Fix UART

1 files changed, 46 insertions, 0 deletions

diff --git a/atk16_fpga/uart_tx.v b/atk16_fpga/uart_tx.v
new file mode 100644
index 0000000..a91c726
--- /dev/null
+++ b/atk16_fpga/uart_tx.v
@@ -0,0 +1,46 @@
+// UART transmitter module.
+// Transmits a frame: start bit (0), 8 data bits (LSB first), stop bit (1)
+// at a baud rate determined by CLKS_PER_BIT (here: 10417 for 100MHz/9600)
+module uart_tx (
+    input            clk,         // system clock: 100 MHz
+    input            start,       // one-cycle pulse to start transmission
+    input      [7:0] data,        // data byte to send
+    output reg       tx = 1'b1,   // serial TX line
+    output reg       busy = 1'b0  // high while transmitting the byte
+);
+
+    // CLK / baudrate cycles per bit.
+    //parameter CLKS_PER_BIT = 10417; // 9600 baud rate
+    parameter CLKS_PER_BIT = 868;  // 115200 baud rate
+
+    reg [13:0] clk_count = 14'b0;  // counter for baud tick (14 bits is enough)
+    reg [ 3:0] bit_index = 4'b0;  // counts from 0 to 9 (10 bits total: start, 8 data, stop)
+    reg [ 9:0] tx_frame;  // complete frame: {stop bit, data[7:0], start bit}
+
+    always @(posedge clk) begin
+        if (!busy) begin
+            if (start) begin
+                // Load frame: start bit (0), 8 data bits (LSB first), stop bit (1)
+                tx_frame  <= {1'b1, data, 1'b0};
+                busy      <= 1'b1;
+                clk_count <= 0;
+                bit_index <= 0;
+                tx        <= 1'b0;  // send start bit immediately
+            end else begin
+                tx <= 1'b1;  // remain idle
+            end
+        end else begin
+            // When busy, count clocks for each bit period.
+            if (clk_count < CLKS_PER_BIT - 1) clk_count <= clk_count + 1;
+            else begin
+                clk_count <= 0;
+                bit_index <= bit_index + 1;
+                if (bit_index < 9) tx <= tx_frame[bit_index+1];
+                else begin
+                    busy <= 1'b0;
+                    tx   <= 1'b1;  // return to idle
+                end
+            end
+        end
+    end
+endmodule