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I currently have a 1Mb Parallel SRAM chip in a DIP-32 package. Putting that on a board is causing me all kinds of routing and space headaches. I know that there are nice 8 pin spi or i2c chips that have that capacity, but it seems that they are all eeproms or flash memory. I plan to overwrite data quite frequently and I know that most persistent storage has write limits and (I would assume) slightly slower speeds than vanilla SRAM. If I had an eeprom or something I would be performing a chip erase on each startup, and don't want a part that will wear out too soon. My attempts to find a suitable part on mouser or digikey has turned up nothing, but I am not sure what I am looking for.

Is there any kind of volatile storage that can be accessed quickly over some serial protocol? My MCU is running at 5v supply and logic, but I am willing to shift voltage down if necessary.

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I remember looking for something like this in the past and not finding it. If they can make large parallel RAM chips, why not SPI or IIC RAM chips with the same capacity? Microchip does make a few, but the capacity is very low compared to the parallel ones. – Olin Lathrop Dec 1 '11 at 19:26
The parallel ram chips are pretty big. Is it a space issue? All required to make it serial is a few shift registers, so it isn't really that difficult, or is it? – CMP Dec 1 '11 at 20:02
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Most of the space of the package goes to all those pins needed for parallel access. A serial RAM chip should be smaller, although the die would be about the same size. It think it's a volume thing. No manufacturer believes there is enough demand for that, and they're probably right. – Olin Lathrop Dec 1 '11 at 20:15
@CMP - A DIP-32 package is rather large. SMD packages are available (TSOP-32, -44, and -54 for example) that are much more compact. Are you willing to use these options? – Kevin Vermeer Dec 2 '11 at 14:00
I could but it is still a pain to route 17 address lines and 8 data lines. An 8 pin would be much simpler. – CMP Dec 2 '11 at 17:14

3 Answers

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The highest standard serial SRAM I have seen available for sale is 256Kb.

1Mb serial is available in FRAM though, here is an example part (from this page)

Note there is a lead time on the 1Mbit parts, but you might possibly be able to grab some samples from Ramtron. They do have the 512Kb part in stock.

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Call me naive, but what is the difference between sram and fram? – CMP Dec 1 '11 at 22:49
I don't know that much about it as it's pretty new, but the main difference from SRAM is probably the fact that it's non volatile. A bit like very fast, low power flash memory. Check the datasheet out, it should go into some detail on the operation. It will certainly do what you ask for in your question. I just had a quick look and it can do up to 40MHz SPI. – Oli Glaser Dec 2 '11 at 0:54
A bit more expensive than most eeproms of that capacity, but looks pretty good. Thanks. – CMP Dec 2 '11 at 20:13
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Check serial SRAM from microchip:

http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=2701&mid=&lang=en

Maybe this is what you need.

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The biggest they appear to have is 32K x 8. I would need 4 of those to get 1Mb. Not a huge problem, but not optimal either. – CMP Dec 1 '11 at 19:27
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The German company IPSilLog has a 512Kb (64x8) SRAM called the IP12B512 with an SPI bus in a TSSOP-8 package that they claim is in production. Can't find a US distributor for it though.

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