Datasheet

31.6 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 application code is loaded, the program can

start executing the application code. The fuses cannot be changed by the MCU itself. This means that

once the boot Reset fuse is programmed, 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.

Table 31-4. Boot Reset Fuse

BOOTRST Reset Address

1 Reset vector = application Reset (address 0x0000)

0 Reset vector = boot loader Reset, as described by the boot loader parameters

Note: '1' means unprogrammed, '0' means programmed.

31.7 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. The number of bits actually used is implementation dependent.

Bit 15 14 13 12 11 10 9 8

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

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.

ATmega48PA/88PA/168PA

Boot Loader Support – Read-While-Write Self-...

Datasheet Complete

DS40002011A-page 351