SDRAM quality

I've got my design talking to some (Kingston) KVR133X64/1G SDRAM modules and running at only 66MHz, doing some simple testing (the data=address or maybe data={address,address}) and have found some funnies.

There are a few dozen single bit errors. There are also several locations that come back with some F digits (as if the cells just don't exist). Also they are mid-burst as much as not which indicates it isn't a timing problem.

I have done testing on different FPGAs with different SDRAM modules and the errors definitely go with the modules and are quite repeatable.

I have checked the refresh timing and it's good.

Is Kingston SDRAM really that bad ? The fact they are made of repainted Infineon chips such that you can't read the original part number makes me suspicious that these might not be the real thing. They claim 100% testing but I wonder what they do with modules tested as bad. Is it because the FPGA can keep memory busy in a way a processor can't ? Should I speed up the refresh ? We intend to try some Micron parts anyway.


Reply to
Jon Schneider
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I'd suspect your design before I'd suspect the modules especially if it's more then one module that's failing. Have you simulated the design using the Micron DRAM models? The Micron models do a very good job of checking that you've met all of the timing requirements. DRAMs have a lot timing requirements, not just set up and hold and refresh intervals, but things like precharge times, RAS to RAS times on the same bank, ...

Reply to
Hiding in Plain Sight

Errors which are apparently 'static' can be due to artifacts of the relative numbers of address/data lines /banks active/transitioning at the time - noise, ground bounce etc. Might be interesting to try different data patterns & see if the errors move. If refresh is WAY out of spec (or not happenning at all) it could be that, however as the refresh interval is specced over the entire temp range I wouldn't expect to see refresh-related failures until it was out of spec by a very big margin.

Reply to
Mike Harrison

What about signal integrity? impedance matching?



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Reply to
Aurelian Lazarut

I agree with the other posters in this thread, and would tend to suspect my own design before suspecting the SDRAM...

.. but ...

A few weeks ago we bought three new PCs for the office. All with 2GB SDRAM, and all but one of the 1GB modules was some brand which I have never heard of, and can't recall (the last was a Corsair). I started having problems with my PC rebooting for no reason. Starting running a comprehensive memory test and it showed hundreds of bit errors overnight. The other guys here ran the same tests and, although not all quite as bad, all had bit errors in an overnight test.

We went back and exchanged the RAM (including the Corsair, which did

*NOT* fail) for another brand, and re-ran the tests. *Zero* errors on all 6 modules.

Now that *could* be put down to crappy mobo design (they're identical machines) or bad SDRAM, but it is food for thought...

Regards, Mark

Reply to
Mark McDougall

Regarding the PC RAM issue, is anything being overclocked? That seems really odd that they would mostly all fail... on all the PC's for that matter. I would try the RAM in another PC and see what happens. But regarding your issue, I highly doubt signal integrity would be a problem at 66 MHz, unless you have seriously long trace-lengths. I have seen SDRAM working on 2 layer boards without any controlled impedance whatsoever, as long as lengths are kept relatively short.

Reply to

No, these are office PCs so everything is factory settings on Gigabyte mobos. The RAM is long gone - returned to the shop for replacement. And going from 100+ errors overnight (6 modules on 3 mobos) to zero errors with the new RAM is pretty damning evidence IMHO.

Regards, Mark

Reply to
Mark McDougall


Isn't there some minimal frequency to respect (I believe it's 75 MHz for DDR, I don't have the specs at hand)? Failing to do this could upset the SDRAM's PLL.

Regards, Alvin.

Reply to
Alvin Andries

Yes, but only for registered or DDR SDRAM - standard SDRAM modules don't have a PLL

Reply to
Mike Harrison

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