ATMEGA649_14 Datasheet, PDF (15/392 Pages) ATMEL Corporation – High Endurance Non-volatile Memory Segments

ATmega329/3290/649/6490
6.7 Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR
CPU is driven by the CPU clock clk_CPU, directly generated from the selected clock source for the
chip. No internal clock division is used.
Figure 6-4 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 1 MIPS per MHz with the corresponding unique results for functions per cost,
functions per clocks, and functions per power-unit.
Figure 6-4. The Parallel Instruction Fetches and Instruction Executions
T1
T2
T3
T4
clk_CPU
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
Figure 6-5 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 destina-
tion register.
Figure 6-5. Single Cycle ALU Operation
T1
T2
T3
T4
clk_CPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
6.8 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 “Memory Program-
ming” on page 293 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 49. The list also
determines the priority levels of the different interrupts. The lower the address the higher is the
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