Cause analysis of serial communication overshoot and method of termination

        First of all, let's take a look at our commonly used serial communication wiring methods ,MCU Of RX,TX Let's add one here 33~100Ω To match the impedance of the transmission line ,USB turn TTL This end is not terminated with any resistance ,MCU Here and USB turn TTL This is through 15cm Long DuPont line connection , The circuit diagram is as follows ,USB turn TTL Connected to the upper computer ,MCU adopt TXD Continuously send data to the upper computer ,RXD For the upper computer MCU Send measurement instructions .

 1、MCU_TX wave form

 2、USB_RX

 3、MCU_RX

 4、USB_TX

        We have found the problem from the above test waveform ,MCU Of TX and USB Of TX There is basically no overshoot ,MCU Of RX and USB Of RX All have relatively large overshoot , This overshoot one 15% One 30%, Overshoot is relatively large , The cause of this overshoot is the impedance mismatch , And the long wire makes us see the reflection of the signal .

      We use MCU_TX（ Proximal end ） and USB_RX（ Distal ） Take this group for example , We put the matching 100R Remove the resistor , Look at the waveform of the remote end as follows ：

It can be found that the overshoot increases again 3% about .

And then we put 100R Resistance welding back , Parallel one at the far end 470p capacitance , This increases the transmission delay , Let us ring and drown in the rising edge , So we can see the waveform at the far end without overshoot .

        Some people may say that the communication signal frequency of the serial port is not high , For example, the baud rate of the serial port above me is ：961200bps, Look at the frequency 500k When , If the 500k It is far from the frequency that requires impedance matching .

My understanding of this question is as follows ：

because commonly No To be terminated Maximum length in Count Is the rising edge of the signal ns Count , That is to say

Len max  >  RT

Len max Is the maximum length of a transmission line that is not terminated ,RT Is the rising edge of the signal . for instance ： The signal 10M The frequency of , The clock period is 100ns, The rising edge is 10ns, Then the maximum length of the transmission line that does not need to be terminated is 10in, Test us MCU Of TX The rising edge time of is shown in the figure below , It can be seen that the rise time is 1.1ns, That is, the maximum length of the transmission line that does not need to be terminated 1.1in, That is to say 2.79cm, The length of the transmission line we use is 15cm about , So we can see the reflection of the signal . We solved the problem of reflection by adding a capacitor at the far end , Is to make the rising time of the signal longer , Submerge the reflection in the rising edge process .