In the reference designs for a few FPGA development boards, I have observed that, there is always a PLL which generates two clocks at the same frequency but not in phase. One clock feeds to the SDRAM controller while the other "delayed" feeds the SDRAM_CLK which is output from the FPGA. I only know that the delayed clock is required since there is propagation delay between the FPGA and the SDRAM and the communication will not be synchronized between the two devices if this phase shifted does not exist. Is this the reason?

What is supposed to be the relationship between these two clocks and how does one determine how much phase difference is required for correct operation?


2 Answers 2


Altera Embedded IP user guide explains the SDRAM clock phase calculation in details in the SDRAM Controller section.

Basically, you want the SDRAM clock to strike right in the middle of the window when data is stable. This interval is calculated for both read and write scenarios, and the smallest values for both lead and lag define the window:

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Typically, the lag values are bigger than lead values, which means the SDRAM clock should be delayed compared to the controller clock. On lower frequencies both lead and lag are positive, which means you can get away with using the same clock for both.


This is most likely related to meta stability. The output of the controller needs to be stable for the hold time. Otherwise you can have a situation as shown in the figure below.

meta stability

If your signal from the controller (A) is not stable for long enough before the memory clock (C2), the latched signal at the memory (B) can be meta stable. It can be high, low, or a voltage between high and low. The behavior is undefined.

To determine the delay required you need to complete the timing closure analysis with the memory (SDRAM) timing constraints using your tool chain.


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