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ATMEGA8L-8AI Datasheet(PDF) 12 Page - ATMEL Corporation |
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ATMEGA8L-8AI Datasheet(HTML) 12 Page - ATMEL Corporation |
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12 / 303 page  12 ATmega8(L) 2486M–AVR–12/03 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 5 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture and the fast-access Register File concept. This is the basic pipelin- ing 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 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 destination register. Figure 6. Single Cycle ALU Operation 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 Programming” on page 219 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 44. 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 clk 1st Instruction Fetch 1st Instruction Execute 2nd Instruction Fetch 2nd Instruction Execute 3rd Instruction Fetch 3rd Instruction Execute 4th Instruction Fetch T1 T2 T3 T4 CPU Total Execution Time Register Operands Fetch ALU Operation Execute Result Write Back T1 T2 T3 T4 clk CPU |
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