Datasheet

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

9.2 Register Description

9.2.1 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 determined by the BOOTSZ Fuses. Refer to the

section “Memory Programming” on page 353 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

the section “Memory Programming” on page 353 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.

Bit 76543210

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