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// uart transmit module  

//

//---------------------------------------------------------------------------------------

module uart_tx

(

        clock, reset,

        ce_16, tx_data, new_tx_data,

        ser_out, tx_busy

);

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// modules inputs and outputs 

input                   clock;                  // global clock input 

input                   reset;                  // global reset input 

input                   ce_16;                  // baud rate multiplyed by 16 - generated by baud module 

input   [7:0]    tx_data;                // data byte to transmit 

input                   new_tx_data;    // asserted to indicate that there is a new data byte for transmission 

output                  ser_out;                // serial data output 

output                  tx_busy;                // signs that transmitter is busy 

// internal wires 

wire ce_1;              // clock enable at bit rate 

// internal registers 

reg ser_out;

reg tx_busy;

reg [3:0]        count16;

reg [3:0]        bit_count;

reg [8:0]        data_buf;

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// module implementation 

// a counter to count 16 pulses of ce_16 to generate the ce_1 pulse 

always @ (posedge clock or posedge reset)

begin

        if (reset)

                count16 <= 4'b0;

        else if (tx_busy & ce_16)

                count16 <= count16 + 4'b1;

        else if (~tx_busy)

                count16 <= 4'b0;

end

// ce_1 pulse indicating output data bit should be updated 

assign ce_1 = (count16 == 4'b1111) & ce_16;

// tx_busy flag 

always @ (posedge clock or posedge reset)

begin

        if (reset)

                tx_busy <= 1'b0;

        else if (~tx_busy & new_tx_data)

                tx_busy <= 1'b1;

        else if (tx_busy & (bit_count == 4'h9) & ce_1)

                tx_busy <= 1'b0;

end

// output bit counter 

always @ (posedge clock or posedge reset)

begin

        if (reset)

                bit_count <= 4'h0;

        else if (tx_busy & ce_1)

                bit_count <= bit_count + 4'h1;

        else if (~tx_busy)

                bit_count <= 4'h0;

end

// data shift register 

always @ (posedge clock or posedge reset)

begin

        if (reset)

                data_buf <= 9'b0;

        else if (~tx_busy)

                data_buf <= {tx_data, 1'b0};

        else if (tx_busy & ce_1)

                data_buf <= {1'b1, data_buf[8:1]};

end

// output data bit 

always @ (posedge clock or posedge reset)

begin

        if (reset)

                ser_out <= 1'b1;

        else if (tx_busy)

                ser_out <= data_buf[0];

        else

                ser_out <= 1'b1;

end

endmodule

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//                                              Th.. Th.. Th.. Thats all folks !!!

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