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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I'm learning how to access SPI-based memory cards like SD or MMC. I have found some picbasic routines that work in FAT16 mode, allowing you to create,detele,open,close,reaname files from the PIC. The drawback of those routines is that almost all of the programming memory is used. I don't need a real file sistem, just need to access a file and append some data (#### <tab> ###.##). Anyone of you have esperimented such devices?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
If the card must be readable in a general purpose reader then you will likely have to make the format of the contents conform to some industrial standard.
If the card will only ever be read or written in your own embedded system, then you can store the data any way you like!
Bare in mind though that structures like FAT were invented to allow you to open files, append data, delete data, close files, recover unused storage space etc. So if you need to be able to perform these functions you might end up reinventing FAT in the long run anyway.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I have done some experiments with memory cards. It is possible to open a device (floppy, harddrive, memory card) in raw mode in most operating systems so you read the bytes on the card without any file systems, so the PIC can just write the data directly to the card. I wrote a program for dumping the raw content of a card some time ago, so I should have the source code somewhere.
You can also put a large file on the card that contains a known pattern. Then the PIC can search for the pattern and with some care put the data there and it will appear in the file when you read it on your PC.
So for simple logging no FAT support in the PIC is really needed.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> You can also put a large file on the card that contains a known pattern. Then the PIC can search for the pattern and with some care put the data there and it will appear in the file when you read it on your PC.
Aren't there things like checksums or CRC's that would also need to be updated when poking values into an existing file in this manner?
Also surely the FAT entry would need to be updated too to indicate the new length of the file when you add more data?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Yes, if all you need to do is transfer data from your pic to your computer its a lot easier than implementing the whole file system. You can simply format the card fat32, and create a empty text file that fills the partition. The starting address for the file is a constant value (google can tell you where to find it) and the file will be stored contiguously on the partition (just don't write past the end of the file). Then you can simply open the file when you bring the card back to a computer and read your data.
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Logging raw lines of data in sd card through pic microcontroller (wind speed datalogger)
