uart: RX support

milkymist/uart/__init__.py

@@ -9,7 +9,8 @@ def __init__(self, address, clk_freq, baud=115200):
 		self._divisor = RegisterField("divisor", 16, reset=int(clk_freq/baud/16))
 
 		self._tx_event = EventSourceLevel()
-		self.events = EventManager(self._tx_event)
+		self._rx_event = EventSourcePulse()
+		self.events = EventManager(self._tx_event, self._rx_event)
 		self.bank = csrgen.Bank([self._rxtx, self._divisor] + self.events.get_registers(),
 			address=address)
 
@@ -28,10 +29,11 @@ def get_fragment(self):
 				enable16_counter.eq(self._divisor.field.r - 1))
 		]
 
+		# TX
 		tx_reg = Signal(BV(8))
 		tx_bitcount = Signal(BV(4))
 		tx_count16 = Signal(BV(4))
-		tx_busy = Signal()
+		tx_busy = self._tx_event.trigger
 		sync += [
 			If(self._rxtx.re,
 				tx_reg.eq(self._rxtx.r),
@@ -55,7 +57,53 @@ def get_fragment(self):
 				)
 			)
 		]
-		comb.append(self._tx_event.trigger.eq(tx_busy))
+
+		# RX
+		rx0 = Signal() # sychronize
+		rx = Signal()
+		sync += [
+			rx0.eq(self.rx),
+			rx.eq(rx0)
+		]
+		rx_r = Signal()
+		rx_reg = Signal(BV(8))
+		rx_bitcount = Signal(BV(4))
+		rx_count16 = Signal(BV(4))
+		rx_busy = Signal()
+		rx_done = self._rx_event.trigger
+		rx_data = self._rxtx.w
+		sync += [
+			rx_done.eq(0),
+			If(enable16,
+				rx_r.eq(rx),
+				If(~rx_busy,
+					If(~rx & rx_r, # look for start bit
+						rx_busy.eq(1),
+						rx_count16.eq(7),
+						rx_bitcount.eq(0)
+					)
+				).Else(
+					rx_count16.eq(rx_count16 + 1),
+					If(rx_count16 == 0,
+						rx_bitcount.eq(rx_bitcount + 1),
+
+						If(rx_bitcount == 0,
+							If(rx, # verify start bit
+								rx_busy.eq(0)
+							)
+						).Elif(rx_bitcount == 9,
+							rx_busy.eq(0),
+							If(rx, # verify stop bit
+								rx_data.eq(rx_reg),
+								rx_done.eq(1)
+							)
+						).Else(
+							rx_reg.eq(Cat(rx_reg[1:], rx))
+						)
+					)
+				)
+			)
+		]
 
 		return self.bank.get_fragment() \
 			+ self.events.get_fragment() \