milkymist/adc/__init__.py: CounterADC - simple counter-based ADC

This is a revised version of the counter-based ADC.

milkymist/adc/__init__.py

@@ -0,0 +1,57 @@
+from migen.fhdl.structure import *
+from migen.fhdl.module import Module
+from migen.bank.description import *
+from migen.genlib.misc import optree
+
+class CounterADC(Module, AutoCSR):
+	def __init__(self, charge, sense, width = 24):
+		if not isinstance(sense, collections.Iterable):
+			sense = [sense]
+
+		channels = len(sense)
+
+		self._start_busy = CSR()
+		self._overflow = CSRStatus(channels)
+		self._polarity = CSRStorage()
+
+		count = Signal(width)
+		busy = Signal(channels)
+
+		res = []
+		for i in range(channels):
+			res.append(CSRStatus(width, name="res"+str(i)))
+			setattr(self, "_res"+str(i), res[-1])
+
+		any_busy = Signal()
+		self.comb += [
+			any_busy.eq(optree("|",
+			    [busy[i] for i in range(channels)])),
+			self._start_busy.w.eq(any_busy)
+		]
+
+		carry = Signal()
+
+		self.sync += [
+			If(self._start_busy.re,
+				count.eq(0),
+				busy.eq((1 << channels)-1),
+				self._overflow.status.eq(0),
+			    	charge.eq(~self._polarity.storage)
+			).Elif(any_busy,
+				Cat(count, carry).eq(count + 1),
+				If(carry,
+					self._overflow.status.eq(busy),
+					busy.eq(0)
+				)
+			).Else(
+				charge.eq(self._polarity.storage)
+			)
+		]
+
+		for i in range(channels):
+			self.sync += If(busy[i],
+				If(sense[i] != self._polarity.storage,
+					res[i].status.eq(count),
+					busy[i].eq(0)
+				)
+			)