Subversion Repositories neorv32

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

// # << NEORV32 - Bootloader >>                                                                    #

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

// # BSD 3-Clause License                                                                          #

// #                                                                                               #

// # Copyright (c) 2021, 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 RISC-V Processor - https://github.com/stnolting/neorv32       (c) Stephan Nolting #

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

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

 * @file bootloader.c

 * @author Stephan Nolting

 * @brief NEORV32 bootloader.

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

// Libraries

#include <stdint.h>

#include <neorv32.h>

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

 * @name Bootloader configuration (override via console to customize)

 * default values are used if not explicitly customized

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

/**@{*/

/* ---- UART interface configuration ---- */

/** Set to 0 to disable UART interface */

#ifndef UART_EN

  #define UART_EN 1

#endif

/** UART BAUD rate for serial interface */

#ifndef UART_BAUD

  #define UART_BAUD 19200

#endif

/* ---- Status LED ---- */

/** Set to 0 to disable bootloader status LED (heart beat) at GPIO.gpio_o(STATUS_LED_PIN) */

#ifndef STATUS_LED_EN

  #define STATUS_LED_EN 1

#endif

/** GPIO output pin for high-active bootloader status LED (heart beat) */

#ifndef STATUS_LED_PIN

  #define STATUS_LED_PIN 0

#endif

/* ---- Boot configuration ---- */

/** Set to 1 to enable automatic (after reset) only boot from external SPI flash at address SPI_BOOT_BASE_ADDR */

#ifndef AUTO_BOOT_SPI_EN

  #define AUTO_BOOT_SPI_EN 0

#endif

/** Set to 1 to enable boot only via on-chip debugger (keep CPU in halt loop until OCD takes over control) */

#ifndef AUTO_BOOT_OCD_EN

  #define AUTO_BOOT_OCD_EN 0

#endif

/** Set to 1 to enable simple UART executable upload (no console, no SPI flash) */

#ifndef AUTO_BOOT_SIMPLE_UART_EN

  #define AUTO_BOOT_SIMPLE_UART_EN 0

#endif

/** Time until the auto-boot sequence starts (in seconds); 0 = disabled */

#ifndef AUTO_BOOT_TIMEOUT

  #define AUTO_BOOT_TIMEOUT 8

#endif

/* ---- SPI configuration ---- */

/** Enable SPI module (default) including SPI flash boot options */

#ifndef SPI_EN

  #define SPI_EN 1

#endif

/** SPI flash chip select (low-active) at SPI.spi_csn_o(SPI_FLASH_CS) */

#ifndef SPI_FLASH_CS

  #define SPI_FLASH_CS 0

#endif

/** SPI flash sector size in bytes */

#ifndef SPI_FLASH_SECTOR_SIZE

  #define SPI_FLASH_SECTOR_SIZE 65536 // default = 64kB

#endif

/** SPI flash clock pre-scaler; see #NEORV32_SPI_CTRL_enum */

#ifndef SPI_FLASH_CLK_PRSC

  #define SPI_FLASH_CLK_PRSC CLK_PRSC_8

#endif

/** SPI flash boot base address */

#ifndef SPI_BOOT_BASE_ADDR

  #define SPI_BOOT_BASE_ADDR 0x08000000

#endif

/**@}*/

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

  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 */

};

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

 * 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

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

 * Helper macros

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

/**@{*/

/** Actual define-to-string helper */

#define xstr(a) str(a)

/** Internal helper macro */

#define str(a) #a

/** Print to UART 0 */

#if (UART_EN != 0)

  #define PRINT_TEXT(...) neorv32_uart0_print(__VA_ARGS__)

  #define PRINT_XNUM(a) print_hex_word(a)

  #define PRINT_GETC(a) neorv32_uart0_getc()

  #define PRINT_PUTC(a) neorv32_uart0_putc(a)

#else

  #define PRINT_TEXT(...)

  #define PRINT_XNUM(a)

  #define PRINT_GETC(a) 0

  #define PRINT_PUTC(a)

#endif

/**@}*/

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

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

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

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

volatile uint32_t exe_available = 0;

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

 * Only set during executable fetch (required for capturing STORE BUS-TIMOUT exception).

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

volatile uint32_t getting_exe = 0;

// Function prototypes

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

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);

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

 * Sanity check: Base ISA only!

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

#if defined __riscv_atomic || defined __riscv_a || __riscv_b || __riscv_compressed || defined __riscv_c || defined __riscv_mul || defined __riscv_m

  #warning In order to allow the bootloader to run on *any* CPU configuration it should be compiled using the base ISA only.

#endif

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

 * Bootloader main.

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

int main(void) {

  // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

  // AUTO BOOT: OCD

  // Stay in endless loop until the on-chip debugger

  // takes over CPU control

  // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

#if (AUTO_BOOT_OCD_EN != 0)

  #warning Custom boot configuration: Boot via on-chip debugger.

  while(1) {

    asm volatile ("nop");

  }

  return 0; // should never be reached

#endif

  // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

  // AUTO BOOT: Simple UART boot

  // Upload executable via simple UART interface, no console, no flash options

  // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

#if (AUTO_BOOT_SIMPLE_UART_EN != 0)

  #warning Custom boot configuration: Auto boot via simple UART interface.

  // setup UART0 (primary UART, no parity bit, no hardware flow control)

  neorv32_uart0_setup(UART_BAUD, PARITY_NONE, FLOW_CONTROL_NONE);

  PRINT_TEXT("\nNEORV32 bootloader\nUART executable upload\n");

  get_exe(EXE_STREAM_UART);

  PRINT_TEXT("\n");

  start_app();

  return 0; // bootloader should never return

#endif

  // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

  // AUTO BOOT: SPI flash only

  // Bootloader will directly boot and execute image from SPI flash

  // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

#if (AUTO_BOOT_SPI_EN != 0)

  #warning Custom boot configuration: Auto boot from external SPI flash.

  // setup UART0 (primary UART, no parity bit, no hardware flow control)

  neorv32_uart0_setup(UART_BAUD, PARITY_NONE, FLOW_CONTROL_NONE);

  // SPI setup

  neorv32_spi_setup(SPI_FLASH_CLK_PRSC, 0, 0, 0);

  PRINT_TEXT("\nNEORV32 bootloader\nLoading from SPI flash at ");

  PRINT_XNUM((uint32_t)SPI_BOOT_BASE_ADDR);

  PRINT_TEXT("...\n");

  get_exe(EXE_STREAM_FLASH);

  PRINT_TEXT("\n");

  start_app();

  return 0; // bootloader should never return

#endif

  // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

  // AUTO BOOT: Default

  // User UART to upload new executable and optionally store it to SPI flash

  // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

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

  getting_exe   = 0; // we are not trying to get an executable yet

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

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

#if (SPI_EN != 0)

  // setup SPI for 8-bit, clock-mode 0

  neorv32_spi_setup(SPI_FLASH_CLK_PRSC, 0, 0, 0);

#endif

#if (STATUS_LED_EN != 0)

  if (neorv32_gpio_available()) {

    // activate status LED, clear all others

    neorv32_gpio_port_set(1 << STATUS_LED_PIN);

  }

#endif

#if (UART_EN != 0)

  // setup UART0 (primary UART, no parity bit, no hardware flow control)

  neorv32_uart0_setup(UART_BAUD, PARITY_NONE, FLOW_CONTROL_NONE);

#endif

  // Configure machine system timer interrupt for ~2Hz

  if (neorv32_mtime_available()) {

    neorv32_mtime_set_timecmp(neorv32_cpu_get_systime() + (NEORV32_SYSINFO.CLK/4));

    // active timer IRQ

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

    neorv32_cpu_eint(); // enable global interrupts

  }

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

  // Show bootloader intro and system info

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

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

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

  PRINT_XNUM(neorv32_cpu_csr_read(CSR_MIMPID));

  PRINT_TEXT("\nCLK:  ");

  PRINT_XNUM(NEORV32_SYSINFO.CLK);

  PRINT_TEXT("\nMISA: ");

  PRINT_XNUM(neorv32_cpu_csr_read(CSR_MISA));

  PRINT_TEXT("\nCPU:  ");

  PRINT_XNUM(NEORV32_SYSINFO.CPU);

  PRINT_TEXT("\nSOC:  ");

  PRINT_XNUM(NEORV32_SYSINFO.SOC);

  PRINT_TEXT("\nIMEM: ");

  PRINT_XNUM(NEORV32_SYSINFO.IMEM_SIZE);

  PRINT_TEXT(" bytes @");

  PRINT_XNUM(NEORV32_SYSINFO.ISPACE_BASE);

  PRINT_TEXT("\nDMEM: ");

  PRINT_XNUM(NEORV32_SYSINFO.DMEM_SIZE);

  PRINT_TEXT(" bytes @");

  PRINT_XNUM(NEORV32_SYSINFO.DSPACE_BASE);

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

  // Auto boot sequence

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

#if (SPI_EN != 0)

#if (AUTO_BOOT_TIMEOUT != 0)

  if (neorv32_mtime_available()) {

    PRINT_TEXT("\n\nAutoboot in "xstr(AUTO_BOOT_TIMEOUT)"s. Press key to abort.\n");

    uint64_t timeout_time = neorv32_cpu_get_systime() + (uint64_t)(AUTO_BOOT_TIMEOUT * NEORV32_SYSINFO.CLK);

    while(1){

      if (neorv32_uart0_available()) { // wait for any key to be pressed

        if (neorv32_uart0_char_received()) {

          break;

        }

      }

      if (neorv32_cpu_get_systime() >= timeout_time) { // timeout? start auto boot sequence

        get_exe(EXE_STREAM_FLASH); // try booting from flash

        PRINT_TEXT("\n");

        start_app();

        while(1);

      }

    }

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

  }

#else

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

#endif

#else

  PRINT_TEXT("\n\n");

#endif

  print_help();

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

  // Bootloader console

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

  while (1) {

    PRINT_TEXT("\nCMD:> ");

    char c = PRINT_GETC();

    PRINT_PUTC(c); // echo

    PRINT_TEXT("\n");

    while (neorv32_uart0_tx_busy());

    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);

    }

#if (SPI_EN != 0)

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

      save_exe();

    }

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

      get_exe(EXE_STREAM_FLASH);

    }

#endif

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

      if (exe_available == 0) {

        PRINT_TEXT("No executable available.");

      }

      else {

        start_app();

      }

    }

    else { // unknown command

      PRINT_TEXT("Invalid CMD");

    }

  }

  return 1; // bootloader should never return

}

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

 * Print help menu.

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

void print_help(void) {

  PRINT_TEXT("Available CMDs:\n"

                     " h: Help\n"

                     " r: Restart\n"

                     " u: Upload\n"

#if (SPI_EN != 0)

                     " s: Store to flash\n"

                     " l: Load from flash\n"

#endif

                     " e: Execute");

}

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

 * Start application program at the beginning of instruction space.

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

void start_app(void) {

  // deactivate global IRQs

  neorv32_cpu_dint();

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

  // wait for UART to finish transmitting

  while (neorv32_uart0_tx_busy());

  // start app at instruction space base address

  register uint32_t app_base = NEORV32_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 Adapt exception PC only for sync exceptions!

 *

 * @note 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) {

  register uint32_t cause = neorv32_cpu_csr_read(CSR_MCAUSE);

  // Machine timer interrupt

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

#if (STATUS_LED_EN != 0)

    if (neorv32_gpio_available()) {

      neorv32_gpio_pin_toggle(STATUS_LED_PIN); // toggle status LED

    }

#endif

    // set time for next IRQ

    if (neorv32_mtime_available()) {

      neorv32_mtime_set_timecmp(neorv32_cpu_get_systime() + (NEORV32_SYSINFO.CLK/4));

    }

  }

  // Bus store access error during get_exe

  else if ((cause == TRAP_CODE_S_ACCESS) && (getting_exe)) {

    system_error(ERROR_SIZE); // -> seems like executable is too large

  }

  // Anything else (that was not expected); output exception notifier and try to resume

  else {

    register uint32_t epc = neorv32_cpu_csr_read(CSR_MEPC);

#if (UART_EN != 0)

    if (neorv32_uart0_available()) {

      PRINT_TEXT("\n[ERR ");

      PRINT_XNUM(cause); // MCAUSE

      PRINT_PUTC(' ');

      PRINT_XNUM(epc); // MEPC

      PRINT_PUTC(' ');

      PRINT_XNUM(neorv32_cpu_csr_read(CSR_MTVAL)); // MTVAL

      PRINT_TEXT("]\n");

    }

#endif

    neorv32_cpu_csr_write(CSR_MEPC, epc + 4); // advance to next instruction

  }

}

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

 * Get executable stream.

 *

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

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

void get_exe(int src) {

  getting_exe = 1; // to inform trap handler we were trying to get an executable

  // flash image base address

  uint32_t addr = (uint32_t)SPI_BOOT_BASE_ADDR;

  // get image from flash?

  if (src == EXE_STREAM_UART) {

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

  }

#if (SPI_EN != 0)

  else {

    PRINT_TEXT("Loading... ");

    // flash checks

    if (spi_flash_read_1st_id() == 0x00) { // check if flash ready (or available at all)

      system_error(ERROR_FLASH);

    }

  }

#endif

  // 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*)NEORV32_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 {

    PRINT_TEXT("OK");

    exe_available = size; // store exe size

  }

  getting_exe = 0; // to inform trap handler we are done getting an executable

}

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

 * Store content of instruction memory to SPI flash.

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

void save_exe(void) {

#if (SPI_EN != 0)

  // size of last uploaded executable

  uint32_t size = exe_available;

  if (size == 0) {

    PRINT_TEXT("No executable available.");

    return;

  }

  uint32_t addr = (uint32_t)SPI_BOOT_BASE_ADDR;

  // info and prompt

  PRINT_TEXT("Write ");

  PRINT_XNUM(size);

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

  PRINT_XNUM(addr);

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

  char c = PRINT_GETC();

  PRINT_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);

  }

  PRINT_TEXT("\nFlashing... ");

  // clear memory before writing

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

  uint32_t sector = (uint32_t)SPI_BOOT_BASE_ADDR;

  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*)NEORV32_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((uint32_t)SPI_BOOT_BASE_ADDR + EXE_OFFSET_CHECKSUM, checksum);

  PRINT_TEXT("OK");

#endif

}

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

 * 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[i] = (uint8_t)PRINT_GETC();

    }

#if (SPI_EN != 0)

    else {

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

    }

#endif

  }

  return data.uint32;

}