ATMEGA325P-20MU(2006) View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

ATMEGA325P-20MU
(Rev.:2006)

8-bit Microcontroller with 32K Bytes In-System Programmable Flash

Atmel Corporation 

ATmega325P/3250P

Figure 23-3. Addressing the Flash During SPM(1)

BIT 15

Z - REGISTER

ZPCMSB

ZPAGEMSB 1 0

0

PCMSB

PROGRAM

COUNTER

PCPAGE

PAGE ADDRESS

WITHIN THE FLASH

PROGRAM MEMORY

PAGE

PAGEMSB

PCWORD

WORD ADDRESS

WITHIN A PAGE

PAGE

INSTRUCTION WORD

PCWORD[PAGEMSB:0]:

00

01

02

PAGEEND

23.8

Notes: 1. The different variables used in Figure 23-3 on page 263 are listed in Table 23-8 on page 269.

2. PCPAGE and PCWORD are listed in Table 24-11 on page 277.

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

buffer 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 267 for an assembly

code example.

8023A–AVR–12/06

263

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