RS-232 transmitter module

We are building an "async transmitter":

It works like that:

• The transmitter takes an 8-bits data, and serializes it (starting when the "TxD_start" signal is asserted).
• The "busy" signal is asserted while a transmission occurs. The "TxD_start" signal is ignored during that time.

The RS-232 parameters used are fixed: 8-bits data, 2 stop bits, no-parity.

Serializing the data

We assume that we have a "BaudTick" signal available, asserted 115200 times a second.

We need to generate the start bit, the 8 data bits, and the stop bits.
A state machine seems appropriate.

reg [3:0] state; always @(posedge clk) case(state)   4'b0000: if(TxD_start) state <= 4'b0100;   4'b0100: if(BaudTick) state <= 4'b1000; // start   4'b1000: if(BaudTick) state <= 4'b1001; // bit 0   4'b1001: if(BaudTick) state <= 4'b1010; // bit 1   4'b1010: if(BaudTick) state <= 4'b1011; // bit 2   4'b1011: if(BaudTick) state <= 4'b1100; // bit 3   4'b1100: if(BaudTick) state <= 4'b1101; // bit 4   4'b1101: if(BaudTick) state <= 4'b1110; // bit 5   4'b1110: if(BaudTick) state <= 4'b1111; // bit 6   4'b1111: if(BaudTick) state <= 4'b0001; // bit 7   4'b0001: if(BaudTick) state <= 4'b0010; // stop1   4'b0010: if(BaudTick) state <= 4'b0000; // stop2   default: if(BaudTick) state <= 4'b0000; endcase

Notice how the state machine starts right when "TxD_start" is asserted, but then only advances when "BaudTick" is asserted.

Now, we just need to generate the "TxD" output.

reg muxbit; always @(state[2:0]) case(state[2:0])   0: muxbit <= TxD_data[0];   1: muxbit <= TxD_data[1];   2: muxbit <= TxD_data[2];   3: muxbit <= TxD_data[3];   4: muxbit <= TxD_data[4];   5: muxbit <= TxD_data[5];   6: muxbit <= TxD_data[6];   7: muxbit <= TxD_data[7]; endcase // combine start, data, and stop bits together assign TxD = (state<4) | (state[3] & muxbit);

That's it! The complete code can be found here.

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This page was last updated on February 15 2008.