Subversion Repositories neorv32

// #################################################################################################

// # << NEORV32 - Bootloader >>                                                                    #

// # ********************************************************************************************* #

// # In order to run the bootloader on any CPU configuration, the bootloader should be compiled    #

// # unsing the base ISA (rv32i/rv32e) only.                                                       #

// # ********************************************************************************************* #

// # Boot from (internal) instruction memory, UART or SPI Flash.                                   #

// #                                                                                               #

// # UART configuration: 8 data bits, NO parity bit, 1 stop bit, 19200 baud (19200-8N1)            #

// # Boot Flash: 8-bit SPI, 24-bit addresses (like Micron N25Q032A) @ neorv32.spi_csn_o(0)         #

// # neorv32.gpio_o(0) is used as high-active status LED (can be disabled via #STATUS_LED_EN).     #

// #                                                                                               #

// # Auto boot sequence (can be disabled via #AUTOBOOT_EN) after timeout (via #AUTOBOOT_TIMEOUT):  #

// #  -> Try booting from SPI flash at spi_csn_o(0).                                               #

// #  -> Permanently light up status led and stall CPU if SPI flash booting attempt fails.         #

// # ********************************************************************************************* #

// # BSD 3-Clause License                                                                          #

// #                                                                                               #

// # Copyright (c) 2020, Stephan Nolting. All rights reserved.                                     #

// #                                                                                               #

// # Redistribution and use in source and binary forms, with or without modification, are          #

// # permitted provided that the following conditions are met:                                     #

// #                                                                                               #

// # 1. Redistributions of source code must retain the above copyright notice, this list of        #

// #    conditions and the following disclaimer.                                                   #

// #                                                                                               #

// # 2. Redistributions in binary form must reproduce the above copyright notice, this list of     #

// #    conditions and the following disclaimer in the documentation and/or other materials        #

// #    provided with the distribution.                                                            #

// #                                                                                               #

// # 3. Neither the name of the copyright holder nor the names of its contributors may be used to  #

// #    endorse or promote products derived from this software without specific prior written      #

// #    permission.                                                                                #

// #                                                                                               #

// # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS   #

// # OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF               #

// # MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE    #

// # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,     #

// # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE #

// # GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED    #

// # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING     #

// # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED  #

// # OF THE POSSIBILITY OF SUCH DAMAGE.                                                            #

// # ********************************************************************************************* #

// # The NEORV32 Processor - https://github.com/stnolting/neorv32              (c) Stephan Nolting #

// #################################################################################################

/**********************************************************************//**

 * @file bootloader.c

 * @author Stephan Nolting

 * @brief Default NEORV32 bootloader.

 **************************************************************************/

// Libraries

#include <stdint.h>

#include <neorv32.h>

/**********************************************************************//**

 * @name User configuration

 **************************************************************************/

/**@{*/

/** UART BAUD rate */

#define BAUD_RATE              (19200)

/** Enable auto-boot sequence if != 0 */

#define AUTOBOOT_EN            (1)

/** Time until the auto-boot sequence starts (in seconds) */

#define AUTOBOOT_TIMEOUT       8

/** Set to 0 to disable bootloader status LED */

#define STATUS_LED_EN          (1)

/** SPI_DIRECT_BOOT_EN: Define/uncomment to enable SPI direct boot (disables the entire user console!) */

//#define SPI_DIRECT_BOOT_EN

/** Bootloader status LED at GPIO output port */

#define STATUS_LED             (0)

/** SPI flash boot image base address (warning! address might wrap-around!) */

#define SPI_FLASH_BOOT_ADR     (0x00800000)

/** SPI flash chip select line at spi_csn_o */

#define SPI_FLASH_CS           (0)

/** Default SPI flash clock prescaler */

#define SPI_FLASH_CLK_PRSC     (CLK_PRSC_8)

/** SPI flash sector size in bytes (default = 64kb) */

#define SPI_FLASH_SECTOR_SIZE  (64*1024)

/** ASCII char to start fast executable upload process (for use with automatic upload scripts) */

#define FAST_UPLOAD_CMD        '#'

/**@}*/

/**********************************************************************//**

  Executable stream source select

 **************************************************************************/

enum EXE_STREAM_SOURCE {

  EXE_STREAM_UART  = 0, /**< Get executable via UART */

  EXE_STREAM_FLASH = 1  /**< Get executable via SPI flash */

};

/**********************************************************************//**

 * Error codes

 **************************************************************************/

enum ERROR_CODES {

  ERROR_SIGNATURE = 0, /**< 0: Wrong signature in executable */

  ERROR_SIZE      = 1, /**< 1: Insufficient instruction memory capacity */

  ERROR_CHECKSUM  = 2, /**< 2: Checksum error in executable */

  ERROR_FLASH     = 3, /**< 3: SPI flash access error */

  ERROR_ROM       = 4, /**< 4: Instruction memory is marked as read-only */

  ERROR_SYSTEM    = 5  /**< 5: System exception */

};

/**********************************************************************//**

 * SPI flash commands

 **************************************************************************/

enum SPI_FLASH_CMD {

  SPI_FLASH_CMD_PAGE_PROGRAM = 0x02, /**< Program page */

  SPI_FLASH_CMD_READ         = 0x03, /**< Read data */

  SPI_FLASH_CMD_READ_STATUS  = 0x05, /**< Get status register */

  SPI_FLASH_CMD_WRITE_ENABLE = 0x06, /**< Allow write access */

  SPI_FLASH_CMD_READ_ID      = 0x9E, /**< Read manufacturer ID */

  SPI_FLASH_CMD_SECTOR_ERASE = 0xD8  /**< Erase complete sector */

};

/**********************************************************************//**

 * NEORV32 executable

 **************************************************************************/

enum NEORV32_EXECUTABLE {

  EXE_OFFSET_SIGNATURE =  0, /**< Offset in bytes from start to signature (32-bit) */

  EXE_OFFSET_SIZE      =  4, /**< Offset in bytes from start to size (32-bit) */

  EXE_OFFSET_CHECKSUM  =  8, /**< Offset in bytes from start to checksum (32-bit) */

  EXE_OFFSET_DATA      = 12, /**< Offset in bytes from start to data (32-bit) */

};

/**********************************************************************//**

 * Valid executable identification signature.

 **************************************************************************/

#define EXE_SIGNATURE 0x4788CAFE

/**********************************************************************//**

 * String output helper macros.

 **************************************************************************/

/**@{*/

/* Actual define-to-string helper */

#define xstr(a) str(a)

/* Internal helper macro */

#define str(a) #a

/**@}*/

/**********************************************************************//**

 * This global variable keeps the size of the available executable in bytes.

 * If =0 no executable is available (yet).

 **************************************************************************/

uint32_t exe_available = 0;

// Function prototypes

void __attribute__((__interrupt__)) bootloader_trap_handler(void);

void fast_upload(int src);

void print_help(void);

void start_app(void);

void get_exe(int src);

void save_exe(void);

uint32_t get_exe_word(int src, uint32_t addr);

void system_error(uint8_t err_code);

void print_hex_word(uint32_t num);

// SPI flash driver functions

uint8_t spi_flash_read_byte(uint32_t addr);

void spi_flash_write_byte(uint32_t addr, uint8_t wdata);

void spi_flash_write_word(uint32_t addr, uint32_t wdata);

void spi_flash_erase_sector(uint32_t addr);

uint8_t spi_flash_read_1st_id(void);

void spi_flash_write_wait(void);

void spi_flash_write_enable(void);

void spi_flash_write_addr(uint32_t addr);

/**********************************************************************//**

 * Bootloader main.

 **************************************************************************/

int main(void) {

#ifdef __riscv_compressed

  #warning In order to allow the bootloader to run on any CPU configuration it should be compiled using the base ISA (rv32i/e) only.

#endif

  // global variable for executable size; 0 means there is no exe available

  exe_available = 0;

  // ------------------------------------------------

  // Minimal CPU hardware initialization

  // - all IO devices are reset and disabled by the crt0 code

  // ------------------------------------------------

  // confiure trap handler (bare-metal, no neorv32 rte available)

  neorv32_cpu_csr_write(CSR_MTVEC, (uint32_t)(&bootloader_trap_handler));

  // ------------------------------------------------

  // Minimal processor hardware initialization

  // - all IO devices are reset and disabled by the crt0 code

  // ------------------------------------------------

  // get clock speed (in Hz)

  uint32_t clock_speed = SYSINFO_CLK;

  // init SPI for 8-bit, clock-mode 0, no interrupt

  if (clock_speed < 40000000) {

    neorv32_spi_setup(SPI_FLASH_CLK_PRSC, 0, 0, 0);

  }

  else {

    neorv32_spi_setup(CLK_PRSC_128, 0, 0, 0);

  }

  if (STATUS_LED_EN == 1) {

    // activate status LED, clear all others

    neorv32_gpio_port_set(1 << STATUS_LED);

  }

  // init UART (no parity bit, no interrupts)

  neorv32_uart_setup(BAUD_RATE, 0, 0, 0);

  // Configure machine system timer interrupt for ~2Hz

  neorv32_mtime_set_timecmp(neorv32_mtime_get_time() + (clock_speed/4));

  neorv32_cpu_csr_write(CSR_MIE, 1 << CSR_MIE_MTIE); // activate MTIME IRQ source

  neorv32_cpu_eint(); // enable global interrupts

  // ------------------------------------------------

  // Fast boot mode: Direct SPI boot

  // Bootloader will directly boot and execute image from SPI memory.

  // No user UART console is available in this mode!

  // ------------------------------------------------

#ifdef SPI_DIRECT_BOOT_EN

  #warning Compiling bootloader in 'SPI direct boot mode'. Bootloader will directly boot from SPI memory. No user UART console will be available.

  neorv32_uart_print("\nNEORV32 bootloader\nAccessing SPI flash at ");

  print_hex_word((uint32_t)SPI_FLASH_BOOT_ADR);

  neorv32_uart_print("\n");

  get_exe(EXE_STREAM_FLASH);

  neorv32_uart_print("\n");

  start_app();

  return 0;

#endif

  // ------------------------------------------------

  // Show bootloader intro and system info

  // ------------------------------------------------

  neorv32_uart_print("\n\n\n\n<< NEORV32 Bootloader >>\n\n"

                     "BLDV: "__DATE__"\nHWV:  ");

  print_hex_word(neorv32_cpu_csr_read(CSR_MIMPID));

  neorv32_uart_print("\nCLK:  ");

  print_hex_word(SYSINFO_CLK);

  neorv32_uart_print(" Hz\nUSER: ");

  print_hex_word(SYSINFO_USER_CODE);

  neorv32_uart_print("\nMISA: ");

  print_hex_word(neorv32_cpu_csr_read(CSR_MISA));

  neorv32_uart_print("\nPROC: ");

  print_hex_word(SYSINFO_FEATURES);

  neorv32_uart_print("\nIMEM: ");

  print_hex_word(SYSINFO_IMEM_SIZE);

  neorv32_uart_print(" bytes @ ");

  print_hex_word(SYSINFO_ISPACE_BASE);

  neorv32_uart_print("\nDMEM: ");

  print_hex_word(SYSINFO_DMEM_SIZE);

  neorv32_uart_print(" bytes @ ");

  print_hex_word(SYSINFO_DSPACE_BASE);

  // ------------------------------------------------

  // Auto boot sequence

  // ------------------------------------------------

#if (AUTOBOOT_EN != 0)

  neorv32_uart_print("\n\nAutoboot in "xstr(AUTOBOOT_TIMEOUT)"s. Press key to abort.\n");

  uint64_t timeout_time = neorv32_mtime_get_time() + (uint64_t)(AUTOBOOT_TIMEOUT * clock_speed);

  while ((UART_DATA & (1 << UART_DATA_AVAIL)) == 0) { // wait for any key to be pressed

    if (neorv32_mtime_get_time() >= timeout_time) { // timeout? start auto boot sequence

      fast_upload(EXE_STREAM_FLASH); // try booting from flash

    }

  }

  neorv32_uart_print("Aborted.\n\n");

  // fast executable upload?

  if (neorv32_uart_char_received_get() == FAST_UPLOAD_CMD) {

    fast_upload(EXE_STREAM_UART);

  }

#else

  neorv32_uart_print("\n\n");

#endif

  print_help();

  // ------------------------------------------------

  // Bootloader console

  // ------------------------------------------------

  while (1) {

    neorv32_uart_print("\nCMD:> ");

    char c = neorv32_uart_getc();

    neorv32_uart_putc(c); // echo

    neorv32_uart_print("\n");

    if (c == FAST_UPLOAD_CMD) { // fast executable upload

      fast_upload(EXE_STREAM_UART);

    }

    else if (c == 'r') { // restart bootloader

      asm volatile ("li t0, %[input_i]; jr t0" :  : [input_i] "i" (BOOTLOADER_BASE_ADDRESS)); // jump to beginning of boot ROM

    }

    else if (c == 'h') { // help menu

      print_help();

    }

    else if (c == 'u') { // get executable via UART

      get_exe(EXE_STREAM_UART);

    }

    else if (c == 's') { // program flash from memory (IMEM)

      save_exe();

    }

    else if (c == 'l') { // get executable from flash

      get_exe(EXE_STREAM_FLASH);

    }

    else if (c == 'e') { // start application program

      start_app();

    }

    else if (c == '?') {

      neorv32_uart_print("by Stephan Nolting");

    }

    else { // unknown command

      neorv32_uart_print("Invalid CMD");

    }

  }

  return 0; // bootloader should never return

}

/**********************************************************************//**

 * Get executable stream and execute it.

 *

 * @param src Source of executable stream data. See #EXE_STREAM_SOURCE.

 **************************************************************************/

void fast_upload(int src) {

  get_exe(src);

  neorv32_uart_print("\n");

  start_app();

  while(1);

}

/**********************************************************************//**

 * Print help menu.

 **************************************************************************/

void print_help(void) {

  neorv32_uart_print("Available CMDs:\n"

                     " h: Help\n"

                     " r: Restart\n"

                     " u: Upload\n"

                     " s: Store to flash\n"

                     " l: Load from flash\n"

                     " e: Execute");

}

/**********************************************************************//**

 * Start application program at the beginning of instruction space.

 **************************************************************************/

void start_app(void) {

  // executable available?

  if (exe_available == 0) {

    neorv32_uart_print("No executable available.");

    return;

  }

  // no need to shut down/reset the used peripherals

  // no need to disable interrupt sources

  // -> crt0 will do a clean CPU/processor reset/setup

  // deactivate global IRQs

  neorv32_cpu_dint();

  neorv32_uart_print("Booting...\n\n");

  // wait for UART to finish transmitting

  while ((UART_CT & (1<<UART_CT_TX_BUSY)) != 0);

  // start app at instruction space base address

  register uint32_t app_base = SYSINFO_ISPACE_BASE;

  asm volatile ("jalr zero, %0" : : "r" (app_base));

  while (1);

}

/**********************************************************************//**

 * Bootloader trap handler. Used for the MTIME tick and to capture any other traps.

 * @warning Since we have no runtime environment, we have to use the interrupt attribute here. Here, and only here!

 **************************************************************************/

void __attribute__((__interrupt__)) bootloader_trap_handler(void) {

  // make sure this was caused by MTIME IRQ

  uint32_t cause = neorv32_cpu_csr_read(CSR_MCAUSE);

  if (cause == TRAP_CODE_MTI) { // raw exception code for MTI

    if (STATUS_LED_EN == 1) {

      // toggle status LED

      neorv32_gpio_pin_toggle(STATUS_LED);

    }

    // set time for next IRQ

    neorv32_mtime_set_timecmp(neorv32_mtime_get_time() + (SYSINFO_CLK/4));

  }

  else if (cause == TRAP_CODE_S_ACCESS) { // seems like executable is too large

    system_error(ERROR_SIZE);

  }

  else {

    neorv32_uart_print("\n\nEXC (");

    print_hex_word(cause);

    neorv32_uart_print(") @ 0x");

    print_hex_word(neorv32_cpu_csr_read(CSR_MEPC));

    system_error(ERROR_SYSTEM);

  }

}

/**********************************************************************//**

 * Get executable stream.

 *

 * @param src Source of executable stream data. See #EXE_STREAM_SOURCE.

 **************************************************************************/

void get_exe(int src) {

  // is MEM implemented and read-only?

  if ((SYSINFO_FEATURES & (1 << SYSINFO_FEATURES_MEM_INT_IMEM_ROM)) &&

      (SYSINFO_FEATURES & (1 << SYSINFO_FEATURES_MEM_INT_IMEM)))  {

    system_error(ERROR_ROM);

  }

  // flash image base address

  uint32_t addr = SPI_FLASH_BOOT_ADR;

  // get image from flash?

  if (src == EXE_STREAM_UART) {

    neorv32_uart_print("Awaiting neorv32_exe.bin... ");

  }

  else {

    neorv32_uart_print("Loading... ");

    // check if flash ready (or available at all)

    if (spi_flash_read_1st_id() == 0x00) { // manufacturer ID

      system_error(ERROR_FLASH);

    }

  }

  // check if valid image

  uint32_t signature = get_exe_word(src, addr + EXE_OFFSET_SIGNATURE);

  if (signature != EXE_SIGNATURE) { // signature

    system_error(ERROR_SIGNATURE);

  }

  // image size and checksum

  uint32_t size  = get_exe_word(src, addr + EXE_OFFSET_SIZE); // size in bytes

  uint32_t check = get_exe_word(src, addr + EXE_OFFSET_CHECKSUM); // complement sum checksum

  // transfer program data

  uint32_t *pnt = (uint32_t*)SYSINFO_ISPACE_BASE;

  uint32_t checksum = 0;

  uint32_t d = 0, i = 0;

  addr = addr + EXE_OFFSET_DATA;

  while (i < (size/4)) { // in words

    d = get_exe_word(src, addr);

    checksum += d;

    pnt[i++] = d;

    addr += 4;

  }

  // error during transfer?

  if ((checksum + check) != 0) {

    system_error(ERROR_CHECKSUM);

  }

  else {

    neorv32_uart_print("OK");

    exe_available = size; // store exe size

  }

}

/**********************************************************************//**

 * Store content of instruction memory to SPI flash.

 **************************************************************************/

void save_exe(void) {

  // size of last uploaded executable

  uint32_t size = exe_available;

  if (size == 0) {

    neorv32_uart_print("No executable available.");

    return;

  }

  uint32_t addr = SPI_FLASH_BOOT_ADR;

  // info and prompt

  neorv32_uart_print("Write 0x");

  print_hex_word(size);

  neorv32_uart_print(" bytes to SPI flash @ 0x");

  print_hex_word(addr);

  neorv32_uart_print("? (y/n) ");

  char c = neorv32_uart_getc();

  neorv32_uart_putc(c);

  if (c != 'y') {

    return;

  }

  // check if flash ready (or available at all)

  if (spi_flash_read_1st_id() == 0x00) { // manufacturer ID

    system_error(ERROR_FLASH);

  }

  neorv32_uart_print("\nFlashing... ");

  // clear memory before writing

  uint32_t num_sectors = (size / SPI_FLASH_SECTOR_SIZE) + 1; // clear at least 1 sector

  uint32_t sector = SPI_FLASH_BOOT_ADR;

  while (num_sectors--) {

    spi_flash_erase_sector(sector);

    sector += SPI_FLASH_SECTOR_SIZE;

  }

  // write EXE signature

  spi_flash_write_word(addr + EXE_OFFSET_SIGNATURE, EXE_SIGNATURE);

  // write size

  spi_flash_write_word(addr + EXE_OFFSET_SIZE, size);

  // store data from instruction memory and update checksum

  uint32_t checksum = 0;

  uint32_t *pnt = (uint32_t*)SYSINFO_ISPACE_BASE;

  addr = addr + EXE_OFFSET_DATA;

  uint32_t i = 0;

  while (i < (size/4)) { // in words

    uint32_t d = (uint32_t)*pnt++;

    checksum += d;

    spi_flash_write_word(addr, d);

    addr += 4;

    i++;

  }

  // write checksum (sum complement)

  checksum = (~checksum) + 1;

  spi_flash_write_word(SPI_FLASH_BOOT_ADR + EXE_OFFSET_CHECKSUM, checksum);

  neorv32_uart_print("OK");

}

/**********************************************************************//**

 * Get word from executable stream

 *

 * @param src Source of executable stream data. See #EXE_STREAM_SOURCE.

 * @param addr Address when accessing SPI flash.

 * @return 32-bit data word from stream.

 **************************************************************************/

uint32_t get_exe_word(int src, uint32_t addr) {

  union {

    uint32_t uint32;

    uint8_t  uint8[sizeof(uint32_t)];

  } data;

  uint32_t i;

  for (i=0; i<4; i++) {

    if (src == EXE_STREAM_UART) {

      data.uint8[3-i] = (uint8_t)neorv32_uart_getc();

    }

    else {

      data.uint8[3-i] = spi_flash_read_byte(addr + i);

    }

  }

  return data.uint32;

}

/**********************************************************************//**

 * Output system error ID and stall.

 *

 * @param[in] err_code Error code. See #ERROR_CODES.

 **************************************************************************/

void system_error(uint8_t err_code) {

  neorv32_uart_print("\a\nERROR_"); // output error code with annoying bell sound

  neorv32_uart_putc('0' + ((char)err_code)); // FIXME err_code should/must be below 10

  neorv32_cpu_dint(); // deactivate IRQs

  if (STATUS_LED_EN == 1) {

    neorv32_gpio_port_set(1 << STATUS_LED); // permanently light up status LED

  }

  while(1); // freeze

}

/**********************************************************************//**

 * Print 32-bit number as 8-digit hexadecimal value (with "0x" suffix).

 *

 * @param[in] num Number to print as hexadecimal.

 **************************************************************************/

void print_hex_word(uint32_t num) {

  static const char hex_symbols[16] = "0123456789ABCDEF";

  neorv32_uart_print("0x");

  int i;

  for (i=0; i<8; i++) {

    uint32_t index = (num >> (28 - 4*i)) & 0xF;

    neorv32_uart_putc(hex_symbols[index]);

  }

}

// -------------------------------------------------------------------------------------

// SPI flash driver functions

// -------------------------------------------------------------------------------------

/**********************************************************************//**

 * Read byte from SPI flash.

 *

 * @param[in] addr Flash read address.

 * @return Read byte from SPI flash.

 **************************************************************************/

uint8_t spi_flash_read_byte(uint32_t addr) {

  neorv32_spi_cs_en(SPI_FLASH_CS);

  neorv32_spi_trans(SPI_FLASH_CMD_READ);

  spi_flash_write_addr(addr);

  uint8_t rdata = (uint8_t)neorv32_spi_trans(0);

  neorv32_spi_cs_dis(SPI_FLASH_CS);

  return rdata;

}

/**********************************************************************//**

 * Write byte to SPI flash.

 *

 * @param[in] addr SPI flash read address.

 * @param[in] wdata SPI flash read data.

 **************************************************************************/

void spi_flash_write_byte(uint32_t addr, uint8_t wdata) {

  spi_flash_write_enable(); // allow write-access

  neorv32_spi_cs_en(SPI_FLASH_CS);

  neorv32_spi_trans(SPI_FLASH_CMD_PAGE_PROGRAM);

  spi_flash_write_addr(addr);

  neorv32_spi_trans(wdata);

  neorv32_spi_cs_dis(SPI_FLASH_CS);

  spi_flash_write_wait(); // wait for write operation to finish

}

/**********************************************************************//**

 * Write word to SPI flash.

 *

 * @param addr SPI flash write address.

 * @param wdata SPI flash write data.

 **************************************************************************/

void spi_flash_write_word(uint32_t addr, uint32_t wdata) {

  union {

    uint32_t uint32;

    uint8_t  uint8[sizeof(uint32_t)];

  } data;

  data.uint32 = wdata;

  int i;

  for (i=0; i<4; i++) {

    spi_flash_write_byte(addr + i, data.uint8[3-i]);