External Memory Interface Handbook Volume 2: Design Guidelines

ID 683385
Date 5/08/2017
Document Table of Contents

2.8.5. Deskew Example4.2.3.4. Deskew Example

Consider an example where you want to deskew an interface with 4 DQ pins, 1 DQS pin, and 1 DQSn pin.

Let’s assume an operating frequency of 667 MHz, and the package lengths for the pins reported in the .pin file as follows:

dq[0] = 120 ps
dq[1] = 120 ps
dq[2] = 100 ps
dq[3] = 100 ps
dqs   = 80 ps
dqs_n = 80 ps

The following figure illustrates this example.

Figure 35. Deskew Example

When you perform length matching for all the traces in the DQS group, you must take package delays into consideration. Because the package delays of traces A and B are 40 ps longer than the package delays of traces E and F, you would need to make the board traces for E and F 40 ps longer than the board traces for A and B.

A similar methodology would apply to traces C and D, which should be 20 ps longer than the lengths of traces A and B.

The following figure shows this scenario with the length of trace A at 450 ps.

Figure 36. Deskew Example with Trace Delay Calculations

When you enter the board skews into the Board Settings tab of the DDR3 parameter editor, you should calculate the board skew parameters as the sums of board delay and corresponding package delay. If a pin does not have a package delay (such as address and command pins), you should use the board delay only.

The example of the preceding figure shows an ideal case where board skews are perfectly matched. In reality, you should allow plus or minus 10 ps of skew mismatch within a DQS group (DQ/DQS/DM).

Did you find the information on this page useful?

Characters remaining:

Feedback Message