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8.2
Moving Interrupts Between Application and Boot Space
The General Interrupt Control Register controls the placement of the Interrupt Vector table.
8.2.1
MCU Control Register – MCUCR
• 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-
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 dis-
abled while executing from the Boot Loader section. Refer to the section
Read-While-Write Self-Programming” on page 321
for details on Boot Lock bits.
Bit
7
6
5
4
3
2
1
0
JTD
–
–
PUD
–
–
IVSEL
IVCE
MCUCR
Read/Write
R/W
R
R
R/W
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
