Changing refresh rate for DRAM while in operation?

On Oct 28, 10:18 pm, davewang202 <davewang...@gmail.com> wrote:
> On Oct 25, 4:20 pm, sendt...@gmail.com wrote:
>
>
>
>
>
> > > Since the OP seems to have disappeared to wherever OPs go, I
> > > suspect we will never find out.
>
> > I didn't disappear, I posted a reply but for some reason it didn't
> > show up... I didn't want to accidentally spam the newsgroups by
> > reposting and figured I'd wait to make sure it wasn't just my
> > newsreader or ISP causing the problem.
>
> > Anyway, I guess I'll answer the reason why I want to do this in the
> > same post.
>
> > I'm trying to characterize a DRAM device in certain environmental
> > (radiation) conditions and see how that effects the retention
> > characteristics. I'm not sure if there tests the industry uses to do
> > this, but I needed to evaluate it realtime.
>
> > I'm using the core Altera provided but all the code is there (except
> > for the NIOS II cpu). So I have direct access to the SDRAM
> > controller.
>
> I think it would be really tough to do what you want to do.  The
> reason is that DRAM cell retention time charcteristics are not always
> deterministic.  Some cells will retain data for hundreds of
> milliseconds, while other cells will retain data for tens of seconds,
> and they don't always stay in the "hundreds of millisecond" bit or the
> "tens of seconds" bin.
>
> Ravi Venkatesan's paper has some numbers of DRAM cell retention time
> characteristics [Venkatesan2006].
>
> What this paper doesn't talk about, and what will hurt you is the
> Variable Retention Time (VRT) characteristics of DRAM cells. That is,
> a given DRAM cell can retain data for tens of seconds most of the
> time, but once in a while, it can become a leaky cell that only
> retains data for tens of milliseconds. End users sometimes refer to
> this as being a "weak bit".
> [Yaney1987,Restle1992,Ueno1998,Mori2005,Kim2004]
>
> Now, if you're trying to use the DRAM device as a SEU detector of some
> sort, it depends on how much radiation you expect.  If there are a lot
> of radiation in your environment, then you don't need to do a lot of
> work beforehand to prepare your sample. If, however, you want to
> measure something that's very subtle, and maybe someone that would
> occur no more frequent than once per X minutes, then you'd really have
> to spend a couple of months with a DRAM device and a tester in a cave
> 50 feet below ground (need to make sure that there are no neutrons
> hitting the DRAM while you're characterising it), then characterise it
> to the level so that you'll be able say with some level of
> mathematical confidence that you know where all the weak bits in the
> DRAM device are.
>
> Then, once you know what your device looks like, then you take it to
> the environment where you want to use it to measure your SEU rate,
> then you'd be able to (to some degree) distinguish between a cell that
> failed "early" because it has some built-in VRT characteristic, as
> opposed to a cell that failed because of a SEU.
>
> Good luck
> David
>
> @INPROCEEDINGS{Venkatesan2006, author = {Ravi K. Venkatesan, Stephen
> Herr, Eric Rotenberg}, title = {Retention-Aware Placement in DRAM
> (RAPID):Software Methods for Quasi-Non-Volatile DRAM}, booktitle =
> {Proceedings of the 12th International Symposium on High Performance
> Computer Architecture}, year = {2006}, pages = {157-167}}
>
> @INPROCEEDINGS{Yaney1987, author = {D. S. Yaney, C. Y. Lu, R. A.
> Kohler, M. J. Kelly, J. T. Nelson}, title = {A Meta-Stable Leakage
> Phenonmenon in DRAM Charge Storage - Variable Hold Time}, booktitle =
> {International Electron Devices Meeting Technical Digest}, year =
> {1987}, pages = {336-338}}
>
> @INPROCEEDINGS{Restle1992, author = {P. J. Restle, J. W. Park, B. F.
> Lloyd}, title = {DRAM Variable Retention Time}, booktitle =
> {International Electron Devices Meeting Technical Digest}, year =
> {1992}, pages = {807-810}}
>
> @INPROCEEDINGS{Ueno1998, author = {S. Ueno, T. Yamashita, H. Oda, S.
> Komori, Y. Inoue, T. Nishimura}, title = {Leakage Current Observation
> on Irregular Local Pn Junction Forming the Tail Distribution of DRAM
> Retention Time Characteristics}, booktitle = {International Electron
> Devices Meeting Technical Digest}, year = {1998}, pages = {153-156}}
>
> @INPROCEEDINGS{Mori2005, author = {Yuki Mori, Kiyonori Ohyu, Kensuke
> Okonogi, Ren-ichi Yamada}, title = {The Origins of Variable Retention
> Time in DRAM}, booktitle = {International Electron Devices Meeting
> Technical Digest}, year = {2005}, pages = {1057-1060}}
>
> @INPROCEEDINGS{Kim2004, author = {Y. I. Kim, K. H. Yang, W. S. Lee},
> title = {Thermal Degradation of DRAM Retention Time: Characterization
> and Improving Techniques}, booktitle = {Proceedings of the 42nd Annual
> International Reliability Physics Symposium}, year = {2004}, pages =
> {667-668}}- Hide quoted text -
>
> - Show quoted text -

You brought up some interesting points that I didn't know. I knew that
different cells had different retention times but I was not aware
there was variation in the same cell. That's definitely a problem...