Last time, we analyzed the results without terminating resistors and after adding terminating resistors, and we concluded that termination resistors are required when the DDR adopts a daisy-chain topologyThis time, let's look at whether the termination resistor at the end of the DDR is a little farther away from the last DDR or a little closer
The case used this time is still our DDR3 one-to-eight positive and negative daisy chain topology in the previous issue.
We still use the same method to extract the topology of the root address line A3, and the excitation signal is 533MHz
When the distance from the last DDR to the terminal resistor is set to 100 mil, it is possible that the eye diagram of the signal is relatively clear, the eye width and eye height are obvious, and the signal upshoot and undershoot are small.
When we increase the distance to 300mil, we will find that the overshoot and undershoot of the signal do not increase significantly, and the signal eye diagram meets the standard.
At a distance of 500 mils, although the signal eye diagram can also meet the requirements, the overshoot and undershoot of the signal will continue to increase as the length increases.
When we do the last **, when the distance is set to 1000mil, we can find that the result is similar to our previous ** results, the overshoot and undershoot have increased, but when the distance is set to 1000mil, we will find that the improvement effect is getting worse and worse, and the width and height of the eyes of the signal have narrowed to a certain extentIn summary, we can conclude that the shorter the length of the termination resistor at the end of the DDR from the last DDR, the better, and the shorter the signal qualityWe have set up 4 different length distances, at least within 500mil The eye diagram of the signal has not changed too obviously, and in practical applications, we are also in accordance with the general specification DDR end of the matching resistor length is best controlled within 500mil.
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