Datasheet

47
ATmega8A [DATASHEET]
8159E–AVR–02/2013
When the BOOTRST Fuse is programmed, the boot section size set to 2K bytes, and the IVSEL bit in the GICR
Register is set before any interrupts are enabled, the most typical and general program setup for the Reset and
Interrupt Vector Addresses is:
AddressLabels Code Comments
;
.org $c00
$c00 rjmp RESET ; Reset handler
$c01 rjmp EXT_INT0 ; IRQ0 Handler
$c02 rjmp EXT_INT1 ; IRQ1 Handler
:. :. :. ;
$c12 rjmp SPM_RDY ; Store Program Memory Ready Handler
$c13 RESET: ldi r16,high(RAMEND); Main program start
$c14 out SPH,r16 ; Set Stack Pointer to top of RAM
$c15 ldi r16,low(RAMEND)
$c16 out SPL,r16
$c17 sei ; Enable interrupts
$c18 <instr> xxx
12.1.1 Moving Interrupts Between Application and Boot Space
The General Interrupt Control Register controls the placement of the Interrupt Vector table.
12.2 Register Description
12.2.1 GICR – General Interrupt Control Register
• Bit 1 – IVSEL: Interrupt Vector Select
When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory. When this
bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of the Flash. The actual
address of the start of the boot Flash section is determined by the BOOTSZ Fuses. Refer to the section “Boot
Loader Support – Read-While-Write Self-Programming” on page 194 for details. To avoid unintentional changes of
Interrupt Vector tables, a special write procedure must be followed to change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the cycle IVCE
is set, and they remain disabled until after the instruction following the write to IVSEL. If IVSEL is not written, inter-
rupts remain disabled for four cycles. The I-bit in the Status Register is unaffected by the automatic disabling.
Note: If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed, interrupts
are disabled while executing from the Application section. If Interrupt Vectors are placed in the Application section
and Boot Lock bit BLB12 is programed, interrupts are disabled while executing from the Boot Loader section. Refer
to the section “Boot Loader Support – Read-While-Write Self-Programming” on page 194 for details on Boot Lock
Bits.
Bit 76543210
INT1 INT0 – – – – IVSEL IVCE GICR
Read/Write R/W R/W R R R R R/W R/W
Initial Value00000000