hdl.dsl: implement FSM.

nmigen/hdl/dsl.py

@@ -2,7 +2,7 @@ from collections import OrderedDict
 from collections.abc import Iterable
 from contextlib import contextmanager
 
-from ..tools import flatten
+from ..tools import flatten, bits_for
 from .ast import *
 from .ir import *
 from .xfrm import *
@@ -211,6 +211,62 @@ class Module(_ModuleBuilderRoot):
             self._ctrl_context = "Switch"
             self._statements = _outer_case
 
+    @contextmanager
+    def FSM(self, reset=None, domain="sync", name="fsm"):
+        self._check_context("FSM", context=None)
+        fsm_data = self._set_ctrl("FSM", {
+            "signal":   Signal(name="{}_state".format(name)),
+            "domain":   domain,
+            "encoding": OrderedDict(),
+            "states":   OrderedDict(),
+        })
+        if reset is not None:
+            fsm_data["encoding"][reset] = 0
+        try:
+            self._ctrl_context = "FSM"
+            self.domain._depth += 1
+            yield
+        finally:
+            self.domain._depth -= 1
+            self._ctrl_context = None
+        self._pop_ctrl()
+
+    @contextmanager
+    def State(self, name):
+        self._check_context("FSM State", context="FSM")
+        fsm_data = self._get_ctrl("FSM")
+        if name in fsm_data["states"]:
+            raise SyntaxError("FSM state '{}' is already defined".format(name))
+        if name not in fsm_data["encoding"]:
+            fsm_data["encoding"][name] = len(fsm_data["encoding"])
+        try:
+            _outer_case, self._statements = self._statements, []
+            self._ctrl_context = None
+            yield
+            self._flush_ctrl()
+            fsm_data["states"][name] = self._statements
+        finally:
+            self._ctrl_context = "FSM"
+            self._statements = _outer_case
+
+    @property
+    def next(self):
+        raise SyntaxError("Only assignment to `m.next` is permitted")
+
+    @next.setter
+    def next(self, name):
+        for ctrl_name, ctrl_data in reversed(self._ctrl_stack):
+            if ctrl_name == "FSM":
+                if name not in ctrl_data["encoding"]:
+                    ctrl_data["encoding"][name] = len(ctrl_data["encoding"])
+                self._add_statement(
+                    assigns=[ctrl_data["signal"].eq(ctrl_data["encoding"][name])],
+                    domain=ctrl_data["domain"],
+                    depth=len(self._ctrl_stack))
+                break
+        else:
+            raise SyntaxError("`m.next = <...>` is only permitted inside an FSM")
+
     def _pop_ctrl(self):
         name, data = self._ctrl_stack.pop()
 
@@ -238,6 +294,13 @@ class Module(_ModuleBuilderRoot):
 
             self._statements.append(Switch(switch_test, switch_cases))
 
+        if name == "FSM":
+            fsm_signal, fsm_encoding, fsm_states = data["signal"], data["encoding"], data["states"]
+            fsm_signal.nbits = bits_for(len(fsm_encoding) - 1)
+            # The FSM is encoded such that the state with encoding 0 is always the reset state.
+            self._statements.append(Switch(fsm_signal,
+                OrderedDict((fsm_encoding[name], stmts) for name, stmts in fsm_states.items())))
+
     def _add_statement(self, assigns, domain, depth, compat_mode=False):
         def domain_name(domain):
             if domain is None: