timer: add documentation

Now that CSRs have documentation support, add documentation to the basic
`Timer` module.

Signed-off-by: Sean Cross <sean@xobs.io>

litex/soc/cores/timer.py

@@ -10,11 +10,22 @@ from litex.soc.interconnect.csr_eventmanager import *
 
 class Timer(Module, AutoCSR):
     def __init__(self, width=32):
-        self._load = CSRStorage(width)
-        self._reload = CSRStorage(width)
-        self._en = CSRStorage()
-        self._update_value = CSR()
-        self._value = CSRStatus(width)
+        self._load = CSRStorage(width, description="""This is the initial value loaded into the
+                                        timer.  You can make a one-shot timer by disabling the
+                                        timer, writing to this register, and then re-enabling
+                                        the timer.  For a recurring timer, set this to the same
+                                        value as `reload`, or to 0.""")
+        self._reload = CSRStorage(width, description="""The internal timer value will be updated
+                                        with this value whenever it reaches 0.  Use this to create
+                                        a periodic timer that fires whenever this transitions from
+                                        0 to >0.  To create a one-shot timer, leave this value as 0.""")
+        self._en = CSRStorage(fields=[CSRField("en", description="Write a `1` here to start the timer running")])
+        self._update_value = CSRStorage(fields=[CSRField("update", description="""Writing to this register causes
+                                        the `value` register to be updated with with the current countdown
+                                        value.""")])
+        self._value = CSRStatus(width, description="""Last snapshotted value of the countdown
+                                        timer.  This value is only updated when a `1` is written
+                                        to `update_value`.""")
 
         self.submodules.ev = EventManager()
         self.ev.zero = EventSourceProcess()