Add read flag to sd controller

Read flag is set when the sd controller reads response data in from the
sd card. When the cpu reads from the controller, the flag is reset.

This flag does not trigger an interrupt, it mmust be polled.

hw/fpga/sd_controller.sv

@@ -12,13 +12,30 @@ module sd_controller(
     output logic o_sd_cmd,
 
     input i_sd_data,
-    output logic o_sd_data
+    output logic o_sd_data,
+
+    output logic [7:0] data_out
 );
 
 logic [31:0] arg;
 logic [5:0] cmd;
 
 logic [47:0] rxcmd_buf;
+logic [31:0] rx_val;
+
+assign rx_val = rxcmd_buf[39:8];
+
+always_comb begin
+    data_out = 'x;
+
+    if (addr < 4'h4) begin
+        data_out = rx_val[8 * addr +: 8];
+    end else if (addr == 4'h4) begin
+        data_out = read_flag;
+    end
+end
+
+logic read_flag, next_read_flag;
 
 typedef enum bit [2:0] {IDLE, LOAD, CRC, TXCMD, RXCMD} macro_t;
 struct packed {
@@ -30,16 +47,25 @@ always_ff @(posedge clk) begin
     if (rst) begin
         state.macro <= IDLE;
         state.count <= '0;
+        read_flag <= '0;
     end else begin
         if (state.macro == TXCMD || state.macro == CRC) begin
             if (sd_clk) begin
                 state <= next_state;
             end
+        end else if (state.macro == RXCMD) begin
+            if (~sd_clk) begin
+                state <= next_state;
+            end
         end else begin
             state <= next_state;
         end
     end
 
+    if (sd_clk) begin
+        read_flag <= next_read_flag;
+    end
+
     if (cs & ~rw) begin
         if (addr < 4'h4) begin
             arg[8 * addr +: 8] <= data;
@@ -71,6 +97,7 @@ crc7 u_crc7(
 
 always_comb begin
     next_state = state;
+    next_read_flag = read_flag;
 
     case (state.macro)
         IDLE: begin
@@ -81,6 +108,10 @@ always_comb begin
             if (addr == 4'h4 & cs & ~rw) begin     // transmit if cpu writes to cmd
                 next_state.macro = LOAD;
             end
+
+            if (addr == 4'h4 & cs & rw) begin
+                next_read_flag = '0;
+            end
         end
 
         LOAD: begin
@@ -104,6 +135,7 @@ always_comb begin
             if (state.count < 47) begin
                 next_state.count = state.count + 6'b1;
             end else begin
+                next_read_flag = '1;
                 next_state.macro = IDLE;
                 next_state.count = '0;
             end