Indiscriminate use of "termination", the signal pours on the street

author ： Jiang Jie （ Team members of Yibo technology we media express Mr ）

The last afterglow of the setting sun , Twilight is everywhere , With the sky darkening down, there is Lei Gong's mood ： Because the clock termination designed with full confidence does not improve the signal quality , Instead, it drags back . Let's start with Mr. Gao's latest short video .
 
Words , The captain of Mr. expressway is really charming , An easy to understand metaphor explains the basic principle of termination clearly （ Why does the signal reflect ？ How to do a good job of termination matching PCB Design ten mistakes video links ）.
The topic of termination matching has aroused Lei Gong's interest , After watching the video, the meaning is still incomplete , I also took the time to learn several common termination again . Opportunity is for the prepared mind , Lei Gong was in the next day Layout There is an opportunity for performance in the design .
 
Data rate is 2400Mbps Of DDR3 The signal , The topology of address and clock routing is one to four 、fly-by. When dealing with differential clock , What Lei Gong saw on the schematic diagram except the termination resistance , There is also a differential signal in parallel P/N Intermediate capacitance .
 
If this capacitor appeared two days earlier , Leigong estimates that it still has to struggle with the location of the link , But , For him who has studied termination seriously , This is no longer a problem , There is a terminating resistance R, Capacitance appears again C, It's not the terminal RC Termination ！ It clearly needs to be put together , The hardware siege lion has to draw pictures separately , It's just too much , Leigong did not hesitate to put this capacitor at the end of the differential clock . Send the simulation to Mr. Gao after the wiring is completed , Lei Gong is complacent , Sit back and wait for PASS.
 
Mr. Gao brought good news and bad news , The good news is the clock designed by Leigong “ Termination ” It worked , Lei Gong's smile began to bloom ; The bad news is ,“ Termination ” It has side effects , Wear down the clock signal , Lei Gong's smile froze on his face . So there was a slightly sad scene at the beginning of the article .
 
Mr. Gao did not leave Lei Gong alone in a mess , But helped him analyze the problem carefully . The key lies in what Leigong calls “ Terminating capacitance ” Is not RC Part of the termination , Actually , Its main function is to reduce the reflection at the source by slowing down the rising edge of the driving signal , It should be close to the layout of the driver chip , Usually used to drive strong chips . therefore , The modifications that need to be made by Leigong are also very simple , It is to adjust the capacitor from the end to the driving end . About this capacitor , Mr. Gao called it “ Differential capacitance ”, There have been special articles before .（《DDR3 Series of clock signal differential capacitance , The average person I don't tell him ！》 The article links ）
 
that , Lei Gong originally intended to show off his skills RC What is the correct opening posture of the terminal ？RC Termination , also called AC Termination , Link commonly used to transmit DC balance signal . For single ended signals ,RC The termination method is to add a capacitor on the basis of the traditional end parallel termination , The biggest advantage is that the DC power consumption is small , At the same time, it will also avoid the phenomenon that the high level of traditional parallel termination is pulled down or the low level is raised .
 
For differential signals RC Termination , Usually in T A DC isolating capacitor is added on the basis of type a termination , Some designs also put R2 Omit .
 
Whether it's a single ended signal , Or differential signal ,RC In termination “C” To cut off the DC circuit , Lei Gong encountered parallel connection in differential clock P/N The capacitance in the middle is obviously not included . After some explanation , Lei Gong suddenly enlightened , Final , Adjust the differential capacitance from the terminal to the driving end according to the simulation suggestions , The clock waveform improves , And meet SPEC requirement . After the problem is solved , The haze in Lei Gong's heart was swept away , Reflect on this lesson , Act rashly without understanding the theory , It turned out to be a joke , I feel that I have a long way to go to advance .