ATmega325P datasheet(261/364 Pages) ATMEL

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261

8023F–AVR–07/09

ATmega325P/3250P

24.6

Entering the Boot Loader Program

Entering the Boot Loader takes place by a jump or call from the application program. This may

be initiated by a trigger such as a command received via USART, or SPI interface. Alternatively,

the Boot Reset Fuse can be programmed so that the Reset Vector is pointing to the Boot Flash

start address after a reset. In this case, the Boot Loader is started after a reset. After the applica-

tion code is loaded, the program can start executing the application code. Note that the fuses

cannot be changed by the MCU itself. This means that once the Boot Reset Fuse is pro-

grammed, the Reset Vector will always point to the Boot Loader Reset and the fuse can only be

changed through the serial or parallel programming interface.

Note:

1. “1” means unprogrammed, “0” means programmed

24.7

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 25-11 on page 276), 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 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 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.

Table 24-4.

Boot Reset Fuse

(1)

BOOTRST

Reset Address

Reset Vector = Application Reset (address 0x0000)

Reset Vector = Boot Loader Reset (see Table 24-6 on page 268)

Bit

151413121110

ZH (R31)

Z15

Z14

Z13

Z12

Z11

Z10

ZL (R30)

Z7Z6Z5

Z4Z3

Z1Z0

765

432

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