I Need a large memory serial static RAM!

Hmmm, why are these so hard to find?

I already have 16megabit SPI serial EEPROMs, cheap enough and they come in a 8pin package.

I'm working on a datalogger app and need similar storage, ie 16Mbit or maybe 32Mbit, but don't want to use EEPROMs as this logger has internal rechargable battery so is never powered down, and I don't want the power and time costs of "saving" data to EEPROM, both of which are significant.

So what I am after is a static RAM that will be permently powered, and draws a handful of uA in standby. That is common enough, but I need it to have serial input so it only uses 3 or 4 input pins, and a small footprint ie 8 pin package preferred.

I checked Microchip as they have a serial static RAM but it is stupidly tiny! Can anyone please suggest a small pin count SPI serial static RAM with a large memory?

Thank you!

 

and I'd be thinking employing mass storage and a USB stick. Added bonus of portability to plug into your analysis software.

 

Weird but the largest chip I could find as serial RAM is only 128K.

Of course, an SD card also has a serial interface and comes in gigabyte sizes.

 

Have you considered SD cards?
http://en.wikipedia.org/wiki/Secure_Digital
Yeah, more pins than you're looking for, but the microSD card is really teensy, and you can interface with them several ways.
[eta]
ErnieM beat me to it...

 

Stupidly expensive, but might fill the bill?, FRAM.

 

Ferroelectric Random Access Memory???

Going to thread a bunch of iron donuts?

 

Correct! but it depends on your appetite.

EDIT: TI (bought out...ooops!) I mean uses Ramtron FRAM, and now you can eat the donuts onboard with a micro..mmmmmm.

 

Yeah, thanks Ernie that was similar to my experience. I can buy a 16000k or 32000k EEPROM in 8pin but am limited to a 128k RAM in 8 pin. Where's the justice?

Thanks for the suggestion (everybody!) of using SD card or uSD card etc, I have used those in a few apps but this is for a small encapsulated weatherproof logger, and that means no card socket.

I also don't particularly like the reliability of SD cards in a hot environment, or the reliability of cards in general, especially when it could never be replaced. I also didn't want the high energy and time costs of EEPROM or cards; 20mA and XmS etc every 512 bytes.

Currently the best bet looks like the 16Mbit 8pin EEPROM, and I may just have to swallow the annoying current and time costs to save data to EEPROM.

It's a real bummer.

PS. Regarding the FRAM, isn't that just another type of EEPROM that still requires extra current and time for a "programming" cycle? If you know a FRAM that has 16Mbit in 8pin package and doesn't need a programming cycle please speak up.

 

Here is the data sheet for the Ramtron 2Mb device in an 8 pin package http://www.ramtron.com/files/datasheets/FM25V20_ds.pdf

I'm not in anyway suggesting FRAM fits all your needs. It is an alternative to eeprom, just like the sd card, or flash drive others have mentioned. It is synchronous to the SPI bus, so it is fast, and just so-so on power it looks like from the data sheet. Never used one, but have had a small I2C Ramtron chip in the parts bin for a long time.

 

SRAM needs at least 1mA while EEPROM needs about 20 to 30 mA when writing.
Byte write for large serial flash chips is about 10uS. I have used all of these 3 different technologies myself, and parallel SRAM as well.

What I suggest is to use SRAM as buffer, and write to serial FLASH when it is full.

Digital cameras etc. which do indeed need large RAM, all use 8bit or 16bit parallel chips.

 

Thanks Nickelflipper for the FRAM info, it's interesting but not ideal for this application I think.

Takao, thanks for the input. I have large parallel RAM chips in stock but they are very heavy on pin count, and size too once you stack up enough chips to get Mbytes.

The 16Mbit SPI EEPROMs I have in stock are 15mS 20mA for a write cycle to write 500 bytes, so by your terms "byte write time" is about 30uS. Still it's a bit power hungry for memory that could be low-current Static RAM.

 

PDIP chips? I have 4-bit SRAM ICs here, SOIC, you would hate them, they need 150mA, while only having 32K x 4 bit.

You can build a bank using serial SRAM, but for as many as you need for 16 Mbits, there is not much current saved anymore.

 

Perhaps an experiment in VHDL is in order -- roll your own SRAM + controller and package it inside a small FPGA with only the necessary pins? Here's some info on the memory controller with VHDL included..

 

To Takao; I have 128kbyte SRAMs here, they are reasonably low power but still high pin count, and multiple packages needed to get 16Mbit.

To Osx-addict; That's a clever suggestion but I don't think it will provide the 16Mbit of RAM needed in a 8pin package!