litex/milkymist/uart/__init__.py

from migen.fhdl.std import *
from migen.genlib.cdc import MultiReg
from migen.bank.description import *
from migen.bank.eventmanager import *

class UART(Module, AutoCSR):
	def __init__(self, pads, clk_freq, baud=115200):
		self._rxtx = CSR(8)
		self._divisor = CSRStorage(16, reset=int(clk_freq/baud/16))
		
		self.submodules.ev = EventManager()
		self.ev.tx = EventSourceProcess()
		self.ev.rx = EventSourcePulse()
		self.ev.finalize()
	
		###

		pads.tx.reset = 1

		enable16 = Signal()
		enable16_counter = Signal(16)
		self.comb += enable16.eq(enable16_counter == 0)
		self.sync += [
			enable16_counter.eq(enable16_counter - 1),
			If(enable16,
				enable16_counter.eq(self._divisor.storage - 1))
		]
		
		# TX
		tx_reg = Signal(8)
		tx_bitcount = Signal(4)
		tx_count16 = Signal(4)
		tx_busy = self.ev.tx.trigger
		self.sync += [
			If(self._rxtx.re,
				tx_reg.eq(self._rxtx.r),
				tx_bitcount.eq(0),
				tx_count16.eq(1),
				tx_busy.eq(1),
				pads.tx.eq(0)
			).Elif(enable16 & tx_busy,
				tx_count16.eq(tx_count16 + 1),
				If(tx_count16 == 0,
					tx_bitcount.eq(tx_bitcount + 1),
					If(tx_bitcount == 8,
						pads.tx.eq(1)
					).Elif(tx_bitcount == 9,
						pads.tx.eq(1),
						tx_busy.eq(0)
					).Else(
						pads.tx.eq(tx_reg[0]),
						tx_reg.eq(Cat(tx_reg[1:], 0))
					)
				)
			)
		]
		
		# RX
		rx = Signal()
		self.specials += MultiReg(pads.rx, rx)
		rx_r = Signal()
		rx_reg = Signal(8)
		rx_bitcount = Signal(4)
		rx_count16 = Signal(4)
		rx_busy = Signal()
		rx_done = self.ev.rx.trigger
		rx_data = self._rxtx.w
		self.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))
						)
					)
				)
			)
		]