Is there any cumulative error in serial communication and the requirements for clock accuracy

• 1. The problem background ：

• 2. Problem analysis :

• 3. summary :

1. The problem background ：

For embedded developers , Serial port should be the most widely used module , In the process of daily customer support, customers often ask some questions about the stability of serial communication , Some typical problems are as follows ：

• stay 9600 Baud rate can work , But switch to 115200 You can't communicate normally , Is it related to the accuracy of the clock ？

• What is the requirement of serial port stable communication for clock accuracy , Whether the higher the baud rate configuration, the higher the clock accuracy requirements ？ Inside RC Can you meet ？

• The serial port communicates normally for a period of time, and the communication error , Is it because the cumulative error is too large ？

Despite the overwhelming information about serial port configuration on the Internet , But there is really not much description of these specific problems , So I studied it carefully .

2. Problem analysis :

To answer these questions, we need to start with the sampling sequence of serial communication , As shown in the figure above , According to the sampling characteristics of the serial port, it is not difficult to see , For the sender , The data is being drive On the bus , The receiving end is at each bit Sampling in the middle of , This means that in the last Bit Bit sampling , The allowable limit offset is 50%, If used 1 Starting bits +9 Data bits +1 Two parity bits +1 Stop bits to calculate , Then each deviation is up to ：

±50% / 12 = ±4.16%

Note: For some serial ports that support oversampling , The sampling point may not be 50% In the middle , Generally, it will be more backward , No discussion here ;

If used 1 Starting bits +8 Data bits +1 Stop bits to calculate , Then each deviation is up to ：±50% / 10 = ±5%

And serial communication , It involves both sending and receiving ends , When one end is 0 error , The maximum error at the other end can reach 4.16%, If the errors at both ends are the same , Then the maximum error at each end can only be 2.08%, This is why many hardware masters suggest that the baud rate error of the serial port be controlled within 2% within .

±4.16% / 2 = ±2.08%

therefore , For serial communication , It can be roughly considered that , The clock error is less than ±2% when , Communication is relatively reliable . For the actual application system of customers , in consideration of UART It is an asynchronous communication , The stability of communication depends on the clock accuracy of both sides , It doesn't mean that only one side's accuracy meets the requirements , The system will work , So it needs to be considered systematically .

3. summary :

Based on the above analysis , It can be concluded as follows ：

• The stable communication of serial port is related to the clock accuracy of both sides , The total error cannot exceed ±4.16%, For most full temperature ranges ±1% Of MCU Come on , Can meet the basic communication needs .

• The cumulative error of serial communication occurs in each data ( It can also be simply understood as each byte , Just the bytes are not precise , Because sometimes the data may be 9 bits) Send internal , There is no cumulative error between successive transmissions , Because consecutive multiple bytes will be sent Start Start signal for resynchronization , That is to say, transmission 1 Error and transmission of bytes 1000 The error of bytes is basically the same ;

• Whether the serial port is stable or not has nothing to do with the baud rate , It is not that the higher the baud rate configuration, the higher the clock accuracy requirements , Only with the length of serial port data sent each time ( Contains the start bit / Data bits / Parity bit / Stop bit length ) of , Longer bit length , The higher the accuracy of the clock ;

• For using serial port LIN Communications , The above conclusion is also basically applicable ;

• The above conclusion is only analyzed from the perspective of sampling timing , Do not consider the influence of external electromagnetic interference , Because usually the higher the baud rate , The shorter the time length of each data , The more vulnerable it is to electromagnetic interference, resulting in unreliable communication .