module i2c_master_bit_ctrl (
    input             clk,      //system clock
    input             rst,      //synchronous active high reset
    input             nReset,   //asynchronous active low reset
    input             ena,      //core enable signal

    input      [15:0] clk_cnt,  //clock prescale value

    input      [ 3:0] cmd,      //command (from byte controller)
    output reg        cmd_ack,  //command complete acknowledge
    output reg        busy,     //i2c bus busy
    output reg        al,       //i2c bus arbitration lost

    input             din,
    output reg        dout,

    input             scl_i,    //i2c clock line input
    output            scl_o,    //i2c clock line output
    output reg        scl_oen,  //i2c clock line output enable (active low)
    input             sda_i,    //i2c data line input
    output            sda_o,    //i2c data line output
    output reg        sda_oen   //i2c data line output enable (active low)
);


    //
    // variable declarations
    //

    reg [1:0] cSCL,cSDA;      //capture SCL and SDA
    reg [2:0] fSCL,fSDA;      //SCL and SDA filter inputs
    reg       sSCL,sSDA;      //filtered and synchronized SCL and SDA inputs
    reg       dSCL,dSDA;      //delayed versions of sSCL and sSDA
    reg       dscl_oen;       //delayed scl_oen
    reg       sda_chk;        //check SDA output (Multi-master arbitration)
    reg       clk_en;         //clock generation signals(LOW active)
    reg       slave_wait;     //slave inserts wait states
    reg [15:0]cnt;            //clock divider counter (synthesis)
    reg [13:0]filter_cnt;     //clock divider for filter


    // state machine variable
    reg [17:0] c_state;       // synopsys enum_state
 
    //
    // module body
    //

    // whenever the slave is not ready it can delay the cycle by pulling SCL low
    // delay scl_oen
    always @(posedge clk)
    dscl_oen <= #1 scl_oen;

    // slave_wait is asserted when master wants to drive SCL high, but the slave pulls it low
    // slave_wait remains asserted until the slave releases SCL
    always @(posedge clk or negedge nReset)
	 begin
       if(!nReset) 
		   slave_wait <= 1'b0;
       else         
		   slave_wait <= (scl_oen & ~dscl_oen & ~sSCL)|(slave_wait & ~sSCL);
	 end

    // master drives SCL high, but another master pulls it low
    // master start counting down its low cycle now (clock synchronization)
    wire scl_sync = dSCL&~sSCL&scl_oen;        


    // generate clk enable signal
    always @(posedge clk or negedge nReset)
	 begin
       if (!nReset)
       begin
          cnt    <= #1 16'h0;
          clk_en <= #1 1'b1;
       end
       else if (rst || ~(|cnt) || (!ena) || scl_sync)
       begin
          cnt    <= #1 clk_cnt;
          clk_en <= #1 1'b1;
       end
       else if (slave_wait)
       begin
          cnt    <= #1 cnt;
          clk_en <= #1 1'b0;    
       end
       else
       begin
          cnt    <= #1 cnt-16'h1;
          clk_en <= #1 1'b0;
       end
	 end


    // generate bus status controller

    // capture SDA and SCL
    // reduce metastability risk
    always @(posedge clk or negedge nReset)
	 begin
       if (!nReset)
       begin
          cSCL <= #1 2'b00;
          cSDA <= #1 2'b00;
       end
       else if (rst)
       begin
          cSCL <= #1 2'b00;
          cSDA <= #1 2'b00;
       end
       else
       begin
          cSCL <= {cSCL[0],scl_i};
          cSDA <= {cSDA[0],sda_i};
       end
	 end
	 
	 /*usually use this mode take some operation
	 always@(posedge clk or negedge rst_n)
	 begin
	    if(!rst_n)
		   cscl<=2'b00;
			csda<=2'b00;
		 else 
		   cscl<={cscl[max:0],scl_i};
			csda<={csda[max:0],sda_i};//cache the input signal
	 end
	 assign post_cscl_en=cscl[max-1]&~cscl[max];//negedge
	 assign post_csda=csda[max+1];//delay (max+1) clock period
	 */

    // filter SCL and SDA signals; (attempt to) remove glitches
	 
    always @(posedge clk or negedge nReset)
	 begin
       if(!nReset) 
		    filter_cnt <= 14'h0;
       else if (rst || !ena ) 
		    filter_cnt <= 14'h0;
       else if (~|filter_cnt)
		    filter_cnt <= clk_cnt >> 2; //16x I2C bus frequency(4*fSCL)
       else                   
		    filter_cnt <= filter_cnt -1;
	 end


    always @(posedge clk or negedge nReset)//fSCL is the Operation control clock
	 begin
       if(!nReset)
       begin
          fSCL <= 3'b111;
          fSDA <= 3'b111;
       end
       else if (rst)
       begin
          fSCL <= 3'b111;
          fSDA <= 3'b111;
       end
       else if (~|filter_cnt)             
       begin
          fSCL <= {fSCL[1:0],cSCL[1]};
          fSDA <= {fSDA[1:0],cSDA[1]};
       end
    end

    // generate filtered SCL and SDA signals
    always @(posedge clk or negedge nReset)
	 begin
       if (~nReset)
       begin
          sSCL <= #1 1'b1;
          sSDA <= #1 1'b1;
     
          dSCL <= #1 1'b1;
          dSDA <= #1 1'b1;
       end
       else if (rst)
       begin
          sSCL <= #1 1'b1;
          sSDA <= #1 1'b1;

          dSCL <= #1 1'b1;
          dSDA <= #1 1'b1;
       end
       else
       begin
		    sSCL <= #1 &fSCL[2:1] | &fSCL[1:0] | (fSCL[2] & fSCL[0]);
          sSDA <= #1 &fSDA[2:1] | &fSDA[1:0] | (fSDA[2] & fSDA[0]);

          dSCL <= #1 sSCL;
          dSDA <= #1 sSDA;
       end
	 end 

    // detect start condition => detect falling edge on SDA while SCL is high
    // detect stop condition => detect rising edge on SDA while SCL is high
    reg sta_condition;
    reg sto_condition;
    always @(posedge clk or negedge nReset)
	 begin
       if(~nReset)
       begin
          sta_condition <= #1 1'b0;
          sto_condition <= #1 1'b0;
       end
       else if (rst)
       begin
          sta_condition <= #1 1'b0;
          sto_condition <= #1 1'b0;
       end
       else
       begin
          sta_condition <= #1 ~sSDA &  dSDA & sSCL;//falling edge,start signal
          sto_condition <= #1  sSDA & ~dSDA & sSCL;//rising edge,stop signal 
       end
  	 end

    // generate i2c bus busy signal
    always @(posedge clk or negedge nReset)
	 begin
       if(!nReset) 
		   busy <= #1 1'b0;
       else if (rst)
		   busy <= #1 1'b0;
       else              
		   busy <= #1 (sta_condition | busy) & ~sto_condition;
	 end


    // generate arbitration lost signal+
    // aribitration lost when:
    // 1) master drives SDA high, but the i2c bus is low
    // 2) stop detected while not requested
    reg cmd_stop;
    always @(posedge clk or negedge nReset)
	 begin
       if(~nReset)
         cmd_stop <= #1 1'b0;
       else if(rst)
         cmd_stop <= #1 1'b0;
       else if(clk_en)
         cmd_stop <= #1 cmd == `I2C_CMD_STOP;
	 end

    always @(posedge clk or negedge nReset)  //仲裁丢失：当检测到停止信号，没有检测到请求信号，主机设置SDA为高电平，但实际上SDA为低电平
	 begin
       if(~nReset)
          al <= #1 1'b0;
       else if(rst)
          al <= #1 1'b0;
       else
          al <= #1 (sda_chk & ~sSDA & sda_oen)|(|c_state & sto_condition & ~cmd_stop);
	 end


    // generate dout signal (store SDA on rising edge of SCL)
    always @(posedge clk)
    if(sSCL & ~dSCL) 
		dout <= #1 sSDA;


    // generate statemachine

    // nxt_state decoder
    parameter [17:0] idle   = 18'b0_0000_0000_0000_0000;
    parameter [17:0] start_a= 18'b0_0000_0000_0000_0001;
    parameter [17:0] start_b= 18'b0_0000_0000_0000_0010;
    parameter [17:0] start_c= 18'b0_0000_0000_0000_0100;
    parameter [17:0] start_d= 18'b0_0000_0000_0000_1000;
    parameter [17:0] start_e= 18'b0_0000_0000_0001_0000;
    parameter [17:0] stop_a = 18'b0_0000_0000_0010_0000;
    parameter [17:0] stop_b = 18'b0_0000_0000_0100_0000;
    parameter [17:0] stop_c = 18'b0_0000_0000_1000_0000;
    parameter [17:0] stop_d = 18'b0_0000_0001_0000_0000;
    parameter [17:0] rd_a   = 18'b0_0000_0010_0000_0000;
    parameter [17:0] rd_b   = 18'b0_0000_0100_0000_0000;
    parameter [17:0] rd_c   = 18'b0_0000_1000_0000_0000;
    parameter [17:0] rd_d   = 18'b0_0001_0000_0000_0000;
    parameter [17:0] wr_a   = 18'b0_0010_0000_0000_0000;
    parameter [17:0] wr_b   = 18'b0_0100_0000_0000_0000;
    parameter [17:0] wr_c   = 18'b0_1000_0000_0000_0000;
    parameter [17:0] wr_d   = 18'b1_0000_0000_0000_0000;

    always @(posedge clk or negedge nReset)
	 begin
       if(!nReset)
       begin
          c_state <= #1 idle;
          cmd_ack <= #1 1'b0;
          scl_oen <= #1 1'b1;
          sda_oen <= #1 1'b1;
          sda_chk <= #1 1'b0;
       end
       else if (rst | al)
       begin
          c_state <= #1 idle;
          cmd_ack <= #1 1'b0;
          scl_oen <= #1 1'b1;
          sda_oen <= #1 1'b1;
          sda_chk <= #1 1'b0;
       end
       else
       begin
          cmd_ack   <= #1 1'b0; //default no command acknowledge + assert cmd_ack only 1 clk cycle

          if(clk_en)
          case (c_state) //synopsys full_case parallel_case
                    // idle state
                    idle:
                    begin
                        case (cmd) //synopsys full_case parallel_case
                             `I2C_CMD_START: c_state <= #1 start_a;
                             `I2C_CMD_STOP:  c_state <= #1 stop_a;
                             `I2C_CMD_WRITE: c_state <= #1 wr_a;
                             `I2C_CMD_READ:  c_state <= #1 rd_a;
                             default:        c_state <= #1 idle;
                        endcase

                        scl_oen <= #1 scl_oen; // keep SCL in same state
                        sda_oen <= #1 sda_oen; // keep SDA in same state
                        sda_chk <= #1 1'b0;    // don't check SDA output
                     end

                    // start
                    start_a:
                    begin
                        c_state <= #1 start_b;
                        scl_oen <= #1 scl_oen; // keep SCL in same state
                        sda_oen <= #1 1'b1;    // set SDA high
                        sda_chk <= #1 1'b0;    // don't check SDA output
                    end

                    start_b:
                    begin
                        c_state <= #1 start_c;
                        scl_oen <= #1 1'b1; // set SCL high
                        sda_oen <= #1 1'b1; // keep SDA high
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    start_c:
                    begin
                        c_state <= #1 start_d;
                        scl_oen <= #1 1'b1; // keep SCL high
                        sda_oen <= #1 1'b0; // set SDA low
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    start_d:
                    begin
                        c_state <= #1 start_e;
                        scl_oen <= #1 1'b1; // keep SCL high
                        sda_oen <= #1 1'b0; // keep SDA low
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    start_e:
                    begin
                        c_state <= #1 idle;
                        cmd_ack <= #1 1'b1;
                        scl_oen <= #1 1'b0; // set SCL low
                        sda_oen <= #1 1'b0; // keep SDA low
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    // stop
                    stop_a:
                    begin
                        c_state <= #1 stop_b;
                        scl_oen <= #1 1'b0; // keep SCL low
                        sda_oen <= #1 1'b0; // set SDA low
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    stop_b:
                    begin
                        c_state <= #1 stop_c;
                        scl_oen <= #1 1'b1; // set SCL high
                        sda_oen <= #1 1'b0; // keep SDA low
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    stop_c:
                    begin
                        c_state <= #1 stop_d;
                        scl_oen <= #1 1'b1; // keep SCL high
                        sda_oen <= #1 1'b0; // keep SDA low
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    stop_d:
                    begin
                        c_state <= #1 idle;
                        cmd_ack <= #1 1'b1;
                        scl_oen <= #1 1'b1; // keep SCL high
                        sda_oen <= #1 1'b1; // set SDA high
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    // read
                    rd_a:
                    begin
                        c_state <= #1 rd_b;
                        scl_oen <= #1 1'b0; // keep SCL low
                        sda_oen <= #1 1'b1; // tri-state SDA
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    rd_b:
                    begin
                        c_state <= #1 rd_c;
                        scl_oen <= #1 1'b1; // set SCL high
                        sda_oen <= #1 1'b1; // keep SDA tri-stated
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    rd_c:
                    begin
                        c_state <= #1 rd_d;
                        scl_oen <= #1 1'b1; // keep SCL high
                        sda_oen <= #1 1'b1; // keep SDA tri-stated
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    rd_d:
                    begin
                        c_state <= #1 idle;
                        cmd_ack <= #1 1'b1;
                        scl_oen <= #1 1'b0; // set SCL low
                        sda_oen <= #1 1'b1; // keep SDA tri-stated
                        sda_chk <= #1 1'b0; // don't check SDA output
                    end

                    // write
                    wr_a:
                    begin
                        c_state <= #1 wr_b;
                        scl_oen <= #1 1'b0; // keep SCL low
                        sda_oen <= #1 din;  // set SDA
                        sda_chk <= #1 1'b0; // don't check SDA output (SCL low)
                    end

                    wr_b:
                    begin
                        c_state <= #1 wr_c;
                        scl_oen <= #1 1'b1; // set SCL high
                        sda_oen <= #1 din;  // keep SDA
                        sda_chk <= #1 1'b0; // don't check SDA output yet
                                            // allow some time for SDA and SCL to settle
                    end

                    wr_c:
                    begin
                        c_state <= #1 wr_d;
                        scl_oen <= #1 1'b1; // keep SCL high
                        sda_oen <= #1 din;
                        sda_chk <= #1 1'b1; // check SDA output
                    end

                    wr_d:
                    begin
                        c_state <= #1 idle;
                        cmd_ack <= #1 1'b1;
                        scl_oen <= #1 1'b0; // set SCL low
                        sda_oen <= #1 din;
                        sda_chk <= #1 1'b0; // don't check SDA output (SCL low)
                    end

              endcase
      end
	end


    // assign scl and sda output (always gnd)
    assign scl_o = 1'b0;
    assign sda_o = 1'b0;

endmodule