REM to allow the receiver position, REM set 1 serial port to allow reception, set to 0 to prohibit reception. REM is the set or cleared by software. If a circuit to receive and transmit pins P3.0, P3.1, and host computer are connected to serial port on the software interrupt handler, when asked when in a particular subroutine is not allowed to host computer serial port to the control characters generate an interrupt, then the beginning of this subroutine by adding REM = 0 to prohibit the receipt, at the end of the subroutine by adding REM = 1 re-open the serial port reception. We can also use the above actual source code by adding REM = 0 for the experiment. 

     TB8 send data bit 8, in mode 2 and 3 is to send the first nine. This bit can be set using the software as needed spaces or removed, usually in the communication protocol to do the parity bit, in a multi-processor communications, which are used to indicate a yes or a data frame address of the frame. 

   RB8 receive data bit 8, in the mode 2 and 3 is already receiving data the first nine. This bit may be parity bits, the address / data identification bits. In the mode 0 in, RB8 to preserve the bits not being used. In mode 1, when SM2 = 0, RB8 is the stop bit of data has been received. 

TI Transmit Interrupt ID bits. In mode 0, sending finished the first 8-bit data from the hardware set. Other models are sending stop bit at the beginning, from the hardware set. TI Set applications after interrupt, CPU to respond to disruption, send the next frame data. In any mode, TI must be cleared by software, which means the data is written to SBUF, the hardware to send data, interrupt response (such as the interruption to open), then TI = 1, that sending has been completed, TI will not be cleared by the hardware, so its time to be cleared in software. 

   RI Receive Interrupt ID bits. In mode 0, the receiving end of the first eight, from the hardware set. Other models are at the receiving end of eighteen months, bits from the hardware set. RI = 1, the application interrupt require CPU removed data. However, in Mode 1, SM2 = 1, when not received a valid stop bit, no would be RI set. Similarly, RI must also rely on the software removed. Commonly used serial port mode 1 is the transfer of 10-bit, 1 start bit for the 0.8 bits of data bits, low first, and a stop bit is 1. Its baud rate is variable, and its rate is determined by Timer 1 or Timer 2, Timer value (overflow rate). AT89S52 and AT89C2051 chips such as 51 only two timers, Timer 0 and Timer 1 and Timer 2 is only available 89C52 chips. 

Done using the serial communication baud rate, a very important parameter is the baud rate, only the upper and lower baud rate the same machine only when they can engage in normal communication. Baud rate refers to the serial port baud per second that can be transmitted within the median. Some beginner's friends that the baud rate refers to the number of bytes transmitted per second, such as standard 9600 would be mistaken for second kinds can be sent 9600 bytes, but actually it refers to can be transmitted per second, binary 9600 , and a byte to eight binary bits, such as with the serial port mode to transmit it together with a start bit and stop bits, each data byte is necessary to occupy 10 binary bits, 9600 baud rate with the model of a transmission , the number of bytes transmitted per second is 9600 ÷ 10 = 960 bytes. 51-chip serial mode 0 baud rate is fixed for the fosc/12, with a 12M crystal to calculate, its baud rate up to 1M. Mode 2 baud rate is fixed at fosc/64 or fosc/32, the specific uses that depend on the SMOD bit in PCON register, such as the SMOD is 0, the baud rate for the focs/64, SMOD is 1, wave Special rate for focs/32. Mode 1 and mode 3 baud rate is variable, depending on Timer 1 or 2 (52 chips) of the overflow rate. 

 

　　随着国内超大规模集成电路的出现，微处理器及其外围芯片有了迅速的发展。集成技术的最新发展之一是将CPU和外围芯片，如程序存储器、数据存储器、并行I/O口、串行I/O口、定时/计数器、中断控制器及其他控制部件集成在一个芯片之中，制成单片计算机（Single-Chip Microcomputer）。而近年来推出的一些高档单片机还包括有许多特殊功能单元，如A/D、D/A转换器、调制解调器、通信控制器、锁相环、DMA、浮点运算单元、PWM控制输出单元、PWM输出时的死区可编程控制功能等。因此，只要外加一些扩展电路及必要的通道接口就可以构成各种计算机应用系统，如工业流水线控制系统、作为家用电器的主控制器、分布式控制系统的终端节点或作为其主控制节点起中继的作用、数据采集系统、自动测试系统等。

　　单片机的出现，并在各技术领域中得到如此迅猛的发展，与单片机构成计算机应用系统所形成的下述特点有关：

　　 2.1、单片机构成的应用系统有较大的可靠性。这些可靠性的获得除了依靠单片机芯片本身的高可靠性以及应用有最少的联接外，还可以方便地采用软、硬件技术。

　　 2.2、系统扩展、系统配置较典型、规范，容易构成各种规模的应用系统，应用系统有较高的软、硬件利用系数。【转载请注明出处：https://www.5ubs.com/bsTemplate/fanyi/538.html】