31.8 Self-Programming the Flash

The program memory is updated in a page-by-page fashion. Before programming a page with the data stored in the temporary page buffer, the page must be erased. The temporary page buffer is filled one word at a time using SPM and the buffer can be filled either before the Page Erase command or between a Page Erase and a Page Write operation:

Alternative 1. Fill the Buffer Before a Page Erase

  • Fill temporary page buffer
  • Perform a Page Erase
  • Perform a Page Write

Alternative 2. Fill the Buffer After Page Erase

  • Perform a Page Erase
  • Fill temporary page buffer
  • Perform a Page Write

If only a part of the page needs to be changed, the rest of the page must be stored (for example in the temporary page buffer) before the erase, and then be rewritten. When using Alternative 1, the Boot Loader provides an effective Read-Modify-Write feature which allows the user software to first read the page, do the necessary changes, and then write back the modified data. If Alternative 2 is used, it is not possible to read the old data while loading since the page is already erased. The temporary page buffer can be accessed in a random sequence. It is essential that the page address used in both the Page Erase and Page Write operations are addressing the same page. Refer to Simple Assembly Code Example for a Boot Loader.