246
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
24.6
Addressing the Flash during Self-Programming
The Z-pointer is used to address the SPM commands.
Since the flash is organized in pages (see
Table 25-12 on page 261), 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 24-3. 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 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.
Figure 24-3. Addressing the Flash during SPM(1)
Note:
1.
24.7
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
Bit
1514
1312
11
10
9
8
ZH (R31)
Z15
Z14
Z13
Z12
Z11
Z10
Z9
Z8
ZL (R30)
Z7
Z6
Z5
Z4
Z3
Z2
Z1
Z0
76
54
321
0
BIT
PAGEMSB
PCMSB
ZPAGEMSB
ZPCMSB
0
1
15
Z-register
Program
counter
Word address
within page
Page address
within the flash
0
PCWORD
PCPAGE
02
01
00
PAGEEND
PCWORD[PAGEMSB : 0]
Page
Program Memory
Instructions Word
Page