Atmel AT90PWM2 Manual page 272

24.7 Self-Programming the Flash
4317I–AVR–01/08
Figure 24-3. Addressing the Flash During SPM
BIT 15 ZPCMSB
Z - REGISTER
PCMSB
PROGRAM
COUNTER
PAGE ADDRESS
WITHIN THE FLASH
PROGRAM MEMORY
PAGE
Note: 1. The different variables used in
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 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 276
assembly code example.
(1)
ZPAGEMSB 1 0
0
PAGEMSB
PCPAGE PCWORD
WORD ADDRESS
WITHIN A PAGE
Figure 24-3 are listed in
AT90PWM2/3/2B/3B
PAGE
PCWORD[PAGEMSB:0]:
00
INSTRUCTION WORD
01
02
PAGEEND
Table 24-8 on page 279.
for an
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