Hi everyone,
I am really new to arduino and I have many questions. I want to save data from a 6-axis accelerometer in 100 Hz frequency for about 2 weeks. This means I need to save 600 numbers per second (something about 726 milion numbers). Now the question is how much memory I need? May I save them continuously on a SD card? The second question is which baord should I use in order to this (XD 240 data logger shield or ethernet shield W5100)?
ebarash:
This means I need to save 600 numbers per second (something about 726 milion numbers). Now the question is how much memory I need?
This depends on how your data is encoded.
If you save them as text, you might need at least 8 GB. On the other hand, if you save them as raw binary; you will only need half of that (assuming 32-bit values).
ebarash:
May I save them continuously on a SD card?
Why not? I mean, most of the known-brand SD cards have a built-in wear leveling system.
If you decide to encode the data as text, keep in mind that due to FAT32 limitations, files can't grow beyond 4294967295 bytes (roughly 4 GB); further write attempts will obviously fail, sometimes wrongly called as a full memory error by certain software (despite of still having free space).
If you still have space to spare, the only workaround will be to eventually split the recording into another file.
Don't even bother formatting the card to NTFS or exFAT (it's a bad idea to use the OS utilities for that anyways), the library doesn't support them.
ebarash:
The second question is which baord should I use in order to this (XD 240 data logger shield or ethernet shield W5100)?
I'm not sure if the hardware quality has any influence; if not, then it depends on your (current and maybe future) needs and budget. In the end, all it's required is just a way to connect the card to the Arduino in their corresponding pins, plus voltage regulation and level shifting because SD cards work at CMOS voltage (aka 3.3 volts or 3V3).
PD: are you sure the program can keep up that sampling rate? Does obtaining data from the accelerometer take less than 5 milliseconds?
I said 5 and not 10 because you need to leave some time for the SD library when it has to its thing (more exactly: a block update that occurs every 512 bytes or 22 samples if encoded as binary 32-bit values).