Timing unit inconsistencies

[gatecat]

The documentation for the units in the https://github.com/SymbiFlow/prjxray/blob/cb5f2a30633bf0d3ac91a9b5802790ac1da86736/docs/dev_database/common/tile_type.rst#pips-section doesn't seem to be correct.

Not only does it mention us (nanoseconds) which must be a typo, it doesn't seem to be consistent with what's actually in the database.

Looking at a random PIP:

        "INT_L.NL1END0->>NE2BEG0": {
            "can_invert": "0",
            "dst_to_src": {
                "delay": [
                    "0.033",
                    "0.041",
                    "0.111",
                    "0.134"
                ],
                "in_cap": "8.473",
                "res": "1185.1186875"
            },
            "dst_wire": "NE2BEG0",
            "is_directional": "1",
            "is_pass_transistor": 0,
            "is_pseudo": "0",
            "src_to_dst": {
                "delay": [
                    "0.033",
                    "0.041",
                    "0.111",
                    "0.134"
                ],
                "in_cap": "8.473",
                "res": "1185.1186875"
            },
            "src_wire": "NL1END0"
        },

firstly the delays are apparently in nanoseconds, so the us is definitely a typo, but in_cap in µF and res in mΩ don't work either.

8.473µF * 1185mΩ = 10.04µS

which is a delay about 5 or 6 orders of magnitude too high.

The absolute values of the capacitance and resistance values don't matter, so long as their product gives a reasonable delay scaling. My awareness of likely ASIC numbers is limited, but something like switching to pF for capacitance and keeping mΩ gives a plausible delay product while staying fairly close to physical reality, I think.