diff --git a/fpga/zsbl/sd.c b/fpga/zsbl/sd.c
new file mode 100644
index 000000000..4ffb53d67
--- /dev/null
+++ b/fpga/zsbl/sd.c
@@ -0,0 +1,31 @@
+#include "sd.h"
+#include "spi.h"
+
+uint8_t crc7(uint8_t prev, uint8_t in) {
+    // CRC polynomial 0x89
+    uint8_t remainder = prev & in;
+    remainder ^= (remainder >> 4) ^ (remainder >> 7);
+    remainder ^= remainder << 4;
+    return remainder & 0x7f;
+}
+
+uint16_t crc16(uint16_t crc, uint8_t data) {
+    // CRC polynomial 0x11021
+    crc = (uint8_t)(crc >> 8) | (crc << 8);
+    crc ^= data;
+    crc ^= (uint8_t)(crc >> 4) & 0xf;
+    crc ^= crc << 12;
+    crc ^= (crc & 0xff) << 5;
+    return crc;
+}
+
+uint8_t sd_cmd(uint8_t cmd, uint32_t arg, uint8_t crc) {
+  spi_send_byte
+}
+
+void init_sd(){
+  init_spi();
+
+  
+}
+
diff --git a/fpga/zsbl/sd.h b/fpga/zsbl/sd.h
new file mode 100644
index 000000000..b1b27db14
--- /dev/null
+++ b/fpga/zsbl/sd.h
@@ -0,0 +1,9 @@
+#pragma once
+
+#include <stdint.h>
+
+uint8_t crc7(uint8_t prev, uint8_t in);
+uint16_t crc16(uint16_t crc, uint8_t data);
+uint8_t sd_cmd(uint8_t cmd, uint32_t arg, uint8_t crc);
+void init_sd();
+
diff --git a/fpga/zsbl/spi.c b/fpga/zsbl/spi.c
index 1400b8881..bf6d629b4 100644
--- a/fpga/zsbl/spi.c
+++ b/fpga/zsbl/spi.c
@@ -48,4 +48,3 @@ uint8_t spi_send_byte(uint8_t byte) {
   return result;
 }
 
-
diff --git a/fpga/zsbl/spi.h b/fpga/zsbl/spi.h
index 2bd2003c5..6bae450ae 100644
--- a/fpga/zsbl/spi.h
+++ b/fpga/zsbl/spi.h
@@ -40,6 +40,7 @@
 void write_reg(uintptr_t addr, uint32_t value);
 uint32_t read_reg(uintptr_t addr);
 uint8_t spi_send_byte(uint8_t byte);
+
 void spi_init();
 
 #endif