关于以下代码的解释：
//============================================
#define LongToBin(n) \ // DEF 1
( \
((n >> 21) & 0x80) | \
((n >> 18) & 0x40) | \
((n >> 15) & 0x20) | \
((n >> 12) & 0x10) | \
((n >> 9) & 0x08) | \
((n >> 6) & 0x04) | \
((n >> 3) & 0x02) | \
((n ) & 0x01) \
)
#define Bin(n) LongToBin(0x##n##L) // DEF 2
//============================================
A. 代码的目的是，将8位十进制(如1111 1111)转为十六进制(如0xFF)。
B. 代码的核心思想是，将8位十进制数视为Long型数(32bits，如1111 1111视为0b 0001 0001 0001 0001 0001 0001 0001 0001)——代码中DEF2的目的即在于此；然后将待转换数(即n)的每 半 个字节(4bits)的最低位(非0即1)按高低顺序，转移到单个字节(8bits)的对应位上，若实现如此转换，Long型由高到低的有效位(半字节最低位)分别需要右移21、18、15、12、9、6、3和0个位置，再与上0xff达到保值的效果——代码中DEF1的目的即在于此。
C. 可能需要解释的符号——
1、 " \ "，由于宏定义规定，为便于阅读，需要换行时，可以在行尾加上符号，表示下一行内容也是本定义中内容；
2、" ## "，属于宏定义中的字符(串)连接符，即，将符号两端的字符(串)接为一个整体，如以上代码中，在调用Bin(n)时，若n=1111 1111，该宏的作用是将1111 1111视为字符串，替换"0x##n##L"中的"n"，并与"0x"和"L"连接为一体，即Bin(1111 1111)等价于调用LongToBin(0x 1111 1111 L)，而符号"0x"和"L"则分别为十六进制数和Long型数的标记符。
综上，DEF1的作用是将Long型数缩为Char(Byte)型数，DEF2的作用是将十进制数(字符)转为十六进制Long型数。
应用串口的初始化：（Atmega128A）
//初始化串口相关寄存器，设置波特率，并开启串口的中断
void init_uart(unsigned char port_no, unsigned char baudrate)
{
static unsigned int __farflash BAUDRATE_SET[] = {B2400,B4800,B9600,B19200,B38400,B57600,B115200,B230400};
//if (baudrate_set == sizeof(BAUDRATE_SET)/sizeof(unsigned int)) return FALSE;
if (port_no == 0)
{
UCSR0B = 0x00; //disable while setting baud rate
UCSR0A = Bin(11000010); //doubling the transfer rate
/* ||||||||_____ MPCMn: Multi-Processor Communication Mode */
/* |||||||______ U2Xn: Double the USART Transmission Speed */
/* ||||||_______ UPEn: Parity Error */
/* |||||________ DORn: Data OverRun */
/* ||||_________ FEn: Frame Error */
/* |||__________ UDREn: USART Data Register Empty */
/* ||___________ TXCn: USART Transmit Complete */
/* |____________ RXCn: USART Receive Complete */
UCSR0C = Bin(00000110); //Asynchronous Operation; none paUCSRA_Bit7ty; 1 stop bits; 8 data bits
/* ||||||||_____ UCPOLn: Clock Polarity */
/* |||||||______ */
/* ||||||_______ Bit 2:1 – UCSZn1:0: Character Size */
/* |||||________ USBSn: Stop Bit Select */
/* ||||_________ */
/* |||__________ Bit 5:4 – UPMn1:0: Parity Mode */
/* ||___________ UMSELn: USART Mode Select */
/* |____________ - */
UBRR0L = BAUDRATE_SET[baudrate]; //set baud rate lo
UBRR0H = BAUDRATE_SET[baudrate]>>8; //set baud rate hi
UCSR0B = Bin(11011000);
/* ||||||||_____ TXB8n: Transmit Data Bit 8 */
/* |||||||______ RXB8n: Receive Data Bit 8 */
/* ||||||_______ UCSZn2: Character Size */
/* |||||________ TXENn: Transmitter Enable */
/* ||||_________ RXENn: Receiver Enable */
/* |||__________ UDRIEn: USART Data Register Empty Interrupt Enable */
/* ||___________ TXCIEn: TX Complete Interrupt Enable */
/* |____________ RXCIEn:RX Complete Interrupt Enable */
}
else
{
UCSR1B = 0x00; //disable while setting baud rate
UCSR1A = Bin(11000010); //doubling the transfer rate
/* ||||||||_____ MPCMn: Multi-Processor Communication Mode */
/* |||||||______ U2Xn: Double the USART Transmission Speed */
/* ||||||_______ UPEn: Parity Error */
/* |||||________ DORn: Data OverRun */
/* ||||_________ FEn: Frame Error */
/* |||__________ UDREn: USART Data Register Empty */
/* ||___________ TXCn: USART Transmit Complete */
/* |____________ RXCn: USART Receive Complete */
UCSR1C = Bin(00000110); //Asynchronous Operation; none paUCSRA_Bit7ty; 1 stop bits; 8 data bits
/* ||||||||_____ UCPOLn: Clock Polarity */
/* |||||||______ */
/* ||||||_______ Bit 2:1 – UCSZn1:0: Character Size */
/* |||||________ USBSn: Stop Bit Select */
/* ||||_________ */
/* |||__________ Bit 5:4 – UPMn1:0: Parity Mode */
/* ||___________ UMSELn: USART Mode Select */
/* |____________ - */
UBRR1L = BAUDRATE_SET[baudrate]; //set baud rate lo
UBRR1H = BAUDRATE_SET[baudrate]>>8; //set baud rate hi
UCSR1B = Bin(11011000);
/* ||||||||_____ TXB8n: Transmit Data Bit 8 */
/* |||||||______ RXB8n: Receive Data Bit 8 */
/* ||||||_______ UCSZn2: Character Size */
/* |||||________ TXENn: Transmitter Enable */
/* ||||_________ RXENn: Receiver Enable */
/* |||__________ UDRIEn: USART Data Register Empty Interrupt Enable */
/* ||___________ TXCIE: TX Complete Interrupt Enable */
/* |____________ RXCIEn:RX Complete Interrupt Enable */
}
}