Serial communication generates an error when the baud rate error exceeds a certain range. The contents of this section are implemented on the 51 MCU with software to achieve automatic baud rate tuning, which minimizes the baud rate error, thus improving the reliability of serial communication.
The 51 MCU has a full-duplex serial interface, which provides multi-machine communication functions, which can easily constitute a multi-machine system (multi-machine fault-tolerant system, distributed multi-level system), providing an effective means for accomplishing large-scale control tasks. . With the multi-machine control system composed of 51 single-chip microcomputers, there have been many cases in China. In such a system, it is important to reliably implement information exchange (serial communication) between multiple processors. The study in this section found that the baud rate as the rate of serial communication has an important impact on the reliability of communication.
First, the realization conditions of normal serial communicationSerial communication is the transfer of data in bits. Each data has 8 or 9 data bits, plus the start and stop bits form a serial frame. Each occupied time T, 1/T is called the baud rate of serial communication. Ideally, the transmitter and receiver settings have the same baud rate. The receiving controller of the receiver samples at the midpoint of each data bit of the serial frame and can receive the correct data, but it is impossible to require the baud rate to be completely consistent. This raises a question: How reliable is the serial communication to allow the baud rate of both senders and receivers? We take a 10-bit serial frame as an example for analysis.
Figure 1 - 85 (b) is the normal working situation, the baud rate is the same. In Figure 1-85(a), the receiving baud rate is unchanged, and the transmission baud rate becomes smaller, and the time T of each bit is extended by 5%. When the 10th bit is sampled, a misalignment occurs and normal communication cannot be performed. . For a 10-bit serial frame, the maximum allowable baud rate error is less than 5%. In the multi-machine communication mode, the serial frame is 11 bits, and the maximum allowable baud rate error does not exceed 4.5%.
What is obtained above is only the upper limit of the baud rate error in the ideal case. In practical applications, due to the limited frequency band of the hardware device, the upper and lower edges of the data bits occupy a certain time, and the effective time of each data bit is shorter than the ideal situation, so there is a stricter requirement on the baud rate error.
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