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--
--	sc_uart.vhd
--
--	8-N-1 serial interface
--	
--	wr, rd should be one cycle long => trde, rdrf goes 0 one cycle later
--
--	Author: Martin Schoeberl	martin@jopdesign.com
--
--
--	resources on ACEX1K30-3
--
--		100 LCs, max 90 MHz
--
--	resetting rts with fifo_full-1 works with C program on pc
--	but not with javax.comm: sends some more bytes after deassert
--	of rts (16 byte blocks regardless of rts).
--	Try to stop with half full fifo.
--
--	todo:
--
--
--	2000-12-02	first working version
--	2002-01-06	changed tdr and rdr to fifos.
--	2002-05-15	changed clkdiv calculation
--	2002-11-01	don't wait if read fifo is full, just drop the byte
--	2002-11-03	use threshold in fifo to reset rts 
--				don't send if cts is '0'
--	2002-11-08	rx fifo to 20 characters and stop after 4
--	2003-07-05	new IO standard, change cts/rts to neg logic
--	2003-09-19	sync ncts in!
--	2004-03-23	two stop bits
--	2005-11-30	change interface to SimpCon
--	2006-08-07	rxd input register with clk to avoid Quartus tsu violation
--	2006-08-13	use 3 FFs for the rxd input at clk
--
 
 
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
 
entity sc_uart is
 
 
generic (addr_bits : integer;
	clk_freq : integer;
	baud_rate : integer;
	txf_depth : integer; txf_thres : integer;
	rxf_depth : integer; rxf_thres : integer);
port (
	clk		: in std_logic;
	reset	: in std_logic;
 
-- SimpCon interface
 
	address		: in std_logic_vector(addr_bits-1 downto 0);
	wr_data		: in std_logic_vector(31 downto 0);
	rd, wr		: in std_logic;
	rd_data		: out std_logic_vector(31 downto 0);
--	rdy_cnt		: out unsigned(1 downto 0);
 
	txd		: out std_logic;
	rxd		: in std_logic;
	ncts	: in std_logic;
	nrts	: out std_logic
);
end sc_uart;
 
architecture rtl of sc_uart is
 
component fifo is
 
generic (width : integer; depth : integer; thres : integer);
port (
	clk		: in std_logic;
	reset	: in std_logic;
 
	din		: in std_logic_vector(width-1 downto 0);
	dout	: out std_logic_vector(width-1 downto 0);
 
	rd		: in std_logic;
	wr		: in std_logic;
 
	empty	: out std_logic;
	full	: out std_logic;
	half	: out std_logic
);
end component;
 
--
--	signals for uart connection
--
	signal ua_dout			: std_logic_vector(7 downto 0);
	signal ua_wr, tdre		: std_logic;
	signal ua_rd, rdrf		: std_logic;
 
	type uart_tx_state_type		is (s0, s1);
	signal uart_tx_state 		: uart_tx_state_type;
 
	signal tf_dout		: std_logic_vector(7 downto 0); -- fifo out
	signal tf_rd		: std_logic;
	signal tf_empty		: std_logic;
	signal tf_full		: std_logic;
	signal tf_half		: std_logic;
 
	signal ncts_buf		: std_logic_vector(2 downto 0);	-- sync in
 
	signal tsr			: std_logic_vector(9 downto 0); -- tx shift register
 
	signal tx_clk		: std_logic;
 
 
	type uart_rx_state_type		is (s0, s1, s2);
	signal uart_rx_state 		: uart_rx_state_type;
 
	signal rf_wr		: std_logic;
	signal rf_empty		: std_logic;
	signal rf_full		: std_logic;
	signal rf_half		: std_logic;
 
	signal rxd_reg		: std_logic_vector(2 downto 0);
	signal rx_buf		: std_logic_vector(2 downto 0);	-- sync in, filter
	signal rx_d			: std_logic;					-- rx serial data
 
	signal rsr			: std_logic_vector(9 downto 0); -- rx shift register
 
	signal rx_clk		: std_logic;
	signal rx_clk_ena	: std_logic;
 
	constant clk16_cnt	: integer := (clk_freq/baud_rate+8)/16-1;
 
 
begin
 
--	rdy_cnt <= "00";	-- no wait states
	rd_data(31 downto 8) <= std_logic_vector(to_unsigned(0, 24));
--
--	The registered MUX is all we need for a SimpCon read.
--	The read data is stored in registered rd_data.
--
 
-- 
process(clk, reset, rdrf, tdre, ua_dout, rd, address)
begin
 
--	if (reset='1') then
--		rd_data(7 downto 0) <= (others => '0');
--	elsif rising_edge(clk) then
		ua_rd <= '0';
		if rd='1' then
			-- that's our very simple address decoder
			if address(0)='0' then
				rd_data(7 downto 0) <= "000000" & rdrf & tdre;
			else
				rd_data(7 downto 0) <= ua_dout;
				ua_rd <= rd;
			end if;
		end if;
--	end if;
 
end process;
 
	-- write is on address offest 1
	ua_wr <= wr and address(0);
 
--
--	serial clock
--
process(clk, reset)
 
	variable clk16		: integer;-- range 0 to clk16_cnt;
	variable clktx		: unsigned(3 downto 0);
	variable clkrx		: unsigned(3 downto 0);
 
begin
	if (reset='1') then
		clk16 := 0;
		clktx := "0000";
		clkrx := "0000";
		tx_clk <= '0';
		rx_clk <= '0';
		rx_buf <= "111";
 
	elsif rising_edge(clk) then
 
		rxd_reg(0) <= rxd;			-- to avoid setup timing error in Quartus
		rxd_reg(1) <= rxd_reg(0);
		rxd_reg(2) <= rxd_reg(1);
 
		if (clk16=clk16_cnt) then		-- 16 x serial clock
			clk16 := 0;
--
--	tx clock
--
			clktx := clktx + 1;
			if (clktx="0000") then
				tx_clk <= '1';
			else
				tx_clk <= '0';
			end if;
--
--	rx clock
--
			if (rx_clk_ena='1') then
				clkrx := clkrx + 1;
				if (clkrx="1000") then
					rx_clk <= '1';
				else
					rx_clk <= '0';
				end if;
			else
				clkrx := "0000";
			end if;
--
--	sync in filter buffer
--
			rx_buf(0) <= rxd_reg(2);
			rx_buf(2 downto 1) <= rx_buf(1 downto 0);
		else
			clk16 := clk16 + 1;
			tx_clk <= '0';
			rx_clk <= '0';
		end if;
 
 
	end if;
 
end process;
 
--
--	transmit fifo
--
	cmp_tf: fifo generic map (8, txf_depth, txf_thres)
			port map (clk, reset, wr_data(7 downto 0), tf_dout, tf_rd, ua_wr, tf_empty, tf_full, tf_half);
 
--
--	state machine for actual shift out
--
process(clk, reset)
 
	variable i : integer range 0 to 11;
 
begin
 
	if (reset='1') then
		uart_tx_state <= s0;
		tsr <= "1111111111";
		tf_rd <= '0';
		ncts_buf <= "111";
 
	elsif rising_edge(clk) then
 
		ncts_buf(0) <= ncts;
		ncts_buf(2 downto 1) <= ncts_buf(1 downto 0);
 
		case uart_tx_state is
 
			when s0 =>
				i := 0;
				if (tf_empty='0' and ncts_buf(2)='0') then
					uart_tx_state <= s1;
					tsr <= tf_dout & '0' & '1';
					tf_rd <= '1';
				end if;
 
			when s1 =>
				tf_rd <= '0';
				if (tx_clk='1') then
					tsr(9) <= '1';
					tsr(8 downto 0) <= tsr(9 downto 1);
					i := i+1;
					if (i=11) then				-- two stop bits
						uart_tx_state <= s0;
					end if;
				end if;
 
		end case;
	end if;
 
end process;
 
	txd <= tsr(0);
	tdre <= not tf_full;
 
 
--
--	receive fifo
--
	cmp_rf: fifo generic map (8, rxf_depth, rxf_thres)
			port map (clk, reset, rsr(8 downto 1), ua_dout, ua_rd, rf_wr, rf_empty, rf_full, rf_half);
 
	rdrf <= not rf_empty;
	nrts <= rf_half;			-- glitches even on empty fifo!
 
--
--	filter rxd
--
	with rx_buf select
		rx_d <=	'0' when "000",
				'0' when "001",
				'0' when "010",
				'1' when "011",
				'0' when "100",
				'1' when "101",
				'1' when "110",
				'1' when "111",
				'X' when others;
 
--
--	state machine for actual shift in
--
process(clk, reset)
 
	variable i : integer range 0 to 10;
 
begin
 
	if (reset='1') then
		uart_rx_state <= s0;
		rsr <= "0000000000";
		rf_wr <= '0';
		rx_clk_ena <= '0';
 
	elsif rising_edge(clk) then
 
		case uart_rx_state is
 
 
			when s0 =>
				i := 0;
				rf_wr <= '0';
				if (rx_d='0') then
					rx_clk_ena <= '1';
					uart_rx_state <= s1;
				else
					rx_clk_ena <= '0';
				end if;
 
			when s1 =>
				if (rx_clk='1') then
					rsr(9) <= rx_d;
					rsr(8 downto 0) <= rsr(9 downto 1);
					i := i+1;
					if (i=10) then
						uart_rx_state <= s2;
					end if;
				end if;
 
			when s2 =>
				rx_clk_ena <= '0';
				if rsr(0)='0' and rsr(9)='1' then
					if rf_full='0' then				-- if full just drop it
						rf_wr <= '1';
					end if;
				end if;
				uart_rx_state <= s0;
 
		end case;
	end if;
 
end process;
 
end rtl;