Rainbow Electronics ATmega128L - Manual (Page 272)

272
ATmega128(L)
2467B–09/01
Figure 134. Addressing the Flash During SPM (1)
Note: 1. The different variables used in Figure 134 are listed in Table 114 on page 277.
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 operation is addressing the same page. See
“Simple Assem-
bly Code Example for a Boot Loader” on page 275 for an assembly code example.
PROGRAM MEMORY
0 1 15
Z - REGISTER
BIT
0
ZPAGEMSB
WORD ADDRESS
WITHIN A PAGE PAGE ADDRESS
WITHIN THE FLASH
ZPCMSB
INSTRUCTION WORD
PAGE
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORD PCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER