--  SPI Read and Read modify Write Access example 
--     Revision 1.0  25th Oct '21 Author  Miky
--
--     This code receives  reset, clk, rw, address(7-0),data(7-0), mask(7-0), en
--     outputs             ssn, mosi, scl 
--     gets data from      miso
--     and returns data    rreg(7-0)
--       The address (7-0) and en must be change their state before 
--       the rising edge of clk, and hold until the busy status returns to '0'  
--       While busy is high, toggling en is ignored.
--       rw = '1' : Read/ rw = '0' : Read Modify Write


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

entity spi_rmw is 
	
	port(
	
	
		clk 				:in std_logic;
		reset 			:in std_logic;
		rw 	 			:in std_logic;
		en				   :in std_logic;
		address,data,mask		:in unsigned(7 downto 0);
		busy				:inout std_logic;
		rreg				:inout unsigned(7 downto 0);		
		
		ssn, sck,mosi	:out std_logic;
		miso				:in std_logic		
			
		
	);
end spi_rmw;


architecture archspi of spi_rmw is


signal		en1, en2			:std_logic;
signal		count				:unsigned(7 downto 0);
signal		sclen				:std_logic;
signal		reg1				:unsigned(16 downto 0);
signal		wreg				:unsigned(15 downto 0);



begin

	spi01:process(clk, reset, rw, en, address, miso)
	begin
	
		if reset = '0' then
			count <= "00000000";
			en1 <= '0';
			en2 <= '0';
			busy <= '0';
			ssn <= '1';
			sck <= '0';
			sclen <= '0';
		elsif (clk'event and clk = '1') then
			en1 <= en;
			en2 <= en1;

			if (en1 = '1' and en2 ='0' and busy = '0') then
				busy <= '1';
				ssn<='0';
				sclen <= '1';
				mosi <= '1';
				wreg(15 downto 8) <= address(7 downto 0);
				wreg(7 downto 0) <= X"FF";
			end if;

			if(busy = '1') then
				count <= count + 1;
			else
				count <= "00000000";
			end if;

			if (count = 33) then
				sclen  <= '0';
			end if;

			if (count = 34) then
				ssn <= '1';
			end if;

			if (count = 63) then
				ssn <= '0';
				sclen <= '1';
				mosi <= '1';
			end if;

			if (count = 97) then
				sclen <= '0';
			end if;

			if (count = 98) then
				ssn <= '1';
			end if;

			if (count = 127) then
				ssn <= '0';
				sclen <= '1';
				mosi <= '0';
			end if;

			if (count = 161) then
				sclen <= '0';
			end if;

			if (count = 162) then
				ssn <= '1';
			end if;

			if (count = 125 and rw = '1') then
				busy <= '0';
			end if;
			if (count = 191 and rw = '0') then
				busy <= '0';
			end if;

			if(count(0) = '0' and sclen = '1') then
				sck <= '1';
			else
				sck <= '0';
			end if;

			if ( count(0)='1' and sclen = '1') then
				wreg(0) <= '1';
				for i in 0 to 14 loop
					wreg(i + 1) <=wreg(i);
				end loop;
				mosi <= wreg(15);
			end if;

			if ( count(0)='0' and sclen = '1') then
				reg1(0) <=miso;
				for i in 0 to 15 loop
					reg1(i + 1) <=reg1(i);
				end loop;
			end if;
			
			if(count = 98) then
				rreg(7 downto 0) <= reg1(7 downto 0);
			end if;

			if(count = 100) then
				wreg(15 downto 8) <= address(7 downto 0);
				for i in 0 to 7 loop
					if( mask(i) = '0' ) then
						wreg(i) <= rreg(i);
					else
						wreg(i) <= data(i);
					end if;
				end loop;
			end if;
			
		end if;	
	end process;
end archspi;