DOC7707 datasheet(233/307 Pages) ETC1

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233

7707F–AVR–11/10

AT90USB82/162

Note:

Only one SPM instruction should be active at any time.

23.6

Addressing the Flash During Self-Programming

The Z-pointer is used to address the SPM commands. The Z pointer consists of the Z-registers

ZL and ZH in the register file. The number of bits actually used is implementation dependent.

Since the Flash is organized in pages (see Table 25-11 on page 248), the Program Counter can

be treated as having two different sections. One section, consisting of the least significant bits, is

addressing the words within a page, while the most significant bits are addressing the pages.

This is shown in Figure 23-4. Note that the Page Erase and Page Write operations are

addressed independently. Therefore it is of major importance that the Boot Loader software

addresses the same page in both the Page Erase and Page Write operation. Once a program-

ming operation is initiated, the address is latched and the Z-pointer can be used for other

operations.

The (E)LPM instruction use the Z-pointer to store the address. Since this instruction addresses

the Flash byte-by-byte, also bit Z0 of the Z-pointer is used.

Figure 23-4. Addressing the Flash During SPM(1)

Note:

1. The different variables used in Figure 23-4 are listed in Table 23-10 on page 240.

23.7

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 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

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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