16
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
Figure 3-5 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 destination register.
Figure 3-5.
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
The lowest addresses in the program memory space are by default defined as the reset and interrupt vectors. The complete list
The lower the address the higher is the priority level. RESET has the highest priority, and next is ANACOMP0 – the analog
comparator 0 interrupt. The interrupt vectors can be moved to the start of the boot flash section by setting the IVSEL bit in the
3.8.1
Interrupt Behavior
When an interrupt occurs, the global interrupt enable I-bit is cleared and all interrupts are disabled. 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 interrupt 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.
The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not necessarily have
interrupt flags. If the interrupt condition disappears before the interrupt is enabled, the interrupt will not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction before any
pending interrupt is served.
Note that the status register is not automatically stored when entering an interrupt routine, nor restored when returning from an
interrupt routine. This must be handled by software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No interrupt will be executed
after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following example shows how this can be
used to avoid interrupts during the timed EEPROM write sequence.
clk
CPU
T1
Register Operands Fetch
Result Write Back
ALU Operation Execute
Total Execution Time
T2
T3
T4