ATmega128具备引导加载支持的用户程序自编程功能

}        
//从RS232发送一个字节
void UART_putchar(char c)
{
    while(!(UCSR0A & 0x20));
    UDR0 = c;
}
//从RS232接收一个字节
int uart_getchar(void)
{
    unsigned char status,res;
    if(!(UCSR0A & 0x80)) return -1;        //no data to be received
    status = UCSR0A;
    res = UDR0;
    if (status & 0x1c) return -1;        // If error, return -1
    return res;
}
//等待从RS232接收一个有效的字节
char uart_waitchar(void)
{
    int c;
    while((c=uart_getchar())==-1);
    return (char)c;
}
//计算CRC
int calcrc(char *ptr, int count)
{
    int crc = 0;
    char i;
    
    while (--count >= 0)
    {
        crc = crc ^ (int) *ptr++ << 8;
        i = 8;
        do
        {
        if (crc & 0x8000)
            crc = crc << 1 ^ 0x1021;
        else
            crc = crc << 1;
        } while(--i);
    }
    return (crc);
}
//退出Bootloader程序，从0x0000处执行应用程序
void quit(void)
{
      uart_putchar('O');uart_putchar('K');
uart_putchar(0x0d);uart_putchar(0x0a);
     while(!(UCSR0A & 0x20));            //等待结束提示信息回送完成
  MCUCR = 0x01;
     MCUCR = 0x00;                    //将中断向量表迁移到应用程序区头部
     RAMPZ = 0x00;                    //RAMPZ清零初始化
     asm("jmp 0x0000\n");                //跳转到Flash的0x0000处，执行用户的应用程序
}
//主程序
void main(void)
{
    int i = 0;
    unsigned char timercount = 0;
    unsigned char packNO = 1;
    int bufferPoint = 0;
    unsigned int crc;
//初始化M128的USART0
    UBRR0H = BAUD_H;    
    UBRR0L = BAUD_L;            //Set baud rate
    UCSR0B = 0x18;            //Enable Receiver and Transmitter
    UCSR0C = 0x0E;            //Set frame format: 8data, 2stop bit
//初始化M128的T/C0，15ms自动重载
  OCR0 = 0xEA;
  TCCR0 = 0x0F;    
//向PC机发送开始提示信息
    while(startupString[i]!='\0')
    {
        uart_putchar(startupString[i]);
        i++;
    }
//3秒种等待PC下发“d”，否则退出Bootloader程序，从0x0000处执行应用程序
    while(1)
    {
        if(uart_getchar()== 'd') break;
        if (TIFR & 0x02)                        //timer0 over flow
        {
               if (++timercount > 200) quit();        //200*15ms = 3s
            TIFR = TIFR|0x02;
        }
    }
    //每秒向PC机发送一个控制字符“C”，等待控制字〈soh〉
    while(uart_getchar()!=XMODEM_SOH)        //receive the start of Xmodem
    {
         if(TIFR & 0x02)                    //timer0 over flow
        {
            if(++timercount > 67)                        //wait about 1 second
            {
                uart_putchar(XMODEM_RECIEVING_WAIT_CHAR);    //send a "C"
                timercount=0;
            }
            TIFR=TIFR | 0x02;
        }
    }
    //开始接收数据块
    do
    {
        if ((packNO == uart_waitchar()) && (packNO ==(~uart_waitchar())))
        {    //核对数据块编号正确
            for(i=0;i<128;i++)                //接收128个字节数据
            {
                data[bufferPoint]= uart_waitchar();
                bufferPoint++;    
            }
            crc = (uart_waitchar()<<8);
            crc += uart_waitchar();            //接收2个字节的CRC效验字
            if(calcrc(&data[bufferPoint-128],128)==crc)    //CRC校验验证
            {    //正确接收128个字节数据
                while(bufferPoint >= SPM_PAGESIZE)
                {    //正确接受256个字节的数据
                    write_one_page();            //收到256字节写入一页Flash中

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