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Operating speed of DDR2

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embeddedlover

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Hi,

I have to interface a DDR2 with processor.

I have selected a part from micron which is DDR2-667 having cycle time of 3ns.

So, the maximum clocking frequency is 333MHz.

I wanted following clarifications on the operating front:

1. Can i operate the module at maximum operating speed that is 333MHz?
2. What happens if i exceed maximum frequency?
3. Are there any rule of thumb frequencies for DDR2 smooth operation?
 

camr

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If it is sold as supporting 3ns cycle time, it should be able to operate trouble-free with a 333MHz clock if used at the specified operating voltage/temperature / latency setting etc.; There are certain normal levels of noise/jitter it is designed to tolerate. the spec sheet will tell you these things.

If you exceed the maximum frequency, you will start seeing incorrect operation: errors in data, bad timing, strange behavior, etc. the actual limitations will vary widely and depend on a lot of different factors. You will also see trouble if your voltage, temperature, jitter, etc. are poorly managed.

If you read the specsheet, it should say what the expected operating frequencies are for different configuration settings.
 

embeddedlover

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It is infact dependent on the specific operating conditions. As you mentioned, i can get the above information from the datasheet.

If i operate the module or chip above specified frequency and have ECC support does this help?
Also, why is it that ECC supported DDR has longer life than non-ECC?
 

camr

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All other things being equal, I can see how relying on ECC *might* indeed allow you to operate a system a little bit further beyond spec than without, in the sense that the ECC might allow you to know when you start getting bad data, so that you could deal with it. But the gain might not be much (depends on the chips' internal details), and may not be something to rely on.

It's like overclocking your processor or video card: on a case by case basis, you can tweak, test, tweak, test, etc., until you get a decent boost and are still (mostly) crash-free... but it's hard to be certain and you might still get errors now and then, plus the result is only true for that specific chip.

On why ECC parts might have longer life... not sure. One guess would be that they're simply built/tested to higher standards because of the type of application / customer they envision for the devices with ECC vs. the basic ones, and the difference in the price people will pay...
 

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