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ATmega1284 Datasheet

8-bit AVR Microcontrollers

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8.7.
Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR CPU is
driven by the CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal
clock division is used. The Figure below shows the parallel instruction fetches and instruction executions
enabled by the Harvard 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 8-5. The Parallel Instruction Fetches and Instruction Executions
T1 T2
T3
T4
clkCPU
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
The following Figure 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 8-6. Single Cycle ALU Operation
T1
T2 T3
T4
clkCPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
8.8.
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.
The lowest addresses in the program memory space are by default defined as the Reset and Interrupt
Vectors. They have determined priority levels: 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 IVSEL bit in the MCU Control Register
(MCUCR). The Reset Vector can also be moved to the start of the Boot Flash section by programming
the BOOTRST Fuse.
Atmel ATmega1284 [DATASHEET]
Atmel-42718B-ATmega1284_Datasheet_Complete-08/2016
26
ATMEL
ATMEL


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