serial: clear shreg for new byte

Author: HarryMakes

nmigen_stdio/serial.py

@@ -81,6 +81,7 @@ def elaborate(self, platform):
             with m.State("IDLE"):
                 with m.If(~self.i):
                     m.d.sync += [
+                        shreg.eq(Repl(0, len(shreg))),
                         bits_left.eq(len(shreg) - 1),
                         timer.eq(self.divisor >> 1)
                     ]
@@ -119,7 +120,7 @@ def elaborate(self, platform):
                     # (useful for divisor == 0)
                     with m.If(~self.i):
                         m.d.sync += [
-                            shreg.eq(Cat(shreg[1:], self.i)),
+                            shreg.eq(Cat(Repl(0, len(shreg)-1), self.i)),
                             bits_left.eq(len(shreg) - 2),
                             timer.eq(self.divisor)
                         ]