Implement a tiny part of i2c

[clump.git] / i2c.v

module i2c_write (
				  input 	   clk,
				  output 	   scl,
				  output 	   sda,
				  input [7:0]  tx_byte,
				  input 	   transmit,
				  input [5:0]  more_bytes, /* 0 or 1 or 16 are reasonable */
				  input [7:0]  mb_in,
				  output [5:0] mb_addr,
				  output 	   is_transmitting);

parameter CLOCK_DIVIDE = 1;

// States for the transmitting state machine.
// We transmit a START, then the address (constant 0xE1), then
// [tx_byte], then [more_bytes] bytes from [mb_in]
parameter TX_IDLE = 0;
parameter TX_ADDRESS = 1;
parameter TX_FIRST_BYTE = 2;
parameter TX_MORE_BYTES = 3;
parameter TX_STOP = 4;

reg [10:0] tx_clk_divider = CLOCK_DIVIDE;

reg data_out = 1'b1;
reg clk_out = 1'b1;

reg [3:0] tx_state = TX_IDLE;
reg [5:0] tx_countdown;
reg [3:0] tx_bits_remaining;
reg [7:0] tx_data;

reg [3:0] step = 0;

wire [7:0]  address = {7'h70, 1'b0}; // address 0x70, write
wire [15:0] address_data = {address, tx_data};

assign sda = data_out;
assign scl = clk_out;
assign is_transmitting = tx_state != TX_IDLE;

always @(posedge clk) begin
	// The clk_divider counter counts down from
	// the CLOCK_DIVIDE constant. Whenever it
	// reaches 0, 1/16 of the bit period has elapsed.
   // Countdown timers for the receiving and transmitting
	// state machines are decremented.
	tx_clk_divider = tx_clk_divider - 1;
	if (!tx_clk_divider) begin
		tx_clk_divider = CLOCK_DIVIDE;
		tx_countdown = tx_countdown - 1;
	end

   // Transmit state machine
   case (tx_state)
	 TX_IDLE: begin
		if (transmit) begin
		   // If the transmit flag is raised in the idle
		   // state, save tx_byte for transmission
		   tx_data = tx_byte;
		   // Send the initial, low pulse of 1 bit period
		   // to signal the start, followed by the data
		   data_out = 0;
		   tx_state = TX_ADDRESS;
		   tx_bits_remaining = 8;
		   step = 0;
		end
	 end // case: TX_IDLE

	 TX_ADDRESS: begin
		if(step == 0) begin
		   clk_out <= 0;
		   step <= 1;
		end else if (tx_bits_remaining == 0) begin
		   if(step == 1) begin
			  data_out <= 0; // really should be z, not 0
			  step <= 2;
		   end else if(step == 2)begin
			  clk_out <= 1;
			  step <= 3;
		   end else begin
			  step <= 0;

			  tx_state <= TX_FIRST_BYTE;
			  tx_bits_remaining <= 8;
		   end
		end else if(step == 1) begin
		   data_out <= address[tx_bits_remaining - 1];
		   step <= 2;
		end else if(step == 2) begin
		   clk_out <= 1;
		   step <= 3;
		end else begin // step == 3
		   tx_bits_remaining = tx_bits_remaining - 1;
		   step <= 0;
		end
	 end // case: TX_ADDRESS

	 TX_FIRST_BYTE: begin
		if(step == 0) begin
		   clk_out <= 0;
		   step <= 1;
		end else if (tx_bits_remaining == 0) begin
		   if(step == 1) begin
			  data_out <= 0; // really should be z, not 0
			  step <= 2;
		   end else if(step == 2)begin
			  clk_out <= 1;
			  step <= 3;
		   end else begin
			  step <= 0;

			  tx_state <= TX_STOP;
		   end
		end else if(step == 1) begin
		   data_out <= tx_data[tx_bits_remaining - 1];
		   step <= 2;
		end else if(step == 2) begin
		   clk_out <= 1;
		   step <= 3;
		end else begin // step == 3
		   tx_bits_remaining = tx_bits_remaining - 1;
		   step <= 0;
		end
	 end // case: TX_FIRST_BYTE

	 TX_STOP: begin
		if(step == 0) begin
		   clk_out <= 0;
		   step <= 1;
		end else if(step == 1) begin
		   step <= 2;
		end else if(step == 2) begin
		   clk_out <= 1;
		   step <= 3;
		end else begin
		   data_out <= 1;
		   step <= 0;
		   tx_state <= TX_IDLE;
		end
	 end
   endcase
end
endmodule // i2c_write