Datasheet

285
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
26.7 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 implemented when the pro-
gram space is larger than 64K bytes.
Note: 1. Apply only to ATmega1284/1284P
Since the Flash is organized in pages (see Table 27-7 on page 301), 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 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 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 26-3. Addressing the flash during SPM
(1)
.
Note: 1. The different variables used in Figure 26-3 are listed in Table 26-12 on page 293.
Bit 2322212019181716
15 14 13 12 11 10 9 8
RAMPZ
(1)
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