gt2n: document GT2.itf source and PDK files in setRC.tcl/itf_to_rc.py

Note where GT2.itf comes from (the StarRC interconnect tech file shipped with
the GT2N PDK, not in this repo) and the matching NXTGRD/QRC files, mirroring
the existing setRC.tcl header, and point readers to the platform README for
additional GT2N information. itf_to_rc.py also flags that __main__ hardcodes a
local path to the ITF that must be adjusted to a local PDK copy. Comments only.

Signed-off-by: mrg <mrg@ucsc.edu>

Author: mguthaus

flow/platforms/gt2n/itf_to_rc.py

@@ -4,6 +4,13 @@
 Units: R in ohm/um for metals (ohm/via for vias), C in pf/um, matching the
 gt2n libs (pulling_resistance_unit=1ohm, capacitive_load_unit=1pf).
 
+Input: GT2N/nxtgrd/GT2.itf -- the StarRC interconnect tech file shipped with
+the GT2N PDK (not in this repo). __main__ hardcodes a local path to it; point
+that at your PDK copy. The matching NXTGRD/QRC files (GT2N/nxtgrd/GT2.nxtgrd,
+GT2N/qrc/{GT2.ict, GT2_qrc.tch}) carry the full data a calibrated extractor
+needs; this script is only the analytical approximation behind setRC.tcl.
+(See the gt2n platform README for additional GT2N information.)
+
 Model:
   R/um  = RPSQ / WMIN
   C/um  = (Ca + Cb)*FRINGE_FACTOR + 2*Cc
@@ -89,6 +96,8 @@ def pp(d):
 
 
 if __name__ == "__main__":
+    # GT2N/nxtgrd/GT2.itf from the PDK (StarRC interconnect tech file); edit
+    # this path to your local PDK checkout.
     entries = parse_itf("/home/mrg/orfs/GT2N/nxtgrd/GT2.itf")
     print("# Per-length metal R and C, derived analytically from GT2.itf.")
     print("# R/um = RPSQ / WMIN  (matches Fig 1(b) of the GT2N paper).")

flow/platforms/gt2n/setRC.tcl

@@ -19,6 +19,7 @@
 # proprietary tool or a 3D field solver (e.g. FasterCap) run on
 # OpenRCX's own pattern DEFs. Until then this set_layer_rc table is
 # the working approximation.
+# (See the gt2n platform README for additional GT2N information.)
 #
 # Liberty units in gt2_6t_w*_*_tt_0p7v25c.lib:
 #   pulling_resistance_unit : 1ohm    -> R values are ohm / um