pdn: Failure due to missing follow pins vias

[titan73]

Describe the bug

pdngen fails with the errors in log below.

The pdn script part is:

add_global_connection -net VDD -pin_pattern {^VDD$} -power
add_global_connection -net GND -pin_pattern {^GND$} -ground

set_voltage_domain -power VDD -ground GND

define_pdn_grid -name "Core"

add_pdn_stripe -followpins -layer M1 -width 2.630

add_pdn_stripe -layer M3     -width 11.280 -pitch 924.960 -spacing 16.920 -offset 37.000
add_pdn_stripe -layer M4 -width 11.280 -pitch 723.800   -spacing 16.920 -offset 107.910

add_pdn_connect -layers {M3 M4}
add_pdn_connect -layers {M1 M4}

pdn::debug_renderer 1
pdn::debug_renderer_update

pdngen

With the debug feature, we can see the missing vias:

Expected Behavior

No missing vias and pdn created succesfully.

Environment

Latest OR

To Reproduce

openroad test.tcl -gui with following test case:

missing_fp_vias.zip

Relevant log output

[INFO PDN-0001] Inserting grid: Core
[WARNING PDN-0178] Remaining channel (0.0000, 17.1850) - (2621.0800, 84.5650) on M1 for nets: VDD, GND
[WARNING PDN-0178] Remaining channel (0.0000, 942.1850) - (1812.5800, 1009.5650) on M1 for nets: VDD, GND
[ERROR PDN-0179] Unable to repair all channels.
[ERROR GUI-0070] Error: test.tcl, 40 PDN-0179

Screenshots

No response

Additional Context

No response

[gadfort]

@titan73 It's not clear to me what the M3 strap is doing for you, but it's the cause of this issue.

#add_pdn_stripe -layer M3     -width 11.280 -pitch 924.960 -spacing 16.920 -offset 37.000
add_pdn_stripe -layer M4 -width 11.280 -pitch 723.800   -spacing 16.920 -offset 107.910

#add_pdn_connect -layers {M3 M4}
add_pdn_connect -layers {M1 M4}

Without the M3 strap it compiles without issue. If you are worried about IR drop you can always reduce the pitch on M4.

[titan73]

@gadfort The pictures show only the lower left part of the floorplan to show the details. There are several M4 strap P/G pairs and only 2 M4 pins in the top edge for power & ground (not in the test case) so these M4 straps need to be connected together with M3 ones.

[gadfort]

Can you provide a complete testcase then? With what is provided, removing the M3 strap is sufficient.

[titan73]

I can't provide the full test case for legal reasons and we don't have support contract with PII yet. The request has been done to my hierarchy but it will be difficult since it's evaluation work. That's why I made a simplified one. That said removing M3 straps is a workaround. Having a power grid using 2 metals is not something rare. The problem comes from the fact that M3 has the same direction as M1 so M1 needs to connect to M4 and pdn does not handle the M3 straps blocking the way while there are part of the same P/G net.

[gadfort]

If you must have the M3 straps, then you can add back:

add_pdn_connect -layers {M1 M3}

That will complete the power grid, but because the M3 and M1 overlap completely you will have one giant via array and a massive routing blockage on M2. To limit the size of M2 you could use of a routing blockage, however the via code will reject the via due to the obstruction. This could be fixed however, but will require some time.

[titan73]

Ok. With this command, I have a power grid closer to the expected one. Thanks. That could be good to have a fix. No hurry though.

[titan73]

I didn't pay attention but with the 2 lines:

add_pdn_connect -layers {M1 M3}
add_pdn_connect -layers {M1 M4}

This create a barrier that prevent routing vertically on M2 on top & bottom edges (M4 is too thick and not much used for routing):

I'll comment out pdngen for now then.

[maliberty]

I believe you can specify the cut spacing now to avoid the via wall with -split_cuts.

[titan73]

Indeed, it does the job. Thanks.

There are some remarks though:

1. -cut_pitch has no effect. I don't know what it is supposed to do. I just use the line below which works as attended: add_pdn_connect -layers {metal1 metal3} -split_cuts [list metal2 $split_pitch]
2. The affected rails go beyond the floorplan boundary and the power/ground pins are removed (see in orange)
3. That would have been nice to be able to specify the vias cut number in addition to the pitch. And also an offset so I could have placed them like the others (green areas)

[titan73]

Actually there is an issue. In the the bottom (little red dots in green area) it worked but not in the top (orange):

The configuration of top and bottom areas are the same (vdd stripes on vdd rails and same for gnd).