ATmega16U4 datasheet(62/408 Pages) ATMEL

62 / 408 page

7766D–AVR–01/09

ATmega16U4/ATmega32U4

Note:

1. The Boot Reset Address is shown in Table 27-8 on page 346. For the BOOTRST Fuse “1”

means unprogrammed while “0” means programmed.

9.1.1

Moving Interrupts Between Application and Boot Space

The General Interrupt Control Register controls the placement of the Interrupt Vector table.

9.1.2

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 “Memory Programming” on page 348 for

details. To avoid unintentional changes of Interrupt Vector tables, a special write procedure must

be followed to change the IVSEL bit:

Write the Interrupt Vector Change Enable (IVCE) bit to one.

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

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 the section “Memory Programming” on page 348

for details on Boot Lock bits.

• Bit 0 – IVCE: Interrupt Vector Change Enable

Table 9-2.

Reset and Interrupt Vectors Placement(1)

BOOTRST

IVSEL

Reset Address

Interrupt Vectors Start Address

0x0000

0x0002

0x0000

Boot Reset Address + 0x0002

Boot Reset Address

0x0002

Boot Reset Address

Boot Reset Address + 0x0002

Bit

JTD

–

PUD

–

IVSEL

IVCE

MCUCR

Read/Write

R/W

Initial Value

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