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ManualsBrandsMICROCHIP TECHNOLOGY ManualsMicrocontrollers, Microprocessors & QuartzesEmbedded microcontroller TQFP 32 (7x7) 8-Bit 16 MHz I/O number 23
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ATmega8A [DATASHEET]
8159E–AVR–02/2013
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 trig-
ger 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. 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 25-5 on page 210), the Program Counter can be treated as hav-
ing 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 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.
Table 24-4. Boot Reset Fuse
(1)
BOOTRST Reset Address
1 Reset Vector = Application Reset (address 0x0000)
0 Reset Vector = Boot Loader Reset (see Table 24-6 on page 204)
Bit 151413121110 9 8
ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8
ZL (R30) Z7Z6Z5Z4Z3Z2Z1Z0
76543210