273
4317I–AVR–01/08
AT90PWM2/3/2B/3B
Figure 24-3. Addressing the Flash During SPM
Note:
1. The different variables used in
are listed in
24.7
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 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
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
Содержание AT90PWM2
Страница 344: ...346 4317I AVR 01 08 AT90PWM2 3 2B 3B 31 1 SO24...
Страница 345: ...347 4317I AVR 01 08 AT90PWM2 3 2B 3B 31 2 SO32...
Страница 346: ...348 4317I AVR 01 08 AT90PWM2 3 2B 3B 31 3 QFN32...
Страница 347: ...349 4317I AVR 01 08 AT90PWM2 3 2B 3B...