We all know that data communication is the transmission and reception of digital signals between two communication entities. Suppose you want to send the following data: 12, 23, 34, 45, 56, 67, 78, 89. To receive this data correctly, the receiver must know when the data begins to be sent and when it ends, otherwise it may receive the data as 22, 33, 44, 55, 66, 77, 88.
For the synchronous communication protocol, the sender sends a special electrical signal before sending the data, so that the receiver is ready to receive the data, and then the sender sends all the above data continuously. After the transmission is completed, a special electrical signal is sent. The data transmission ends. We can use the following diagram to represent the synchronization signal communication.
data pack
Then, the receiver divides the data packet into one value according to a prior agreement, that is, every two digits.
For the asynchronous communication protocol, the sender sends a "start" flag every time a data is sent, and an "end" flag is sent after each data transmission. The following figure shows the asynchronous communication signal:
Therefore, synchronization and asynchronous in POS communication are two different communication protocols, which are an agreement between the sender of the signal and the receiver of the signal. The role of the synchronization signal is very important during communication, especially for serial data communication. The role of the synchronization signal is simply twofold. 1 is the role of the "ruler" to measure the width of a bit; 2 is the "starting point" position that determines the measurement. Both the transceiver and the receiver must use the same “ruler†to measure the level change on the data line from the same “starting point†to ensure the correct data communication.
Therefore, in any way of serial communication, the process of synchronization must be there, that is, both parties must remain "synchronized". Let's look at the most typical SPI and I2C. In these two modes, there is a dedicated CLK signal line. One of the CLKs is generated by the communication side, and the CLK signal line is transmitted to the other side. The two sides work according to the control of this CLK. The width of CLK is the width of a data bit, and the rising or falling edge of CLK is the "starting point" flag. This way of obviously transmitting a synchronization signal by a CLK line during communication is synchronous communication. Since synchronous communication has a dedicated CLK line control, it is relatively easy for both parties to communicate to achieve "synchronization", so the speed is relatively fast. However, for long-distance communication, the synchronization method will not work. 1 is because a special signal line is needed, and the cost is increased. 2 is that the longer the communication line, the more interference there is, and the speed of communication is not going up. Therefore, the synchronization method is mostly used as a communication interface between chip levels on the same PCB. Long-distance communication mostly uses "asynchronous" communication mode. Here, "asynchronous" does not mean that the communication parties do not need to synchronize, but means that the communication parties do not use a dedicated synchronous signal line to transmit CLK, but each depends on only their own. The system clock (this is asynchronous!), according to the agreed procedures, adjust their "step" to achieve synchronization between the two sides. Synchronous communication is usually much faster than asynchronous communication. The receiver does not have to start and stop each character. Once the frame sync character is detected, it receives them as the next data arrives. In addition, the overhead of synchronous communication is also relatively small. For example, a typical frame may have 500 bytes (ie, 4000 bits) of data, which may only contain 100 bits of overhead. At this point, the increased bits increase the total number of bits of communication by 2.5%, which is much less than the 25% increase in asynchronous communication. As the actual data bits in the data frame increase, the percentage of overhead bits will decrease accordingly. However, the longer the data bits, the larger the buffer required to buffer the data, which limits the size of one frame. In addition, the larger the frame, the longer it takes to occupy the communication medium. In extreme cases, this will cause other users to wait too long.
If you master these basic concepts, then you can understand and face the problems that arise in communication and find solutions. For example, for UART communication, you know why you want to specify the same frame structure and baud rate. What happens if you set the error? At this time, the system clocks of both parties are not only required to be more accurate and stable, but also to use these special crystals of 11.0592, what will happen if 12M crystals are used, and so on.
To summarize, 1, asynchronous communication is character-oriented communication, and synchronous communication is bit-oriented communication. 2. The unit of asynchronous communication is a character and the unit of synchronous communication is æ¡¢. 3. Asynchronous communication seizes the opportunity of resynchronization by the start and stop codes of the start and stop of the character, while the synchronous communication extracts the synchronization information by the data. 4. Asynchronous communication requires less timing, and synchronous communication often coordinates timing through a specific clock line. 5. Asynchronous communication is less efficient than synchronous communication.
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