ATMEGA8 datasheet(14/331 Pages) ATMEL

14 / 331 page

2486AA–AVR–02/2013

ATmega8(L)

Figure 5 shows the parallel instruction fetches and instruction executions enabled by the Har-

vard architecture and the fast-access Register File concept. This is the basic pipelining concept

to obtain up to 1MIPS per MHz with the corresponding unique results for functions per cost,

functions per clocks, and functions per power-unit.

Figure 5. The Parallel Instruction Fetches and Instruction Executions

Figure 6 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 6. Single Cycle ALU Operation

Reset and

Interrupt Handling

The Atmel

®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 inter-

rupts 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

“Memory Programming” on page 215 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 “Interrupts” on page 46. 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 Inter-

rupt Vector Select (IVSEL) bit in the General Interrupt Control Register (GICR). Refer to

“Interrupts” on page 46 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 202.

clk

1st Instruction Fetch

1st Instruction Execute

2nd Instruction Fetch

2nd Instruction Execute

3rd Instruction Fetch

3rd Instruction Execute

4th Instruction Fetch

CPU

Total Execution Time

ALU Operation Execute

Result Write Back

clk

CPU

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