Is all SRAM created equal?

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For our prototypes we used STMicroelectronics SRAM 4Mbit, 512Kx8, 55ns
(M68AW511AL).

Now it is getting near production I find that they have stopped making that
memory. Lots of other companies make equivalent SRAM, but can I assume it is
going to work ok? Our application requires an access time of 60ns to the
SRAM, so within the SRAM spec.

In an ideal world I'd get one of the new types and prototype with it before
ordering, but lead times are so long at the moment. So I want to place an
order (1000 units) for our production run asap.

Is that a bad idea?

Thanks,

Jim



Re: Is all SRAM created equal?
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It is a bad idea to prototype something to "see if it works".  You need
to sit down with a paper and pencil (or spread sheet) and do the math on
all the timing requirements to see if the part will work in your
design.  SRAMs are very simple to analyze compared to DRAMs.  Just
hooking it up won't tell you unless there is a gross error.  The figures
in the data sheets are max over voltage, temp and process.  Any given
chip will run faster at room temp than worse case so you aren't really
testing it.  

If you don't know how to analyze the timing, you need to ask for help on
that.  If it passes timing analysis, then I seriously doubt that you
need to test a prototype to make sure it works.  But with a 1000 piece
order, wouldn't you feel safer?  You can always place the order and then
cancel it if the parts fail tests.  I don't think a vendor will give you
a hard time for that.  

--

Rick "rickman" Collins

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Re: Is all SRAM created equal?
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<snip>
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before
an

Thanks Rick for your advice - it sounds extremely sensible all that you say.
Yes, on paper we've calculated it should work fine. This is my first design
to go into production, so I'm pretty tentative about taking it forward
without testing the actual part. In my naivity I didn't even realise we
could cancel a parts order once it had gone through without a lot of
hassle/penalties. That would be worst case scenario and hopefully it
wouldn't come to that.

Jim




Re: Is all SRAM created equal?
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I won't say it is a bad idea, but if it was me, I would burn up the
phone lines trying to get my hands on some samples of the part to test
and also somebody that has some (any) stock on the part.  If the
samples check out then release the PO for the stock to ship.  You
might loose a week or two of your manufacturing time.

But even before I do that I'd take a very close look at the data
sheet(s) and compare timing diagrams and specs.  In general SRAM data
sheets of similar parts will have specs and timing diagrams done the
same way but sometimes you find somebody giving the same information
but in a slightly different way.  Be careful.  If I do get a warm
fuzzy feeling from the data sheet(s) then I would order the parts.
Still want the sample parts ASAP for testing and if I do find a
problem then I may have enough time to halt production or find a
work-around that can be integrated into that production run.

And finally I'd try to identify two or three (or more in some cases)
substitutes so I don't get pinched down the road.  Not always possible
but for SRAM I'd definately try.

By the way, one more thing.  If your company doesn't mind buying old
inventory you could try one of the hundreds of companies that deal in
surplus and seconded inventory.  One such broker I've had luck with is
M & R Communications 512-502-0783 and ask for Willy.  There are others
like America II, Harry Krantz, etc.  (Disclaimer: I don't work for M&R
or otherwise bennifit from mentioning their name, just gotten good
service).

None of these suggestions are pretty or will help you sleep at night
but it sounds like you are getting pinched.

Re: Is all SRAM created equal?
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Thanks Rob for your good advice. I hadn't realised at the design stage about
6 months ago that SRAM would be so hard to get hold of come production. Our
product is quite low cost so I picked the cheapest I could find at the time
(from STM).

I've got a few samples of various brands that should work on paper but won't
be able to try the out until our next prototype is ready in 3 weeks or so.
I'm itching to get the order in before then in case the situation gets
worse. As well as that, being in the UK, the dollar is so cheap for us at
the moment I want to spend some money now! ;)

I know I should probably hold off but don't want to hold up production when
the time comes.

Jim



Re: Is all SRAM created equal?
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You should get in touch with the FAE at your distributors.  They can
work with you to get parts to test and will be able to help you identify
any possible troble areas.  SRAMs are actually very, very compatible
parts.  So it is unlikely that you will see any problems switching to a
different brand.  Like Rob said, you need to qualify multiple brands of
parts for every socket on your board if possible.  

The distis like for you to lock in their brand, but they understand that
most manufacturers won't do that.  So work with them and get parts
qualified.  The shortage will only get worse, not better... at least for
the next few months.  

--

Rick "rickman" Collins

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Re: Is all SRAM created equal?
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Thanks for the insight Rick. I definitely don't want to limit the brands if
at all possible. This is the first time I've used a SRAM this big (to me
it's big anyway) so it's good to know, in general, they are pretty
interchangeable. We're not using them beyond their specs so there is no
reason why they shouldn't work ok - just a bit new to all this ;) Yes, I
suspected the shortage would get worse, so am going to try to get on top of
this before it bites us!

Jim



Re: Is all SRAM created equal?


<snip> I've got a few samples of various brands that should work on
paper but won't
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  You can always make the order conditional.
  Quote the device you have qualified, and also your timing (& any
other, such as Icc?) targets, and give an order, and also ask for
samples to fully qualify.
  That way, the wheels turn in parallel, and there is more clear
incentive for the disti to get samples - much more than 'might order if
they are OK'.

  -jg


Re: Is all SRAM created equal?
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Thanks Jim - that sounds like good advice. You can probably tell I am pretty
new to this so advice from people like yourself is very much appreciated.

Jim



Re: Is all SRAM created equal?
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Just be careful with this approach.  If you make the order conditional,
they may hold placing the order with the manufacturer until you
"approve" the parts.  Make it very clear what you expect to happen and
make sure the "product manager" at the disti is ok with it.  That is the
guy who will say yes or no to placing the order before you have
committed.  

--

Rick "rickman" Collins

snipped-for-privacy@XYarius.com
We've slightly trimmed the long signature. Click to see the full one.

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