6.4.2 Procedure

There are several options to implement file appending from the host application. All options are based on read → modify → write.

The assumption is that the host processor does not have enough resources to read the entire file to its space so the serial flash needs to be used as a mediator. The proposed approach is to keep two files that hold the same copy but in different time instances. The update/append is applied to the “older” instance (based on the “newer” instance) and upon successful update/append, it becomes the “newer” instance. So practically this approach implements rollback option in host level and does not make use of the file system fail-safe option.

The procedure for file appending is as follows:

1. Create a file and write some content into it. The file would be denoted as file1.
2. Initialize a counter to file1 denoted as file1_counter and initialize it to 1.
3. The other instance is denoted as file2 and its counter file2_counter is initialized to 0. The “newer” file at this point is file1 and its counter indicates it (file1_counter> file2_counter).
4. 4. Now, if an update/append is required:
   1. Look for the “older” instance. If (file1_counter<file2_counter) file1 is the “older” instance, otherwise it is file2.
   2. Open the “newer” file for read.
   3. Open the “older” file for write.
   4. Read from the “newer” and write to the “older”. At this point, any power fail would not corrupt the file as the “newer” instance is opened for read.
   5. When reading from “newer” and writing to “older” is done, apply the followings:
      1. Update/append to “older” instance.
      2. Close the “newer” instance.
      3. Close the “older” instance.
      4. Increase the counter of the “older” instance by 2 so it becomes the “newer” instance.
5. Go to step 4 for further operations.

Note that this approach does not use more space than a regular fail-safe file. The main difference is that this method implements fail-safe functionality in the host application. Additionally, write operations and flash erases are kept to a minimum.

Figure 4 illustrates the procedure:

Figure 4. File Appending Flow