ATmega48PA/88PA/168PA/328P
26.7 Addressing the Flash During Self-Programming
The Z-pointer is used to address the SPM commands.
Bit
15
Z15
Z7
7
14
Z14
Z6
6
13
Z13
Z5
5
12
Z12
Z4
4
11
Z11
Z3
3
10
Z10
Z2
2
9
Z9
Z1
1
8
Z8
Z0
0
ZH (R31)
ZL (R30)
Since the Flash is organized in pages (see Table 27-11 on page 299), 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 Figure 26-3. 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 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.
Figure 26-3. Addressing the Flash During SPM(1)
BIT 15
ZPCMSB
ZPAGEMSB
1
0
0
Z - REGISTER
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
Note:
1. The different variables used in Figure 26-3 are listed in Table 26-9 on page 289.
26.8 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 buf-
fer 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:
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8161D–AVR–10/09
ATMEL [ ATMEL ]
