8-bit AVR Microcontrollers

MCU Control Register

When addressing I/O registers as data space using LD and ST instructions, the provided offset must be used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an I/O address offset within 0x00 - 0x3F.

The device is a complex microcontroller with more peripheral units than can be supported within the 64 locations reserved in Opcode for the IN and OUT instructions. For the extended I/O space from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.

When addressing as I/O register: address offset is 0x35

Bit 6 – BODS: BOD Sleep

BOD Sleep

The BODS bit must be written to '1' in order to turn off BOD during sleep. Writing to the BODS bit is controlled by a timed sequence and the enable bit BODSE. To disable BOD in relevant sleep modes, both BODS and BODSE must first be written to '1'. Then, BODS must be written to '1' and BODSE must be written to zero within four clock cycles.

The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS is active in order to turn off the BOD for the actual sleep mode. The BODS bit is automatically cleared after three clock cycles.

Bit 5 – BODSE: BOD Sleep Enable

BOD Sleep Enable

BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable is controlled by a timed sequence.

Bit 4 – PUD: Pull-up Disable

Pull-up Disable

When this bit is written to one, the pull ups in the I/O ports are disabled even if the DDxn and PORTxn registers are configured to enable the pull ups ({DDxn, PORTxn} = 0b01).

Bit 1 – IVSEL: Interrupt Vector Select

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. To avoid unintentional changes of interrupt vector tables, a special write procedure must be followed to change the IVSEL bit:

  1. 1.Write the Interrupt Vector Change Enable (IVCE) bit to one.
  2. 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 same cycle as IVCE is written, and interrupts 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 programmed, interrupts are disabled while executing from the Boot Loader section.

Bit 0 – IVCE: Interrupt Vector Change Enable

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 the code example below.
Assembly Code Example
in    r16, MCUCR
mov   r17, r16
; Enable change of Interrupt Vectors
ori   r16, (1<<IVCE)
out   MCUCR, r16
; Move interrupts to Boot Flash section
ori   r17, (1<<IVSEL)
out   MCUCR, r17
C Code Example
void Move_interrupts(void)
uchar temp;
temp = MCUCR;
/* Enable change of Interrupt Vectors */
MCUCR = temp|(1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = temp|(1<<IVSEL);