Subversion Repositories neorv32

// #################################################################################################
// # << NEORV32 - Bootloader >>                                                                    #
// # ********************************************************************************************* #
// # THE BOOTLOADER SHOULD BE COMPILED USING THE BASE ISA ONLY (rv32i or rv32e)!                   #
// # ********************************************************************************************* #
// # Boot from (internal) instruction memory, UART or SPI Flash.                                   #
// #                                                                                               #
// # UART configuration: 8N1 at 19200 baud                                                         #
// # 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.                                          #
// #                                                                                               #
// # Auto boot sequence after timeout:                                                             #
// #  -> Try booting from SPI flash at spi_csn_o(0).                                               #
// #  -> Permanently light up status led and freeze 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. Compile only for rv32i or rv32e (better).
 **************************************************************************/
 
// Libraries
#include <stdint.h>
#include <neorv32.h>
 
 
/**********************************************************************//**
 * @name User configuration
 **************************************************************************/
/**@{*/
/** UART BAUD rate */
#define BAUD_RATE              (19200)
/** 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)
/** Bootloader status LED at GPIO output port */
#define STATUS_LED             (0)
/** SPI flash boot image base address */
#define SPI_FLASH_BOOT_ADR     (0x00800000)
/** SPI flash chip select at spi_csn_o */
#define SPI_FLASH_CS           (0)
/** Default SPI flash clock prescaler for serial peripheral interface */
#define SPI_FLASH_CLK_PRSC     (CLK_PRSC_8)
/** SPI flash sector size in bytes */
#define SPI_FLASH_SECTOR_SIZE  (64*1024)
/**@}*/
 
 
/**********************************************************************//**
  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 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 access
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_status(void);
uint8_t spi_flash_read_1st_id(void);
void spi_flash_write_enable(void);
void spi_flash_write_addr(uint32_t addr);
 
 
/**********************************************************************//**
 * Bootloader main.
 **************************************************************************/
int main(void) {
 
  // ------------------------------------------------
  // 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, MSB-first, no interrupt
  if (clock_speed < 40000000) {
    neorv32_spi_setup(SPI_FLASH_CLK_PRSC, 0, 0, 0, 0);
  }
  else {
    neorv32_spi_setup(CLK_PRSC_128, 0, 0, 0, 0);
  }
 
  // init UART (no interrupts)
  neorv32_uart_setup(BAUD_RATE, 0, 0);
 
  // Configure machine system timer interrupt for ~2Hz
  neorv32_mtime_set_timecmp(neorv32_mtime_get_time() + (clock_speed/4));
 
  // confiure trap handler (bare-metal, no neorv32 rte available)
  neorv32_cpu_csr_write(CSR_MTVEC, (uint32_t)(&bootloader_trap_handler));
 
  neorv32_cpu_csr_write(CSR_MIE, 1 << CPU_MIE_MTIE); // activate MTIME IRQ source
  neorv32_cpu_eint(); // enable global interrupts
 
  if (STATUS_LED_EN == 1) {
    // activate status LED, clear all others
    neorv32_gpio_port_set(1 << STATUS_LED);
  }
 
  // global variable to executable size; 0 means there is no exe available
  exe_available = 0;
 
 
  // ------------------------------------------------
  // Show bootloader intro and system info
  // ------------------------------------------------
  neorv32_uart_print("\n\n\n\n<< NEORV32 Bootloader >>\n\n"
                     "BLDV: "__DATE__"\nHWV:  ");
  neorv32_rte_print_hw_version();
  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("\nCONF: ");
  print_hex_word(SYSINFO_FEATURES);
  neorv32_uart_print("\nIMEM: ");
  print_hex_word(SYSINFO_ISPACE_SIZE);
  neorv32_uart_print(" bytes @ ");
  print_hex_word(SYSINFO_ISPACE_BASE);
  neorv32_uart_print("\nDMEM: ");
  print_hex_word(SYSINFO_DSPACE_SIZE);
  neorv32_uart_print(" bytes @ ");
  print_hex_word(SYSINFO_DSPACE_BASE);
 
 
  // ------------------------------------------------
  // Auto boot sequence
  // ------------------------------------------------
  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
      get_exe(EXE_STREAM_FLASH); // try loading from spi flash
      neorv32_uart_print("\n");
      start_app();
    }
  }
  neorv32_uart_print("Aborted.\n\n");
  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 == 'r') { // restart bootloader
      neorv32_cpu_dint(); // disable global interrupts
      asm volatile ("li t0, %[input_i]; jr t0" :  : [input_i] "i" (BOOTLOADER_BASE_ADDRESS)); // jump to beginning of boot ROM
      while(1); // just for the compiler
    }
    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 EEPROM from RAM
      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
}
 
 
/**********************************************************************//**
 * 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 shutdown or reset the used peripherals
  // -> this will be done by application's crt0
 
  // deactivate IRQs and IRQ sources
  neorv32_cpu_dint();
  neorv32_cpu_csr_write(CSR_MIE, 0);
 
  neorv32_uart_print("Booting...\n\n");
 
  // wait for UART to finish transmitting
  while ((UART_CT & (1<<UART_CT_TX_BUSY)) != 0);
 
  // reset performance counters (to benchmark actual application)
  asm volatile ("csrw mcycle,    zero"); // will also clear 'cycle'
  asm volatile ("csrw mcycleh,   zero"); // will also clear 'cycleh'
  asm volatile ("csrw minstret,  zero"); // will also clear 'instret'
  asm volatile ("csrw minstreth, zero"); // will also clear 'instreth'
 
  // 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.
 * Primarily used for the MTIME tick.
 * @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
    neorv32_uart_print("\n\nEXCEPTION (");
    print_hex_word(cause);
    neorv32_uart_print(") @ 0x");
    print_hex_word(neorv32_cpu_csr_read(CSR_MEPC));
    system_error(ERROR_SYSTEM);
    while(1); // freeze
  }
  else {
    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));
  }
}
 
 
/**********************************************************************//**
 * Get executable stream.
 *
 * @param src Source of executable stream data. See #EXE_STREAM_SOURCE.
 **************************************************************************/
void get_exe(int src) {
 
  // is instruction memory (IMEM) read-only?
  if (SYSINFO_FEATURES & (1 << SYSINFO_FEATURES_MEM_INT_IMEM_ROM)) {
    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
 
  // executable too large?
  uint32_t imem_size = SYSINFO_ISPACE_SIZE;
  if (size > imem_size) {
    system_error(ERROR_SIZE);
  }
 
  // 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\nBootloader ERR_"); // 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
  }
 
  asm volatile ("wfi"); // power-down
  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 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);
 
  while (1) {
    uint8_t tmp = spi_flash_read_status();
    if ((tmp & 0x01) == 0) { // write in progress flag cleared?
      break;
    }
  }
}
 
 
/**********************************************************************//**
 * 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;
 
  uint32_t i;
  for (i=0; i<4; i++) {
    spi_flash_write_byte(addr + i, data.uint8[3-i]);
  }
}
 
 
/**********************************************************************//**
 * Erase sector (64kB) at base adress.
 *
 * @param[in] addr Base address of sector to erase.
 **************************************************************************/
void spi_flash_erase_sector(uint32_t addr) {
 
  spi_flash_write_enable(); // allow write-access
 
  neorv32_spi_cs_en(SPI_FLASH_CS);
 
  neorv32_spi_trans(SPI_FLASH_CMD_SECTOR_ERASE);
  spi_flash_write_addr(addr);
 
  neorv32_spi_cs_dis(SPI_FLASH_CS);
 
  while (1) {
    uint8_t tmp = spi_flash_read_status();
    if ((tmp & 0x01) == 0) { // write in progress flag cleared?
      break;
    }
  }
}
 
 
/**********************************************************************//**
 * Read status register.
 *
 * @return Status register.
 **************************************************************************/
uint8_t spi_flash_read_status(void) {
 
  neorv32_spi_cs_en(SPI_FLASH_CS);
 
  neorv32_spi_trans(SPI_FLASH_CMD_READ_STATUS);
  uint8_t status = (uint8_t)neorv32_spi_trans(0);
 
  neorv32_spi_cs_dis(SPI_FLASH_CS);
 
  return status;
}
 
 
/**********************************************************************//**
 * Read first byte of ID (manufacturer ID), should be != 0x00.
 *
 * @note The first bit of the manufacturer ID is used to detect if a Flash is connected at all.
 *
 * @return First byte of ID.
 **************************************************************************/
uint8_t spi_flash_read_1st_id(void) {
 
  neorv32_spi_cs_en(SPI_FLASH_CS);
 
  neorv32_spi_trans(SPI_FLASH_CMD_READ_ID);
  uint8_t id = (uint8_t)neorv32_spi_trans(0);
 
  neorv32_spi_cs_dis(SPI_FLASH_CS);
 
  return id;
}
 
 
/**********************************************************************//**
 * Enable flash write access.
 **************************************************************************/
void spi_flash_write_enable(void) {
 
  neorv32_spi_cs_en(SPI_FLASH_CS);
  neorv32_spi_trans(SPI_FLASH_CMD_WRITE_ENABLE);
  neorv32_spi_cs_dis(SPI_FLASH_CS);
}
 
 
/**********************************************************************//**
 * Send address word to flash.
 *
 * @param[in] addr Address word.
 **************************************************************************/
void spi_flash_write_addr(uint32_t addr) {
 
  union {
    uint32_t uint32;
    uint8_t  uint8[sizeof(uint32_t)];
  } address;
 
  address.uint32 = addr;
 
  neorv32_spi_trans(address.uint8[2]);
  neorv32_spi_trans(address.uint8[1]);
  neorv32_spi_trans(address.uint8[0]);
}