When DDR adopts daisy-chain topology, due to the long signal transmission line, it is usually necessary to add a terminal matching resistor at the end of DDR, and there are many ways to terminate, but they are all to solve the problem of signal reflection, usually in order to eliminate the reflection of the signal can be solved at the source or terminal of the signal, the way to eliminate the reflection at the source is to use the resistor series mode, and the way to eliminate the reflection at the terminal is to use the resistor parallel connection (there are many kinds of termination),Today, let's take a look at how much difference the quality of the signal is between the end and the non-matching resistor when using a daisy-chain topology.
Signal end termination matching resistor.
This time, we will extract the address line topology from this memory module PCB board to observe the transmission quality of the signal when adding matching resistors and not adding matching resistors with SIGXPlorer software, this case is a dual-channel DDR3 one-to-8 front and back daisy chain topology, and the topology after we have sorted out is extracted by the software as shown in the following figure
The signal is transmitted from the drive terminal U21, and the DDR particles that pass through it are U14 and U5 U6 and U15 U7 and U16 U8 and U17 in turn, and finally connected to the 39OHM terminal matching resistor RN106, we extract one of the address lines A3, the excitation signal is 533MHz, and observe the results of his eye diagram.
According to the signal flow direction of the daisy chain, we can distinguish that the leftmost yellow signal is the driving end of the signal, and the right signal is the signal of 8 DDR3 particles.
After adding the termination resistor, the signal quality of the two DDR particles closest to the driver end and the two DDR particles close to the matching resistor will be compared, and the signal quality of the two DDR particles close to the matching resistor is poor, and the overshoot and undershoot are more serious. However, the signal quality is acceptable, and there is no case of falling below VIH and VIL.
The above figure shows the results of the signal eye diagram without the termination resistor, we can see that the eye width and eye height of the signal are very narrow, the vih and vil of the signal are kept for a short time after crossing the line, and the signal quality is poor, we observe the eye diagram results of the two DDR particles near the driving end and the eye diagram results of the two DDR particles at the very end again, we will find that the eye diagram results are opposite when the termination resistor is not added, the quality of the two DDR particles near the driving end is very poor, the eye width and eye height are very narrow, but the quality of the two DDR particles at the most far end will be much better, although there are also obvious upshoot and undershoot, but the eye width and eye height are relatively clear.
We can analyze the above ** results,When using a daisy-chain topology, it is necessary to add a termination resistor, the termination resistance has a very obvious improvement effect,However, it is important to note that the termination resistor must match the impedance of the transmission line to achieve the best improvement.
Declaration:This article is an original article by Fanyi Education, **please indicate**!