Datasheet

11.12.2. 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.
Name: MCUCR
Offset: 0x55
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x35
Bit 7 6 5 4 3 2 1 0
JTD PUD IVSEL IVCE
Access
R/W R/W R/W R/W
Reset 0 0 0 0
Bit 7 – JTD
When this bit is zero, the JTAG interface is enabled if the JTAGEN Fuse is programmed. If this bit is one,
the JTAG interface is disabled. In order to avoid unintentional disabling or enabling of the JTAG interface,
a timed sequence must be followed when changing this bit: The application software must write this bit to
the desired value twice within four cycles to change its value. Note that this bit must not be altered when
using the On-chip Debug system.
Bit 4 – PUD: 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
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. 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.
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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