build.plat,vendor: automatically create sync domain from default_clk.

But only if it is not defined by the programmer.

Closes #57.

nmigen/build/plat.py

@@ -7,6 +7,7 @@ import jinja2
 
 from .. import __version__
 from ..hdl.ast import *
+from ..hdl.cd import *
 from ..hdl.dsl import *
 from ..hdl.ir import *
 from ..back import rtlil, verilog
@@ -71,11 +72,27 @@ class Platform(ResourceManager, metaclass=ABCMeta):
 
         self.toolchain_program(products, name, **(program_opts or {}))
 
+    @abstractmethod
+    def create_missing_domain(self, name):
+        if name == "sync" and self.default_clk is not None:
+            clk_i = self.request(self.default_clk).i
+            if self.default_rst is not None:
+                rst_i = self.request(self.default_rst).i
+
+            m = Module()
+            m.domains += ClockDomain("sync", reset_less=self.default_rst is None)
+            m.d.comb += ClockSignal("sync").eq(clk_i)
+            if self.default_rst is not None:
+                m.d.comb += ResetSignal("sync").eq(rst_i)
+            return m
+
     def prepare(self, fragment, name="top", **kwargs):
         assert not self._prepared
         self._prepared = True
 
         fragment = Fragment.get(fragment, self)
+        fragment = fragment.prepare(ports=list(self.iter_ports()),
+                                    missing_domain=self.create_missing_domain)
 
         def add_pin_fragment(pin, pin_fragment):
             pin_fragment = Fragment.get(pin_fragment, self)
@@ -226,9 +243,8 @@ class TemplatedPlatform(Platform):
         autogenerated = "Automatically generated by nMigen {}. Do not edit.".format(__version__)
 
         def emit_design(backend):
-            return {"rtlil": rtlil, "verilog": verilog}[backend].convert(
-                fragment, name=name, platform=self, ports=list(self.iter_ports()),
-                missing_domain=lambda name: None)
+            return {"rtlil": rtlil, "verilog": verilog}[backend].convert(fragment, name=name,
+                ports=list(self.iter_ports()), missing_domain=lambda name: None)
 
         def emit_commands(format):
             commands = []