Memory Stick Drive Buffering

[LoneWolf]

Hi guys, so i have this idea (not new) to use Micro sdhc memory cards to stack them up and make a "MEMORY DRIVE" the reason for this is to learn, since i am limited as far as resources and money, i thought something like this should be cheap but fun, the concept is easy, get at least 4x 32GB Micro SDHC memory sticks, and make a circuit to connect them all in a single board (or memory stick if you'd like) so all of them can be accessed through a single usb, i was thinking about using demultiplexers, but that would require to manually switch the memory to be accessed, by using dipswitches or pushbuttons to grant access to an output port of the demultiplexer, I know there's not much point in doing this project other than learning, but that's the whole point, any suggestions on how to connect memories together but acces them through a single usb? in advance, thank you.

[Wilksey]

I am not 100% sure you can join multiple cards together like that natively.

My best suggestions are:
1) Use a uC to falsely report a large drive and use SPI on the SD cards to get the uC to write quickly to them, the uC can then check the address range that is being written / read and use the appropriate chip select.
2) Use a USB hub chip or a ATA controller chip and use the OS to try and make it see it as one big drive (like RAID maybe).

I would have thought the second option would be easiest.

Someone may have a better suggestion though?

Regards

Wilksey

[mike_au]

3) Use a USB hub controller and a number of card reader chips and present them to the OS as multiple devices, then use software RAID to combine them into a single drive.

3 would be easiest but it relies on software on the PC, and really it is a lot of work for what is essentially a USB hub with some card readers plugged into it.
1 would be doable, it would take a lot of work and a whole lot reading but I'm sure you would learn a lot about mass storage, USB, file systems, etc. in the process
I don't think 2 is possible

[LoneWolf]

I looked into the USB hub controller chip, and although a single chip could do all the work, it seems like a lot to learn before even start wiring one, i looked at the schematics of a USB controller chip and there's just too many things going on at the same time, and yes, essentially what im trying to do is to daisychain USB memory sticks, i guess using a USB hub and just use some sd card readers should do it, i'll go for these ones: http://www.amazon.com/SODIAL-TM-Memory-MicroSD-T-Flash/dp/B008YCHL76/ref=sr_sp-atf_title_1_19?ie=UTF8&qid=1375286332&sr=8-19&keywords=micro+sd+card+reader

[Gallymimu]

you shouldn't use SPI it is much much slower than the native SD card interface.  If you want to run the memory at full speed you probably need an FPGA or CPLD for the data transfers.  An ARM processor might be okay to but I don't use them often so I don't know their perpheral capabilities.

if you wand to present the array as a single usb drive you'll need to write your own filesystem drivers on the sd card side to create a JBOD array (which is probably easiest to implement).  It will be a good learning experience to fully understand a FAT filesystem, how to store it and remap things across multiple drives.  Not a small project though.