Figure 103. Addressing the Flash during SPM(1)
BIT 15
Z - REGISTER
ZPCMSB
ZPAGEMSB
1
0
0
PCMSB
PAGEMSB
PCWORD
PROGRAM
COUNTER
PCPAGE
PAGE ADDRESS
WITHIN THE FLASH
WORD ADDRESS
WITHIN A PAGE
PROGRAM MEMORY
PAGE
PAGE
INSTRUCTION WORD
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
Notes: 1. The different variables used in the figure are listed in Table 84 on page 218.
2. PCPAGE and PCWORD are listed in Table 93 on page 224.
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 alternative 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
212
ATmega8(L)
2486M–AVR–12/03
ATMEL [ ATMEL ]
