15
7679H–CAN–08/08
AT90CAN32/64/128
shows the internal timing concept for the Register File. In a single clock cycle an ALU
operation using two register operands is executed, and the result is stored back to the destina-
tion register.
Figure 3-6.
Single Cycle ALU Operation
3.8
Reset and Interrupt Handling
The AVR provides several different interrupt sources. These interrupts and the separate Reset
Vector each have a separate program vector in the program memory space. All interrupts are
assigned individual enable bits which must be written logic one together with the Global Interrupt
Enable bit in the Status Register in order to enable the interrupt. Depending on the Program
Counter value, interrupts may be automatically disabled when Boot Lock bits BLB02 or BLB12
are programmed. This feature improves software security. See the section
for details.
The lowest addresses in the program memory space are by default defined as the Reset and
Interrupt Vectors. The complete list of vectors is shown in
. The list also
determines the priority levels of the different interrupts. The lower the address the higher is the
priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request
0. The Interrupt Vectors can be moved to the start of the Boot Flash section by setting the IVSEL
bit in the MCU Control Register (MCUCR). Refer to
for more information.
The Reset Vector can also be moved to the start of the Boot Flash section by programming the
BOOTRST Fuse, see
“Boot Loader Support – Read-While-Write Self-Programming” on page
3.8.1
Interrupt Behavior
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are dis-
abled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled
interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a
Return from Interrupt instruction – RETI – is executed.
There are basically two types of interrupts. The first type is triggered by an event that sets the
interrupt flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vector
in order to execute the interrupt handling routine, and hardware clears the corresponding inter-
rupt flag. Interrupt flags can also be cleared by writing a logic one to the flag bit position(s) to be
cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is cleared,
the interrupt flag will be set and remembered until the interrupt is enabled, or the flag is cleared
by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt Enable
bit is cleared, the corresponding interrupt flag(s) will be set and remembered until the Global
Interrupt Enable bit is set, and will then be executed by order of priority.
T
otal Execution T
ime
Register Operands Fetch
ALU Operation Execute
Result W
rite Back
T1
T2
T3
T4
clk
CPU
