ATmega328P datasheet(282/448 Pages) ATMEL

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282

8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

26.7

Addressing the Flash During Self-Programming

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

Since the Flash is organized in pages (see Table 27-11 on page 299), 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 is1 shown in Figure 26-3. 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 only SPM operation that does not use the Z-pointer is Setting the Boot Loader Lock bits.

The content of the Z-pointer is ignored and will have no effect on the operation. The LPM

instruction does also use the Z-pointer to store the address. Since this instruction addresses the

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

Figure 26-3. Addressing the Flash During SPM(1)

Note:

1. The different variables used in Figure 26-3 are listed in Table 26-9 on page 289.

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:

Bit

151413121110

ZH (R31)

Z15

Z14

Z13

Z12

Z11

Z10

ZL (R30)

Z7Z6Z5

Z4Z3Z2Z1

76543210

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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