add IOBUF

diff --git a/techlibs/gowin/cells_sim.v b/techlibs/gowin/cells_sim.v
index cc1ac48..a67855d 100644
--- a/techlibs/gowin/cells_sim.v
+++ b/techlibs/gowin/cells_sim.v

@@ -308,6 +308,14 @@
   assign O = OEN ? I : 1'bz;
 endmodule
 
+module IOBUF (O, IO, I, OEN);
+  input I,OEN;
+  output O;
+  inout IO;
+  assign IO = OEN ? I : 1'bz;
+  assign I = IO;
+endmodule
+
 module GSR (input GSRI);
 	wire GSRO = GSRI;
 endmodule

diff --git a/techlibs/gowin/synth_gowin.cc b/techlibs/gowin/synth_gowin.cc
index d541edd..a70ff93 100644
--- a/techlibs/gowin/synth_gowin.cc
+++ b/techlibs/gowin/synth_gowin.cc

@@ -230,7 +230,8 @@
 			run("techmap -map +/gowin/cells_map.v");
 			run("setundef -undriven -params -zero");
 			run("hilomap -singleton -hicell VCC V -locell GND G");
-			run("iopadmap -bits -inpad IBUF O:I -outpad OBUF I:O, -toutpad TBUF OEN:I:O", "(unless -noiopads)");
+			run("iopadmap -bits -inpad IBUF O:I -outpad OBUF I:O "
+				"-toutpad TBUF OEN:I:O -tinoutpad IOBUF OEN:O:I:IO", "(unless -noiopads)");
 			run("dffinit  -ff DFF Q INIT");
 			run("clean");