8.1.1 Moving Interrupts Between Application and Boot Space
The MCU control register controls the placement of the interrupt vector table.
8.1.2 MCU Control Register – MCUCR
Bit
7
SPIPS
R/W
0
6
–
5
–
4
3
–
2
–
1
IVSEL
R/W
0
0
IVCE
R/W
0
PUD
R/W
0
MCUCR
Read/Write
Initial Value
R
0
R
0
R
0
R
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 determined by the BOOTSZ fuses. Refer to Section 24. “Boot Loader Support – Read-while-write
Self-Programming ATmega16/32/64/M1/C1” on page 241 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 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 Section 24. “Boot Loader Support – Read-while-write Self-Programming ATmega16/32/64/M1/C1” on
page 241 for details on boot lock bits.
• Bit 0 – IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware four cycles after it
is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as explained in the IVSEL description above.
See code example below.
Assembly Code Example
Move_interrupts:
; Enable change of Interrupt Vectors
ldi
out
r16, (1<<IVCE)
MCUCR, r16
; Move interrupts to Boot Flash section
ldi
out
ret
r16, (1<<IVSEL)
MCUCR, r16
C Code Example
void Move_interrupts(void)
{
/* Enable change of Interrupt Vectors */
MCUCR = (1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = (1<<IVSEL);
}
50
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
