ATmega1284P datasheet(281/380 Pages) ATMEL

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281

8059D–AVR–11/09

ATmega1284P

24.7

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, and RAMPZ in the I/O space. The number of bits actually used is

implementation dependent. Note that the RAMPZ register is only implemented when the pro-

gram space is larger than 64K bytes.

Since the Flash is organized in pages (see Table 25-7 on page 294), 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 24-3. 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 (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 24-3. Addressing the Flash During SPM(1)

Note:

1. The different variables used in Figure 24-3 are listed in Table 24-9 on page 288.

Bit

22212019

1716

RAMPZ

RAMPZ7

RAMPZ6

RAMPZ5

RAMPZ4

RAMPZ3

RAMPZ2

RAMPZ1

RAMPZ0

ZH (R31)

Z15

Z14

Z13

Z12

Z11

Z10

ZL (R30)

Z7Z6Z5Z4Z3Z2Z1Z0

765

43210

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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