m1/tools/: xdltap and xdlfixes, to add taps to FPGA signals

m1/tools/README

@@ -20,3 +20,34 @@ You need to have fjmem.bit somwehere. It searches for it under
 $HOME/.qi/milkymist/, where reflash_m1.sh caches it. You can also
 specify the location directly by setting the environment variable
 FJMEM_BIT.
+
+
+xdltap - Route nets from inside the FPGA to I/O pads
+====================================================
+
+xdltap adds routes from signals inside the FPGA to I/O pads. It
+does this by converting the .ncd file generated by synthesis 
+
+xdltap signal=output ... [xdl-file]
+
+The path to the signal to be tapped has to be fully qualified.
+Note that not all signals are listed in the .pcf file. Some may
+only show up in .xdl
+
+The output is the name of a global net that has to be connected
+to an input (!) pad.
+
+Signal names containing < and > can be written with { and }, to
+avoid extra escaping for the shell.
+
+Since xdl chokes on its own .xdl files, xdlfixes applies a number
+of ugly hacks to either avoid errors or to propagate them to a
+later stage, where they can be ignored.
+
+Example:
+
+map ... foo.ngd			# generates foo.ncd
+xdl -ncd2xdl foo.ncd		# generates foo.xdl
+xdltap usb/sie/rx_pending=exp{8} foo.xdl | xdlfixes >foo-dbg.xdl
+xdl -xdl2ncd foo-dbg.xdl	# generates foo-dbg.ncd
+par ... foo-dbg.ncd ...