Datasheet
BLB1
Mode
BLB12 BLB11 Protection
3 0 0 SPM is not allowed to write to the Boot Loader section, and LPM executing
from the Application section is not allowed to read from the Boot Loader
section. If Interrupt Vectors are placed in the Application section, interrupts
are disabled while executing from the Boot Loader section.
4 0 1 LPM executing from the Application section is not allowed to read from the
Boot Loader section. If Interrupt Vectors are placed in the Application
section, interrupts are disabled while executing from the Boot Loader
section.
Note: “1” means unprogrammed, “0” means programmed.
27.7. 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 program space is larger than
64Kbytes.
ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8
ZL (R30) Z7 Z6 Z5 Z4 Z3 Z2 Z1 Z0
7 6 5 4 3 2 1 0
RAMPZ7
RAMPZ6 RAMPZ5
RAMPZ4
RAMPZ3
RAMPZ2 RAMPZ1
RAMPZ0
Bit
23
22 21
20 19 18
17
16
15
14
13
12 11
10 9 8
Since the Flash is organized in pages, 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 the following figure. 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.
Atmel ATmega644A [DATASHEET]
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