ATMEGA8_08 Datasheet, PDF (49/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

ATmega8(L)
Moving Interrupts
Between Application
and Boot Space
General Interrupt
Control Register –
GICR
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
$c01
rjmp RESET
rjmp EXT_INT0
; Reset handler
; 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
The General Interrupt Control Register controls the placement of the Interrupt Vector table.
Bit
7
6
5
4
3
2
1
0
INT1
INT0
–
–
–
–
IVSEL
IVCE
GICR
Read/Write
R/W
R/W
R
R
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
• 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 deter-
mined by the BOOTSZ Fuses. Refer to the section “Boot Loader Support – Read-While-Write
Self-Programming” on page 209 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, interrupts 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 pro-
grammed, 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 209 for details on Boot Lock Bits.
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