ATmega64A datasheet(287/392 Pages) ATMEL

287 / 392 page

287

8160C–AVR–07/09

ATmega64A

Figure 26-3. Addressing the Flash during SPM

(1)Table 2 on page 287

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.

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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