ATMEGA8L ATMEL [ATMEL Corporation], ATMEGA8L Datasheet - Page 12

• Manufacturer Part Number: ATMEGA8L
• Description: 8-bit AVR with 8K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Instruction Execution
Timing
Reset and Interrupt
Handling
12
ATmega8(L)
This section describes the general access timing concepts for instruction execution. The
AVR CPU is driven by the CPU clock clk
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
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
Register Operands Fetch
2nd Instruction Execute
3rd Instruction Execute
1st Instruction Execute
ALU Operation Execute
2nd Instruction Fetch
3rd Instruction Fetch
4th Instruction Fetch
1st Instruction Fetch
Total Execution Time
Result Write Back
clk
clk
CPU
CPU
T1
T1
CPU
, directly generated from the selected clock
T2
T2
T3
T3
2486M–AVR–12/03
T4
T4