写一段代码基于AT90USB1286的bootloader功能实现重启程序时读取代码并写入app区

对于AT90USB1286的bootloader功能，通常使用avr-libc库提供的bootloader模板进行开发。下面是一个基于模板的重启程序时读取代码并写入app区的示例代码：

#include <avr/io.h>
#include <avr/boot.h>
#include <avr/interrupt.h>

#define APP_SECTION_START   0x1800
#define APP_SECTION_SIZE    (32 * 1024)
#define APP_PAGE_SIZE       SPM_PAGESIZE

void app_start(void) __attribute__ ((noreturn)) {
    void (*start)(void) = (void *) APP_SECTION_START;
    start();
}

int main(void) {
    // Initialize UART and other peripherals as needed
    // ...

    // Check if the application section is valid
    uint8_t *app_ptr = (uint8_t *) APP_SECTION_START;
    if (*app_ptr == 0xff) {
        // Application section is blank, stay in bootloader
        while (1) {
            // Wait for commands from the host
            // ...
        }
    }

    // If the reset was caused by a watchdog timeout, clear the watchdog flag
    if (MCUSR & (1 << WDRF)) {
        MCUSR &= ~(1 << WDRF);
    }

    // Jump to the application section
    app_start();
}

void erase_app(void) {
    // Erase the entire application section
    for (uint32_t addr = APP_SECTION_START; addr < APP_SECTION_START + APP_SECTION_SIZE; addr += APP_PAGE_SIZE) {
        boot_page_erase(addr);
        boot_spm_busy_wait();
    }
}

void write_app(uint8_t *data, uint32_t len) {
    uint32_t addr = APP_SECTION_START;
    uint32_t end_addr = APP_SECTION_START + APP_SECTION_SIZE;
    uint8_t *ptr = data;
    while (addr < end_addr && ptr < data + len) {
        // Write one page at a time
        boot_page_erase(addr);
        boot_spm_busy_wait();
        for (uint16_t i = 0; i < APP_PAGE_SIZE && ptr < data + len; i += 2) {
            uint16_t word = *ptr++;
            word |= (*ptr++) << 8;
            boot_page_fill(addr + i, word);
        }
        boot_page_write(addr);
        boot_spm_busy_wait();
        addr += APP_PAGE_SIZE;
    }
}

void restart_app(void) {
    // Disable interrupts
    cli();

    // Clear the reset vector
    boot_page_erase(0);
    boot_spm_busy_wait();

    // Set the reset vector to the start of the application section
    boot_page_fill(0, APP_SECTION_START >> 1);
    boot_page_write(0);
    boot_spm_busy_wait();

    // Enable interrupts
    sei();

    // Jump to the reset vector
    asm("jmp 0");
}

void handle_command(uint8_t cmd, uint8_t *data, uint16_t len) {
    switch (cmd) {
        case 'E': // Erase application section
            erase_app();
            // Send acknowledgement
            // ...
            break;
        case 'W': // Write data to application section
            write_app(data, len);
            // Send acknowledgement
            // ...
            break;
        case 'R': // Restart application
            restart_app();
            break;
        // Handle other commands as needed
        // ...
    }
}

void receive_command(void) {
    uint8_t cmd;
    uint16_t len;
    uint8_t data[APP_PAGE_SIZE];
    // Receive command from UART
    // ...
    handle_command(cmd, data, len);
}

在这个示例中，bootloader首先检查应用程序区域是否为空。如果为空，则停留在bootloader中等待命令。如果不是空的，则检查是否由看门狗超时引起重启，并清除看门狗标志。然后，bootloader跳转到应用程序区域。

bootloader支持三个命令：擦除应用程序区域（E），将数据写入应用程序区域（W）和重启应用程序（R）。在收到命令后，bootloader会执行相应的操作。写入数据时，bootloader将数据写入应用程序区域的一页，然后将其擦除并写入下一页，直到所有数据都被写入。重启应用程序时，bootloader将重置向量设置为应用程序区域的起始地址，然后跳转到重置向量。

请注意，这只是一个示例代码，需要根据具体的需求进行修改和完善。特别是，需要实现与主机通信的UART接口和命令协议。