microsd flash structure

[SirNick]

If you have some sort of dying-gasp notification anyway, would it be possible to log to RAM and bulk-write on shutdown?

[trevwhite]

Just a follow up on this thread. I used a dedicated flash ic in the end and rolled my own very simple filesystem. I only needed append and read operations so it was relatively straight forward.

Thanks everyone for your comments. I think you saved me a lot of problems with the SD Card approach and this flash ic based version is a lot faster and more robust.

[Jeroen3]

What technique did you use for wear leveling?

[trevwhite]

I do not have wear issues to worry about. My need is just to keep taking measurements and append the data each time to my current position memory. I will probably erase data several times a year. When I do I will just erase all sectors and start again. I am not taxing this memory at all..

[tonyarkles]

My somewhat hacky solution would be to have a series of headers like this:

Code: [Select]

header:
- block number
- address of next block
- checksum

record:
- num bytes of data
- [data]
- checksum

When you power on, you read the first sector of your file, and follow the pointer to the address of the next header. Try reading the data there, and if it passes the checksum, repeat until you reach a block that doesn't pass the checksum. You now know that you've found the last block of valid data. Go back to the last valid header, and start reading data records. Keep going until you find a record that doesn't pass its checksum. That's where you start writing from.

It's not perfect, but given the constraints it should probably work reasonably well.

[Jeroen3]

A linked list approach is an nice linear way of using the available flash. Better than FAT.
I would also use fixed sized blocks, if for some reason the crc fails the next/prev address could be invalid. You don't want to lose all data in the following sectors. If it does, you can still reassemble the remaining blocks by scanning *all* supposed block positions.