This custom library addendum to gf180mcu_fd_io contains an overlay
pad for the bidirectional GPIO pad featuring an additional pin
with a connection directly to the pad on the near side of the 100
ohm ESD resistor, making an analog connection to the pad through
a series 100 ohm resistor.

There are three sets of pads in the PDK for 3, 4, and 5 layers of
metal, which correspond to open_pdks' gf180mcuA, B, and C/D,
respectively.  The base cellname is the same in each case but the
LEF and GDS file names are given a suffix _3lm, _4lm, or _5lm.
Since the magic database file is always the same as the cell name,
the three versions have been placed in separate subdirectories.