ATMEGA640 datasheet(108/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

14.4 Register Description

14.4.1

MCUCR – MCU 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 “Memory

Programming” on page 325 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 Sec-

tion” on page 107):

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 dis-

abled 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 sec-

tion “Memory Programming” on page 325 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

654

321

0x35 (0x55)

JTD

–

PUD

–

IVSEL

IVCE

MCUCR

Read/Write

R/W

Initial Value

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