module mkm_refdes
(
    output [3:0] leds,  // {blue, red, yellow, green}
                        
    input   mkm_cs_n, mkm_sclk, mkm_di,
    output  mkm_do,
    
    input   btn_panic
);

    //
    // Internal High-Speed Oscillator
    //
    wire clk_osc_lf;    // 48 MHz
    SB_LFOSC SB_LFOSC_inst
    (
        .CLKLFPU    (1'b1),
        .CLKLFEN    (1'b1),
        .CLKLF      (clk_osc_lf)
    ) /* synthesis ROUTE_THROUGH_FABRIC = 0 */;
    
    
    //
    // Blinking Green LED
    //
    reg [13:0] led_cnt   = 14'd0;
    reg        led_green = 1'b0;
    
    always @(posedge clk_osc_lf) begin
        led_cnt   <= led_cnt + 1'b1;
        led_green <= &led_cnt[13:12];
    end
    
    
    //
    // "Panic" Red LED
    //
    reg led_red = 1'b0;
        
    always @(posedge clk_osc_lf)
        led_red <= btn_panic;
        
    
    //
    // Blue LED to indicate MKM access over SPI
    //
    reg led_blue = 1'b0;
    
    reg mkm_cs_n_sync1 = 1'b1;
    reg mkm_cs_n_sync2 = 1'b1;
    
    always @(posedge clk_osc_lf)
        {mkm_cs_n_sync1, mkm_cs_n_sync2} <=
        {mkm_cs_n,       mkm_cs_n_sync1};
        
    reg [12:0] spi_cnt = 13'd0;
    
    always @(posedge clk_osc_lf)
        //
        if (spi_cnt > 13'd0)
            spi_cnt <= spi_cnt - 13'd1;
        else if (!mkm_cs_n_sync2)
            spi_cnt <= {13{1'b1}};

    always @(posedge clk_osc_lf)
        //
        led_blue <= spi_cnt[10];
    
    
    //
    // serial memory follows
    //
    

    //
    // operation mode
    //
    reg [1:0] opmode = 2'b00;
    
    
    //
    // bit counter
    //
    localparam integer BIT_CNT_W = 16;
    localparam [BIT_CNT_W-1:0] bit_cnt_zero = {BIT_CNT_W{1'b0}};
    reg        [BIT_CNT_W-1:0] bit_cnt      = bit_cnt_zero;
    wire       [BIT_CNT_W-1:0] bit_cnt_next = bit_cnt + 16'd1;
    
    always @(posedge mkm_sclk or posedge mkm_cs_n)
        //
        if (mkm_cs_n) bit_cnt <= bit_cnt_zero;
        else bit_cnt <= bit_cnt_next;
    
    
    //
    // input shifter
    //
    reg [15:0] di_shreg;
    always @(posedge mkm_sclk)
        //
        if (!mkm_cs_n) di_shreg <= {di_shreg[14:0], mkm_di};


    //
    // instruction decoder
    //
    reg [7:0] instr_latch;
    
    wire instr_is_read  = instr_latch == 8'b0000_0011;
    wire instr_is_write = instr_latch == 8'b0000_0010;
    wire instr_is_rdsr  = instr_latch == 8'b0000_0101;
    wire instr_is_wrsr  = instr_latch == 8'b0000_0001;
    
    always @(posedge mkm_sclk)
        //
        if (bit_cnt == 7) instr_latch <= {di_shreg[6:0], mkm_di};

    //
    // address latch
    //
    reg [15:0] addr_latch;
    
    always @(posedge mkm_sclk) begin
        //
        if (bit_cnt == 23) addr_latch <= {di_shreg[14:0], mkm_di};
        //
        if ((bit_cnt >= 31) && (bit_cnt[2:0] == 3'b111) & instr_is_write) begin
            //
            if (opmode == 2'b10) // page mode
                addr_latch[4:0] <= addr_latch[4:0] + 1'b1;
            else if (opmode == 2'b01) // seq mode
                addr_latch[7:0] <= addr_latch[7:0] + 1'b1;
            //
        end
        //
        if ((bit_cnt >= 24) && (bit_cnt[2:0] == 3'b000) && instr_is_read) begin
            //
            if (opmode == 2'b10) // page
                addr_latch[4:0] <= addr_latch[4:0] + 1'b1;
            else if (opmode == 2'b01) // seq
                addr_latch[7:0] <= addr_latch[7:0] + 1'b1;
        end
        //
    end
    

    //
    // mode change
    //
    always @(posedge mkm_sclk)
        //
        if ((bit_cnt == 15) && instr_is_wrsr && (di_shreg[4:0] == {5{1'b0}}))
            //
            opmode <= di_shreg[6:5];

            
    //
    // memory
    //
    localparam MEM_ADDR_W = 6;  // 64 bytes
    
    reg [7:0] sram[0:2**MEM_ADDR_W-1] /* synthesis syn_ramstyle="registers" */; 


    //
    // write logic
    //
    always @(posedge mkm_sclk)
        //
        if ((bit_cnt >= 31) && (bit_cnt[2:0] == 3'b111) & instr_is_write)
            //
            sram[addr_latch[MEM_ADDR_W-1:0]] <= {di_shreg[6:0], mkm_di};


    //
    // serial output
    //
    reg [7:0] do_shreg;
    assign mkm_do = do_shreg[7];
    
    always @(negedge mkm_sclk)
        //
        if (bit_cnt[2:0] == 3'b000) begin
            //
            if ((bit_cnt >= 24) && instr_is_read)
                //
                do_shreg <= sram[addr_latch[MEM_ADDR_W:0]];
                //
            else if ((bit_cnt > 7) && instr_is_rdsr)
                do_shreg <= {opmode, 5'b00000, 1'b1};
            //
        end else
            do_shreg <= {do_shreg[6:0], 1'bX};


    //
    // Map LEDs
    //
    assign leds[0] = led_green; //
    assign leds[1] = 1'b0;      // unused (yellow)
    assign leds[2] = led_red;   //
    assign leds[3] = led_blue;  //


endmodule