Benjamin Bromilow wrote:
> Hi all,
>
> I'm looking for some *relatively* large memory (eg around 1M)
> As I see it, DRAM has the advantage of fewer pins but the disadvantage of
> needing refreshing. SRAMs need more pins (nothing a multiplexer or a 16F877
> can't solve), but seem to be much much cheaper. I was amazed at how much
> the DRAMs where for a similar storage capacity. Why then would anyone therefore
> use DRAMs?? Am I missing something?

DRAMs tend to be ridiculously expensive when bought in small quantities,
because nobody buys them that way. The people who buy DRAM chips
purchase them in very large quantities (tens of thousands and up). Small
users are typically buying their DRAM in the form of things like memory
modules, not buying chips directly from the manufacturers.

Furthermore, a measly 1Mbit chip is a very SMALL one by current DRAM
standards; it's hopelessly obsolete. (About the smallest DRAM chip that
will actually find its way into a PC these days, for instance, is
64Mbits.) So you're paying a premium for buying something that they're
only reluctantly keeping in the product line at all, for the sake of
people who need them to repair old equipment.

Note that I stated the sizes above in megaBITS, not megaBYTES; convert
if needed.

Most gadgets that need only a small amount of RAM (and 1 meg is a small
amount now) use SRAM because it's easier to work with; no refresh to
worry about, and no critical row/column timing. But DRAM is cheaper when
you want a LOT of memory. Just compare the price of 256 megabytes of
SRAM chips to the price of a 256MB DIMM. The largest SRAM chip currently
in the Digi-Key catalog, for instance, is a Toshiba 8Mbit (512Kx16)
SRAM; in other words, one megabyte. It costs $20 in quantity 500, and
not much less in quantity 5000. To make 256 MB, you would need 256 of
them, for a total cost of over $5,000. You can buy a 256MB DIMM for
under $100 in quantity one; a 256MB DDR SDRAM non-parity DIMM sells for
$85.49 on crucial.com.

But suppose you only need one megabyte? The smallest module currently
available from Crucial is a 32 megabyte module; it sells for $20.69,
which is right around the same price as the 1 megabyte Toshiba chip, and
it will hook up to your microprocessor with less glue and less hassle[.
And if need less than a megabyte, you can buy a smaller SRAM chip
(Digikey offers a 4MB Toshiba chip that is about half the price of the
8MB), but you've bottomed out on the DRAM options.

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