From f89ecfa038309666b4a1458212b4007f13eb5133 Mon Sep 17 00:00:00 2001
From: Byron Lathi <bslathi19@gmail.com>
Date: Fri, 8 Apr 2022 12:28:17 -0500
Subject: [PATCH] Add SD Card controller for sending commands

Adds the start of the SD card controller which is capable of sending
commands using the SD protocol.

It is accessed by writing the arguments first and triggered by writing
the command number.
---
 hw/fpga/sd_controller.sv | 124 +++++++++++++++++++++++++++++++++++++++
 hw/fpga/super6502.qsf    |   1 +
 2 files changed, 125 insertions(+)
 create mode 100644 hw/fpga/sd_controller.sv

diff --git a/hw/fpga/sd_controller.sv b/hw/fpga/sd_controller.sv
new file mode 100644
index 0000000..b898803
--- /dev/null
+++ b/hw/fpga/sd_controller.sv
@@ -0,0 +1,124 @@
+module sd_controller(
+    input clk,
+    input rst,
+
+    input [3:0] addr,
+    input [7:0] data,
+    input cs,
+
+    input i_sd_cmd,
+    output logic o_sd_cmd,
+
+    input i_sd_data,
+    output logic o_sd_dat
+);
+
+logic [31:0] arg;
+logic [5:0] cmd;
+
+logic [47:0] rxcmd_buf;
+
+typedef enum bit [1:0] {IDLE, CRC, TXCMD, RXCMD} macro_t;
+struct packed {
+    macro_t macro;
+    logic [5:0] count;
+} state, next_state;
+
+always_ff @(posedge clk) begin
+    if (rst) begin
+        state.macro <= IDLE;
+        state.count <= '0;
+    end else begin
+        state <= next_state;
+    end
+
+    if (state.macro == IDLE) begin
+        if (cs) begin
+            if (addr < 4'h4) begin
+                arg[8 * addr +: 8] <= data;
+            end else if (addr == 4'h4) begin
+                cmd <= data;
+            end
+        end
+    end else if (state.macro == RXCMD) begin
+        rxcmd_buf[6'd46-state.count] <= i_sd_cmd;   //we probabily missed bit 47
+    end
+end
+
+logic [6:0] crc;
+logic load_crc;
+logic crc_valid;
+logic [39:0] _packet;
+assign _packet = {1'b0, 1'b1, cmd, arg};
+logic [47:0] packet_crc;
+assign packet_crc = {_packet, crc, 1'b1};
+
+crc7 u_crc7(
+    .clk(clk),
+    .rst(rst),
+    .load(load_crc),
+    .data_in(_packet),
+    .crc_out(crc),
+    .valid(crc_valid)
+);
+
+always_comb begin
+    next_state = state;
+
+    case (state.macro)
+        IDLE: begin
+            if (~i_sd_cmd) begin        // receive data if sd pulls cmd low
+                next_state.macro = RXCMD;
+            end
+
+            if (addr == 4'h4 & cs) begin     // transmit if cpu writes to cmd
+                next_state.macro = CRC;
+            end
+        end
+
+        CRC: begin
+            next_state.macro = TXCMD;
+        end
+
+        TXCMD: begin
+            if (state.count < 47) begin
+                next_state.count = state.count + 6'b1;
+            end else begin
+                next_state.macro = IDLE;
+                next_state.count = '0;
+            end
+        end
+
+        RXCMD: begin
+            if (state.count < 47) begin
+                next_state.count = state.count + 6'b1;
+            end else begin
+                next_state.macro = IDLE;
+                next_state.count = '0;
+            end
+        end
+    endcase
+end
+
+always_comb begin
+    o_sd_cmd = '1;  //default to 1
+    o_sd_dat = '1;
+
+    load_crc = '0;
+
+    case (state.macro)
+        IDLE:;
+
+        CRC: begin
+            load_crc = '1;
+        end
+
+        TXCMD: begin
+            o_sd_cmd = packet_crc[6'd47 - state.count];
+        end
+
+        RXCMD:;
+    endcase
+end
+
+endmodule
diff --git a/hw/fpga/super6502.qsf b/hw/fpga/super6502.qsf
index 053e5d2..bd10e36 100644
--- a/hw/fpga/super6502.qsf
+++ b/hw/fpga/super6502.qsf
@@ -350,6 +350,7 @@ set_location_assignment PIN_V22 -to DRAM_LDQM
 set_location_assignment PIN_U22 -to DRAM_RAS_N
 set_location_assignment PIN_J21 -to DRAM_UDQM
 set_location_assignment PIN_V20 -to DRAM_WE_N
+set_global_assignment -name SYSTEMVERILOG_FILE sd_controller.sv
 set_global_assignment -name SYSTEMVERILOG_FILE crc7.sv
 set_global_assignment -name SYSTEMVERILOG_FILE memory_mapper.sv
 set_global_assignment -name SYSTEMVERILOG_FILE board_io.sv