Electronics > Projects, Designs, and Technical Stuff
DDR2 chip how slow can you go?
jbb:
Some words of warning when using external SDRAM:
* Thorough testing is suggested with low / high supply voltage and low / high temperature to make sure everything is configured correctly
* It's likely slower than internal RAM, so you'll need to do some reading about linker behaviour make sure your most critical storage - e.g. stack and critical RTOS tables - are in on-chip RAM
* If you don't want people copying your code, don't run code from external RAM
* If you're doing any cryptographic security, make sure the keys stay in internal RAM
* SDRAM will increase your standby power consumption
nctnico:
--- Quote from: JohanHoltby on August 28, 2019, 09:53:57 pm ---Hi
I'm investigating how to store a lot of data. and need an as cheap as possible data storage solution. Flash is not an option since I will write it over multiple times every minute.
I'm lookng att an DDR2 chip but since I'm using a low voltage STM32 device which only has a clock speed of about 72mhz. How slow can I go without any problem?
--- End quote ---
IIRC DDR2 has two modes: with PLL enabled and PLL disabled. In the latter mode it can work at very slow speeds. The slow speeds are used to load test patterns into the memory which are then used to compensate the delays at the normal operating speed. Needless to say the memory will work just fine for as long as you adhere to the refresh intervals (and auto refresh is an easy way to do this). However a big nand-flash connected to SPI might be another option. You'll need to have BCH error correction and be able to detect bad blocks but even at many overwrites it will take long to wear the flash out. If you don't need any long term data retention then flash wear may not even be an issue.
ejeffrey:
--- Quote from: jbb on August 29, 2019, 07:12:53 pm ---
* If you're doing any cryptographic security, make sure the keys stay in internal RAM
--- End quote ---
Internal keys are only slightly more secure than external keys. If you need physical tamper-proof cypto you should be using a special purpose crypto processor.
daqq:
How about using a small FPGA? If this is for your all-seeing-scope project ( Here ) then things might actually get simpler (and comparably priced) with the proper FPGA. Look around, lattice has fairly cheap FPGAs ( https://www.digikey.sk/products/en/integrated-circuits-ics/embedded-fpgas-field-programmable-gate-array/696?FV=ffe002b8&quantity=0&ColumnSort=1000011&page=1&pageSize=25 ).
SiliconWizard:
--- Quote from: JohanHoltby on August 28, 2019, 09:53:57 pm ---Flash is not an option since I will write it over multiple times every minute.
--- End quote ---
You may want to consider whether you really need to do that to begin with, or at least explain to us why. DO you really need to store that much data multiple times every minute? What are you going to do with this data? Can you not store smaller chunks and process them accordingly instead?
Once you're sure you have no choice, you could still consider using Flash for that. Using an SD card or something would get you for relatively cheap so much more memory than a single RAM chip (the one you showed is 128MBytes if I'm not mistaken), that you could just use a very simple wear-leveling scheme (writing your blocks of data to a different area every time, and then cycle when you've reached the end of the SD card) that would get you a pretty decent lifetime.
Otherwise, if you're sure this is really not an option, SDRAM would be more adequate here than DDR2 as already said (but you'll have a hard time finding single SDRAM chips with that much memory, so maybe that's why you targetted DDR2?)
PSRAM would otherwise be an option too, but they usually come in more limited memory size. How much memory do you REALLY need?
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