What is the difference between asynchronous and synchronous transmission signals (electronic hardware)

In signal protocol , There are many kinds , Such as UART,I2C,SPI,PCIe wait , They have their own set of rules , But in the process of making the agreement , You must choose one of the synchronous and asynchronous transmission rules .

So what is the difference between synchronous transmission and asynchronous transmission ？

To understand synchronous transmission and asynchronous transmission , First of all, understand “ Sync ” and “ asynchronous ” The meaning of these two words .

Sync ： It refers to two or more quantities that change with time and maintain a certain relative relationship in the process of change .

asynchronous ： Asynchronous parties do not need a common clock , That is, the receiver doesn't know when the sender will send , Therefore, in the message sent, there must be a message to prompt the receiver to start receiving .

So let's look at synchronous transmission and asynchronous transmission , What is synchronization ？ Who is the object of synchronization ？

In electronic circuits , The object it synchronizes is the clock signal , Also called CLK The signal , The synchronization signal needs to refer to the clock signal , Asynchronous signals do not need reference CLK The signal .

So you could say , Distinguish between synchronous signals and asynchronous signals , Make sure that when the signal becomes effective , Whether it is affected by CLK The limitation of , Or is there CLK The signal .

In normal operation , Asynchronous signals are independent of clock signals , So when the input signal becomes effective , The device starts to work normally , Its state will change accordingly .

The synchronization signal adds a new reference factor —— Clock signal , The input signal needs to be connected with the clock signal “ And ” Operation or “ And non ” operation , When the operation result of the input signal and the clock signal after synthesis is in the effective state , The device can work normally by using the protocol transmission .

There is no time for reference , How does asynchronous signal judge whether information transmission starts ？

For asynchronous signals , Because there is no clock signal as a reference , Therefore, when formulating, there should be a signal in the sent information that can prompt the receiver to start accepting , Such as UART In the protocol, a logic is sent in the starting bit “0” Low level signal , Indicates the beginning of the transmission character . Of course, there is the stop bit at the end , In this way , Asynchronous signals can also transmit data well without clock signals .

A typical synchronous transmission signal ：I2C,SPI,SPMI,PCIe

Typical asynchronous transmission signal ：UART