timer: add prescaler

misoclib/cpu/peripherals/timer/__init__.py

@@ -1,12 +1,14 @@
 from migen.fhdl.std import *
 from migen.bank.description import *
 from migen.bank.eventmanager import *
+from migen.genlib.misc import Counter
 
 class Timer(Module, AutoCSR):
-	def __init__(self, width=32):
+	def __init__(self, width=32, prescaler_width=32):
 		self._load = CSRStorage(width)
 		self._reload = CSRStorage(width)
 		self._en = CSRStorage()
+		self._prescaler = CSRStorage(prescaler_width, reset=1)
 		self._update_value = CSR()
 		self._value = CSRStatus(width)
 
@@ -15,14 +17,28 @@ class Timer(Module, AutoCSR):
 		self.ev.finalize()
 
 		###
+		enable = self._en.storage
+		tick = Signal()
+
+		counter = Counter(prescaler_width)
+		self.submodules += counter
+		self.comb += [
+			If(enable,
+				tick.eq(counter.value >= (self._prescaler.storage-1)),
+				counter.ce.eq(1),
+				counter.reset.eq(tick),
+			).Else(
+				counter.reset.eq(1)
+			)
+		]
 
 		value = Signal(width)
 		self.sync += [
-			If(self._en.storage,
+			If(enable,
 				If(value == 0,
 					# set reload to 0 to disable reloading
 					value.eq(self._reload.storage)
-				).Else(
+				).Elif(tick,
 					value.eq(value - 1)
 				)
 			).Else(