Add UART interrupts

Currently an interrupt is triggered any time there is any activity on
the UART_RXD line, but later it will only trigger once there is data
ready to be read.

hw/fpga/super6502.sv

@@ -117,6 +117,8 @@ SevenSeg segs(
     .HEX0(HEX0), .HEX1(HEX1), .HEX2(HEX2), .HEX3(HEX3), .HEX4(HEX4), .HEX5(HEX5)
 );
 
+logic uart_irq;
+
 uart uart(
     .clk_50(clk_50),
     .clk(clk),
@@ -127,6 +129,7 @@ uart uart(
     .addr(cpu_addr[1:0]),
     .RXD(UART_RXD),
     .TXD(UART_TXD),
+    .irq(uart_irq),
     .data_out(uart_data_out)
 );
 
@@ -135,8 +138,13 @@ always_ff @(posedge clk_50) begin
         irq_data_out <= '0;
     else if (irq_cs && ~cpu_rwb)
         irq_data_out <= irq_data_out & cpu_data_in;
-    else if (~button_1)
-        irq_data_out <= {irq_data_out[7:1], ~button_1};
+
+    else begin
+        if (~button_1)
+            irq_data_out[0] <= '1;
+        if (uart_irq)
+            irq_data_out[1] <= '1;
+    end
 
 end
  

hw/fpga/uart.sv

@@ -12,9 +12,13 @@ module uart(
     
     output logic TXD,
     
+    output logic irq,
     output logic [7:0] data_out
 );
 
+//Temporary!
+assign irq = ~RXD;
+
 //Handle reading and writing registers
 
 logic [7:0] tx_buf;

sw/interrupt.h

@@ -4,6 +4,7 @@
 #include <stdint.h>
 
 #define BUTTON  (1 << 0)
+#define UART    (1 << 1)
 
 void irq_int();
 void nmi_int();

sw/irq.c

@@ -17,4 +17,8 @@ void handle_irq() {
         puts("Button Interrupt!\n");
         irq_set_status(status & ~BUTTON);
     }
+    if (status & UART) {
+        puts("UART Interrupt!\n");
+        irq_set_status(status & ~UART);
+    }
 }