uSD card + board power off + prevent data loss

Posted on July 23, 2014 at 22:35

One of the real problems is that writing the data doesn't update the file system data in the directory, you'd need to periodically flush or close the file.

You might also want to preallocate the file space so all the FAT entries are assigned and written, and then you back fill the data, and truncate later.

To restart you either need to hold a value, or be able to navigate through the file to find where you stopped.

You might want to add your own fsck/chkdsk type functionality to recover cards where data/fat entries are present, but not reflected in the directory.

Finally, one really might want to look at having some amount of backup power, and the ability to detect if the primary supply is going down. Once sensed you should commit all pending data, and close gracefully, before shutting down. See also the Card Detect pin and if you can see that is changing before the card is removed to the point the connection is lost.

Posted on July 25, 2014 at 02:37

Thanks. To some extent I have solved the problems 1 and 3 written above, using card detect pin, f_sync function periodically. Also I was able to create & write to two files ( debug msg, data log).

Still experimenting the code on the problem 2. 

Posted on July 25, 2014 at 17:16

Is the problem with remembering the name or the last write location?

Seeking to the end of a file shouldn't be that hard. For a file name perhaps you could have your own private tracking file, or use NVRAM to store things, or structures as space permits.

Depending on the naming, or support for timing, you could enumerate files in a directory and find the last one.