ATmega64A
Figure 26-3. Addressing the Flash during SPM(1)Table 2 on page 287
BIT 15
Z - REGISTER
ZPCMSB
ZPAGEMSB
1
0
0
PCMSB
PAGEMSB
PCWORD
PROGRAM
COUNTER
PCPAGE
PAGE ADDRESS
WITHIN THE FLASH
WORD ADDRESS
WITHIN A PAGE
PROGRAM MEMORY
PAGE
PAGE
INSTRUCTION WORD
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
Notes: 1. The different variables used in Figure 26-3 are listed in Table 26-7 on page 292.
2. PCPAGE and PCWORD are listed in Table 27-10 on page 300.
26.8 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 buf-
fer 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 Alter-
native 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 Assembly Code Example for a Boot Loader” on page 290 for an assembly
code example.
287
8160C–AVR–07/09
ATMEL [ ATMEL ]
