--###############################
--# Project Name : I2C slave
--# File         : i2cslave.vhd
--# Project      : i2c slave for FPGA
--# Engineer     : Philippe THIRION
--# Modification History
--###############################


--	copyright Philippe Thirion
--	github.com/tirfil
--
--    Copyright 2016 Philippe THIRION
--
--    This program is free software: you can redistribute it and/or modify
--    it under the terms of the GNU General Public License as published by
--    the Free Software Foundation, either version 3 of the License, or
--    (at your option) any later version.

--    This program is distributed in the hope that it will be useful,
--    but WITHOUT ANY WARRANTY; without even the implied warranty of
--    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
--    GNU General Public License for more details.

--    You should have received a copy of the GNU General Public License
--    along with this program.  If not, see <http://www.gnu.org/licenses/>.


library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity I2CSLAVE is
	generic(
		DEVICE 		: std_logic_vector(7 downto 0) := x"38"
	);
	port(
		MCLK		: in	std_logic;
		nRST		: in	std_logic;
		SDA_IN		: in	std_logic;
		SCL_IN		: in	std_logic;
		SDA_OUT		: out	std_logic;
		SCL_OUT		: out	std_logic;
		ADDRESS		: out	std_logic_vector(7 downto 0);
		DATA_OUT	: out	std_logic_vector(7 downto 0);
		DATA_IN		: in	std_logic_vector(7 downto 0);
		WR			: out	std_logic;
		RD			: out	std_logic
	);
end I2CSLAVE;

architecture rtl of I2CSLAVE is

	type tstate is ( S_IDLE, S_START, S_SHIFTIN, S_RW, S_SENDACK, S_SENDACK2, S_SENDNACK,
		S_ADDRESS, S_WRITE, S_SHIFTOUT, S_READ, S_WAITACK
	);

	type toperation is (OP_READ, OP_WRITE);
	
	signal state : tstate;
	signal next_state : tstate;
	signal operation : toperation;

	signal rising_scl, falling_scl : std_logic;
	signal address_i : std_logic_vector(7 downto 0);
	signal next_address : std_logic_vector(7 downto 0);
	signal counter : integer range 0 to 7;
	signal start_cond : std_logic;
	signal stop_cond  : std_logic;
	signal sda_q, sda_qq, sda_qqq : std_logic;
	signal scl_q, scl_qq, scl_qqq : std_logic;
	signal shiftreg : std_logic_vector(7 downto 0);
	signal sda: std_logic;
	signal address_incr : std_logic;
	signal rd_d : std_logic;
begin

	ADDRESS <= address_i;
	
	next_address <= (others=>'0') when (address_i = x"FF") else
		std_logic_vector(to_unsigned(to_integer(unsigned( address_i )) + 1, 8));
	
	S_RSY: process(MCLK,nRST)
	begin
		if (nRST = '0') then
			sda_q <= '1';
			sda_qq <= '1';
			sda_qqq <= '1';
			scl_q <= '1';
			scl_qq <= '1';
			scl_qqq <= '1';
		elsif (MCLK'event and MCLK='1') then
			sda_q <= SDA_IN;
			sda_qq <= sda_q;
			sda_qqq <= sda_qq;
			scl_q <= SCL_IN;
			scl_qq <= scl_q;
			scl_qqq <= scl_qq;
		end if;
	end process S_RSY;

	rising_scl <= scl_qq and not scl_qqq;
	falling_scl <= not scl_qq and scl_qqq;
		
	START_BIT: process(MCLK,nRST)
	begin
		if (nRST = '0') then
			start_cond <= '0';
		elsif (MCLK'event and MCLK='1') then
			if (sda_qqq = '1' and sda_qq = '0' and scl_qq = '1') then
				start_cond <= '1';
			else	
				start_cond <= '0';
			end if;
		end if;
	end process START_BIT;
	
	STOP_BIT: process(MCLK,nRST)
	begin
		if (nRST = '0') then
			stop_cond <= '0';
		elsif (MCLK'event and MCLK='1') then
			if (sda_qqq = '0' and sda_qq = '1' and scl_qq = '1') then
				stop_cond <= '1';
			else	
				stop_cond <= '0';
			end if;
		end if;
	end process STOP_BIT;
	
	sda <= sda_qq;
	
	RD_DELAY: process(MCLK, nRST)
	begin
		if (nRST = '0') then
			RD <= '0';
		elsif (MCLK'event and MCLK='1') then
			RD <= rd_d;
		end if;
	end process RD_DELAY;

	OTO: process(MCLK, nRST)
	begin
		if (nRST = '0') then
			state <= S_IDLE;
			SDA_OUT <= '1';
			SCL_OUT <= '1';
			WR <= '0';
			rd_d <= '0';
			address_i <= (others=>'0');
			DATA_OUT <= (others=>'0');
			shiftreg <= (others=>'0');
		elsif (MCLK'event and MCLK='1') then
			if (stop_cond = '1') then
				state <= S_IDLE;
				SDA_OUT <= '1';
				SCL_OUT <= '1';
				operation <= OP_READ;
				WR <= '0';
				rd_d <= '0';
				address_incr <= '0';
			elsif(start_cond = '1') then
				state <= S_START;
				SDA_OUT <= '1';
				SCL_OUT <= '1';
				operation <= OP_READ;
				WR <= '0';
				rd_d <= '0';
				address_incr <= '0';
			elsif(state = S_IDLE) then
				state <= S_IDLE;
				SDA_OUT <= '1';
				SCL_OUT <= '1';
				operation <= OP_READ;
				WR <= '0';
				rd_d <= '0';
				address_incr <= '0';
			elsif(state = S_START) then
				shiftreg <= (others=>'0');
				state <= S_SHIFTIN;
				next_state <= S_RW;
				counter <= 6;
			elsif(state = S_SHIFTIN) then
				if (rising_scl = '1') then
					shiftreg(7 downto 1) <= shiftreg(6 downto 0);
					shiftreg(0) <= sda;
					if (counter = 0) then
						state <= next_state;
						counter <= 7;
					else
						counter <= counter - 1;
					end if;
				end if;
			elsif(state = S_RW) then
				if (rising_scl = '1') then
					if (shiftreg = DEVICE) then
						state <= S_SENDACK;
						if (sda = '1') then
							operation <= OP_READ;
							-- next_state <= S_READ; -- no needed
							rd_d <= '1';
						else
							operation <= OP_WRITE;
							next_state <= S_ADDRESS;
							address_incr <= '0';
						end if;
					else
						state <= S_SENDNACK;
					end if;
				end if;
			elsif(state = S_SENDACK) then
				WR <= '0';
				rd_d <= '0';
				if (falling_scl = '1') then
					SDA_OUT <= '0';
					counter <= 7;
					if (operation= OP_WRITE) then
						state <= S_SENDACK2;
					else -- OP_READ
						state <= S_SHIFTOUT;
						shiftreg <= DATA_IN;
					end if;
				end if;
			elsif(state = S_SENDACK2) then
				if (falling_scl = '1') then
					SDA_OUT <= '1';
					state <= S_SHIFTIN;
					shiftreg <= (others=>'0');
					if (address_incr = '1') then
						address_i <= next_address;
					end if;
				end if;
			elsif(state = S_SENDNACK) then
				if (falling_scl = '1') then
					SDA_OUT <= '1';
					state <= S_IDLE;
				end if;
			elsif(state = S_ADDRESS) then
				address_i <= shiftreg;
				next_state <= S_WRITE;
				state <= S_SENDACK;
				address_incr <= '0';
			elsif(state = S_WRITE) then
				DATA_OUT <= shiftreg;
				next_state <= S_WRITE;
				state <= S_SENDACK;
				WR <= '1';
				address_incr <= '1';
			elsif(state = S_SHIFTOUT) then
				if (falling_scl = '1') then
					SDA_OUT <= shiftreg(7);
					shiftreg(7 downto 1) <= shiftreg(6 downto 0);
					shiftreg(0) <= '1';
					if (counter = 0) then
						state <= S_READ;
						address_i <= next_address;
						rd_d <= '1';
					else
						counter <= counter - 1;
					end if;
				end if;
			elsif(state = S_READ) then
				rd_d <= '0';
				if (falling_scl = '1') then
					SDA_OUT <= '1';
					state <= S_WAITACK;
				end if;
			elsif(state = S_WAITACK) then
				if (rising_scl = '1') then
					if (sda = '0') then
						state <= S_SHIFTOUT;
						counter <= 7;
						shiftreg <= DATA_IN;
					else
						state <= S_IDLE;
					end if;
				end if;
			end if;
		end if;
	end process OTO;
					

end rtl;