ATmega165 datasheet(239/336 Pages) ATMEL

239 / 336 page

239

ATmega165/V

2573G–AVR–07/09

Figure 110. Addressing the Flash During SPM

(1)

Note:

1. The different variables used in Figure 110 are listed in Table 98 on page 245.

PCPAGE and PCWORD are listed in Table 105 on page 249.

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

Assembly Code Example for a Boot Loader” on page 243 for an assembly code

example.

PROGRAM MEMORY

Z - REGISTER

BIT

ZPAGEMSB

WORD ADDRESS

WITHIN A PAGE

PAGE ADDRESS

WITHIN THE FLASH

ZPCMSB

INSTRUCTION WORD

PAGE

PCWORD[PAGEMSB:0]:

PAGEEND

PAGE

PCWORD

PCPAGE

PCMSB

PAGEMSB

PROGRAM

COUNTER

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