Merge branch 'uart-irq' into 'master'

Add UART Receive logic

See merge request bslathi19/super6502!5

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

@@ -12,6 +12,7 @@ module uart(
     
     output logic TXD,
     
+    output logic irq,
     output logic [7:0] data_out
 );
 
@@ -22,18 +23,24 @@ logic [7:0] rx_buf;
 logic [7:0] status;
 
 logic tx_flag;
+logic rx_flag;
 
 logic tx_flag_set;
 logic tx_flag_clear;
+logic rx_flag_set;
+logic rx_flag_clear;
 
 assign status[0] = tx_flag | tx_flag_clear;
+assign status[1] = rx_flag | rx_flag_set;
+
+assign irq = status[1];
 
 always_ff @(posedge clk) begin
     if (rst) begin
         tx_flag_set <= '0;
+        rx_flag_clear <= '0;
         tx_buf <= '0;
-        rx_buf <= '0;
+        status[7:2] <= '0;
-        status[7:1] <= '0;
     end
 
     if (cs) begin
@@ -52,26 +59,31 @@ always_ff @(posedge clk) begin
         tx_flag_set <= '1;
     else
         tx_flag_set <= '0;
+
+    if (rw & cs && addr == 0)
+        rx_flag_clear <= '1;
+    else
+        rx_flag_clear <= '0;
 end
 
-// state controller
+// tx state controller
 typedef enum bit [2:0] {START, DATA, PARITY, STOP, IDLE} macro_t;
 struct packed {
     macro_t macro;
     logic [3:0] count;
-} state, next_state;
+} tx_state, tx_next_state, rx_state, rx_next_state;
 localparam logic [3:0] maxcount = 4'h7;
 
 // baud rate: 9600
 localparam baud = 9600;
 localparam count = (50000000/baud)-1;
-logic [14:0] clkdiv;
+logic [14:0] tx_clkdiv;
 
 always_ff @(posedge clk_50) begin
     if (rst) begin
-        clkdiv <= 0;
+        tx_clkdiv <= 0;
-        state.macro <= IDLE;
+        tx_state.macro <= IDLE;
-        state.count <= 3'b0;
+        tx_state.count <= 3'b0;
         tx_flag <= '0;
     end else begin
         if (tx_flag_set)
@@ -79,43 +91,43 @@ always_ff @(posedge clk_50) begin
         else if (tx_flag_clear)
             tx_flag <= '0;
 
-        if (clkdiv == count) begin
+        if (tx_clkdiv == count) begin
-            clkdiv <= 0;
+            tx_clkdiv <= 0;
-            state <= next_state;
+            tx_state <= tx_next_state;
         end else begin
-            clkdiv <= clkdiv + 15'b1;
+            tx_clkdiv <= tx_clkdiv + 15'b1;
         end
     end
 end
 
 always_comb begin
-    next_state = state;
+    tx_next_state = tx_state;
 
-    unique case (state.macro)
+    unique case (tx_state.macro)
         START: begin
-            next_state.macro = DATA;
+            tx_next_state.macro = DATA;
-            next_state.count = 3'b0;
+            tx_next_state.count = 3'b0;
         end
         DATA: begin
-            if (state.count == maxcount) begin
+            if (tx_state.count == maxcount) begin
-                next_state.macro = STOP;    // or PARITY
+                tx_next_state.macro = STOP;    // or PARITY
-                next_state.count = 3'b0;
+                tx_next_state.count = 3'b0;
             end else begin
-                next_state.count = state.count + 3'b1;
+                tx_next_state.count = tx_state.count + 3'b1;
-                next_state.macro = DATA;
+                tx_next_state.macro = DATA;
             end
         end
         PARITY: begin
         end
         STOP: begin
-            next_state.macro = IDLE;
+            tx_next_state.macro = IDLE;
-            next_state.count = '0;
+            tx_next_state.count = '0;
         end
         IDLE: begin
             if (tx_flag)
-                next_state.macro = START;
+                tx_next_state.macro = START;
             else
-                next_state.macro = IDLE;
+                tx_next_state.macro = IDLE;
         end
     
         default:;
@@ -126,12 +138,12 @@ always_comb begin
     TXD = '1;
     tx_flag_clear = '0;
 
-    unique case (state.macro)
+    unique case (tx_state.macro)
         START: begin
             TXD = '0;
         end
         DATA: begin
-            TXD = tx_buf[state.count];
+            TXD = tx_buf[tx_state.count];
         end 
         PARITY: begin
 
@@ -148,4 +160,69 @@ always_comb begin
     endcase
 end
 
+//basically in idle state we need to sample RXD very fast,
+//then as soon as we detect that RXD is low, we start clkdiv
+//going and then go into the start state.
+
+logic [14:0] rx_clkdiv;
+
+always_ff @(posedge clk_50) begin
+    if (rst) begin
+        rx_buf <= '0;
+        rx_clkdiv <= 0;
+        rx_state.macro <= IDLE;
+        rx_state.count <= 3'b0;
+    end else begin
+        if (rx_flag_set)
+            rx_flag <= '1;
+        else if (rx_flag_clear)
+            rx_flag <= '0;
+
+        if (rx_state.macro == IDLE) begin           // Sample constantly in idle state
+            rx_state <= rx_next_state;
+            rx_clkdiv <= count/15'h2;                   // offset rx clock by 1/2 phase
+        end else begin              
+            if (rx_clkdiv == count) begin           // other states are as usual
+                rx_clkdiv <= 0;
+                rx_state <= rx_next_state;
+                if (rx_state.macro == DATA)
+                    rx_buf[rx_state.count] = RXD;
+            end else begin
+                rx_clkdiv <= rx_clkdiv + 15'b1;
+            end
+        end
+    end
+end
+
+always_comb begin
+    rx_next_state = rx_state;
+    rx_flag_set = '0;
+
+    unique case (rx_state.macro)
+        IDLE: begin
+            if (~RXD)
+                rx_next_state.macro = START;
+        end
+        START: begin
+            rx_next_state.macro = DATA;
+            rx_next_state.count = 3'b0;
+        end
+        DATA: begin
+            if (rx_state.count == maxcount) begin
+                rx_next_state.macro = STOP;
+                rx_next_state.count = 3'b0;
+            end else begin
+                rx_next_state.count = rx_state.count + 3'b1;
+                rx_next_state.macro = DATA;
+            end
+        end
+        PARITY: begin
+        end
+        STOP: begin
+            rx_flag_set = '1;
+            rx_next_state.macro = IDLE;
+        end
+    endcase
+end
+
 endmodule

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

@@ -2,12 +2,14 @@
 #include <stdint.h>
 
 #include "interrupt.h"
+#include "uart.h"
 
 // This is defined in main.c
 void puts(const char* s);
 
 void handle_irq() {
     uint8_t status;
+    char c;
 
     puts("Interrupt Detected!\n");
 
@@ -17,4 +19,12 @@ void handle_irq() {
         puts("Button Interrupt!\n");
         irq_set_status(status & ~BUTTON);
     }
+    if (status & UART) {
+        puts("UART Interrupt!\n");
+        c = uart_rxb();
+        puts("Received: ");
+        puts(&c);
+        puts("\n");
+        irq_set_status(status & ~UART);
+    }
 }

sw/uart.h

@@ -6,6 +6,8 @@
 void uart_txb(uint8_t val);
 void uart_txb_block(uint8_t val);
 
+uint8_t uart_rxb();
+
 uint8_t uart_status();
 
 #endif

sw/uart.s

@@ -2,8 +2,8 @@
 
 .importzp sp, sreg
 
-.export _uart_txb
+.export _uart_txb, _uart_txb_block
-.export _uart_txb_block
+.export _uart_rxb
 .export _uart_status
 
 .autoimport	on
@@ -23,6 +23,11 @@ _uart_txb_block:
         bne @1
         rts
 
+_uart_rxb:
+        lda UART_RXB        ; Read value
+        ldx #$00
+        rts
+
 _uart_status:
         lda UART_STATUS
         ldx #$00