Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet

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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

Atmel-8271H-AVR- ATmega-Datasheet_08/2014

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

Since the Flash is organized in pages (see 

), 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 

. 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 
programming 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.

Addressing the Flash During SPM

Note:

1. The different variables used in 

 are listed in 

Bit

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

PROGRAM MEMORY

0

1

15

Z - REGISTER

BIT

0

ZPAGEMSB

WORD ADDRESS
WITHIN A PAGE

PAGE ADDRESS
WITHIN THE FLASH

ZPCMSB

INSTRUCTION WORD

PAGE

PCWORD[PAGEMSB:0]:

00

01

02

PAGEEND

PAGE

PCWORD

PCPAGE

PCMSB

PAGEMSB

PROGRAM
COUNTER