Datasheet
328
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
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 imple-
mented when the program space is larger than 64K bytes.
Since the Flash is organized in pages (see Table 155 on page 345), 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 137. Note that the Page Erase
and Page Write operations are addressed independently. Therefore it is of major impor-
tance 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 (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 137. Addressing the Flash During SPM
(1)
Note: 1. The different variables used in Figure 137 are listed in Table 142 on page 335.
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:
Bit 2322212019181716
15 14 13 12 11 10 9 8
RAMPZ RAMPZ7 RAMPZ6 RAMPZ5 RAMPZ4 RAMPZ3 RAMPZ2 RAMPZ1 RAMPZ0
ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8
ZL (R30)Z7Z6Z5Z4Z3Z2Z1Z0
76543210
PROGRAM MEMORY
0115
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
PCWORDPCPAGE
PCMSB
PAGEMSB
PROGRAM
COUNTER