DDR2 at 125MHz or lower with Cyclone2

Jun 28, 2006 2 Replies
V

Hello all,



I am starting a brand new design, memory speed is not important (but depth is). This is my first time designing with any high speed memory. I have two questions:


  1. Why does Micron data sheet say minimum clock is 8ns (125MHz). What happens when you run DDR2 at lower speed (refresh problems?)
  2. My simulation (Hyperlynx) looks OK with no termination. Has any body tried running DDR2 at 125MHz with no termination whatsoever (only

10 ohm series with DDR2 inputs)


I would really appreciate any response



Thanks,


-- Naveed



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J

The DDR2s use Delay Locked Loops to produce strobes and data in specific timing relative to the system clock. Specifying a lower limit in the DDR2 definition allows the silicon vendors to design a delay line of specific length to implement the DLL function.

Terminations probably are not required at 125 MHz; you can terminate in and only in the DDR2 itself by always driving the On Die Termination.

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T

The DLL in the memory chip won't lock. In practice you can bend the spec a little.

Samsung have an app note on using DDR at slower speeds in embedded apps with short PCB tracks and no terminations.

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