradeoff can be design specific.

[oharboe]

Gotit.

So if I dont need or want to route on top of macros and want to use all metal layers for routing, is there a PDN .tcl file I could use?

Or adapt easily(as in with limited skills and much guesswork)?

[maliberty]

Just modify a copy of the existing pdn script to change the relevant layers. It is possible to connect the power grid to the block in planar sense but it requires that the straps line up exactly with the power pins. It is somewhat easier to leave two layers at the top for power connection from above.

[oharboe]

What about all the magic numbers?

Are they not layer specific?

[maliberty]

A mix of layer specific and design specific. The values we give are just reasonable guesses. Really this is driven by IR drop analysis and human judgement of acceptable results. Often foundries have recommended grids but asap7 has no foundry.

[oharboe]

Thanks! Will give it a go.