Refactor uart.sv to better allow cpu control

Added new signal tx_flag, which indicates whether the transmitter is
ready for new data.

Added status register, which when read will return the tx_flag bit, as
well as others that can be implemented.

Added new state IDLE, which resets the TX flag and allows new data to be
written.

Added code to allow for different baud rates, though it is still fixed
currently.

hw/fpga/uart.sv

@@ -17,42 +17,71 @@ module uart(
 
 //Handle reading and writing registers
 
-logic [7:0] _data [3:0];
+logic [7:0] tx_buf;
+logic [7:0] rx_buf;
+logic [7:0] status;
 
-assign data_out = _data[addr];
+logic tx_flag;
+
+logic tx_flag_set;
+logic tx_flag_clear;
+
+assign status[0] = tx_flag | tx_flag_clear;
 
 always_ff @(posedge clk) begin
-    if (rst)
-        _data = '{default:'0};
-    if (~rw & cs)
-        _data[addr] <= data_in;
+    if (rst) begin
+        tx_flag_set <= '0;
+        tx_buf <= '0;
+        rx_buf <= '0;
+        status[7:1] <= '0;
+    end
+
+    if (cs) begin
+        if (~rw) begin
+            if (addr == 0)
+                tx_buf <= data_in;
+        end else begin
+            if (addr == 0)
+                data_out <= rx_buf;
+            if (addr == 1)
+                data_out <= status;
+        end
+    end
+
+    if (~rw & cs && addr == 0)
+        tx_flag_set <= '1;
+    else
+        tx_flag_set <= '0;
 end
 
 // state controller
-typedef enum bit [1:0] {START, DATA, PARITY, STOP} macro_t;
+typedef enum bit [2:0] {START, DATA, PARITY, STOP, IDLE} macro_t;
 struct packed {
     macro_t macro;
     logic [3:0] count;
 } state, next_state;
 localparam logic [3:0] maxcount = 4'h7;
 
-logic [7:0] testval, next_testval;
-
-
 // baud rate: 9600
+localparam baud = 9600;
+localparam count = (50000000/baud)-1;
 logic [14:0] clkdiv;
 
 always_ff @(posedge clk_50) begin
     if (rst) begin
         clkdiv <= 0;
-        state.macro <= STOP;
+        state.macro <= IDLE;
         state.count <= 3'b0;
-        testval <= '0;
+        tx_flag <= '0;
     end else begin
-        if (clkdiv == 5207) begin
+        if (tx_flag_set)
+            tx_flag <= '1;
+        else if (tx_flag_clear)
+            tx_flag <= '0;
+
+        if (clkdiv == count) begin
             clkdiv <= 0;
             state <= next_state;
-            testval <= next_testval;
         end else begin
             clkdiv <= clkdiv + 15'b1;
         end
@@ -79,30 +108,43 @@ always_comb begin
         PARITY: begin
         end
         STOP: begin
-            next_state.macro = START;
+            next_state.macro = IDLE;
             next_state.count = '0;
         end
+        IDLE: begin
+            if (tx_flag)
+                next_state.macro = START;
+            else
+                next_state.macro = IDLE;
+        end
+    
+        default:;
     endcase
 end
 
 always_comb begin
     TXD = '1;
-    next_testval = testval;
+    tx_flag_clear = '0;
 
     unique case (state.macro)
         START: begin
             TXD = '0;
         end
         DATA: begin
-            TXD = testval[state.count];
+            TXD = tx_buf[state.count];
         end 
         PARITY: begin
 
         end
         STOP: begin
-        next_testval = testval + 8'b1;
+            tx_flag_clear = '1;
             TXD = '1;
         end
+        IDLE: begin
+            TXD = '1;
+        end
+
+        default:;
     endcase
 end