24.6 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 25-12 on page 260), 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)
BIT
15
ZPCMSB
ZPAGEMSB 1 0
0
Z-register
PCMSB
PAGEMSB
PCWORD
Program
counter
PCPAGE
Page address
within the flash
Word address
within page
Program Memory
Page
Page
PCWORD[PAGEMSB : 0]
00
Instructions Word
01
02
PAGEEND
Note:
1. The different variables used in Figure 24-3 are listed in Table 24-9 on page 252.
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
246
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
