Added documentation to AsyncResetSynchronizer (#230)

migen/genlib/resetsync.py

@@ -4,8 +4,24 @@
 
 __all__ = ["AsyncResetSynchronizer"]
 
-
 class AsyncResetSynchronizer(Special):
+    """AsyncResetSynchronizer
+    Connects a synchronized reset signal to the provided ClockDomain. Synchronizing is done with two 
+    D-Flip-Flops in the provided ClockDomain. The output of the second flip-flop is directly connected
+    to the reset signal of the provided ClockDomain.
+    
+    Asynchronous rising edge of the async_reset signal propagates directly to the output. Clearing the
+    async_reset signal is synchronized by the two flip-flops.
+    
+    Needs to be added to the specials list: self.specials += AsyncResetSynchronizer(cd, async_reset)
+    
+    cd : in
+        ClockDomain the reset signal shall be connected to
+    async_reset : in
+        Reset signal (potentially) asynchronous to the ClockDomain provided in parameter "cd".
+        Synchronized instance of the reset signal is connected to the ClockDomains "rst" signal.
+        async_reset is high active.
+    """
     def __init__(self, cd, async_reset):
         Special.__init__(self)
         self.cd = cd