Datasheet

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

MCUCR – MCU Control

• 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 begin-

ning 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 “Memory Pro-

gramming” on page 342 for details. To avoid unintentional changes of Interrupt Vector

tables, a special write procedure must be followed to change the IVSEL bit (see “Moving

Interrupts Between Application and Boot Section” on page 74):

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 fol-

lowing 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 pro-

gramed, interrupts are disabled while executing from the Boot Loader section. Refer to

the section “Memory Programming” on page 342 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.

Bit 76543210

0x35 (0x55)

JTD – – PUD – – IVSEL IVCE MCUCR

Read/Write R/W RRR/W RRR/W R/W

Initial Value 0 0 0 0 0 0 0 0