AT43USB325 datasheet(8/109 Pages) ATMEL

8 / 109 page

3355C–USB–4/05

AT43USB325

The I/O memory space contains 64 addresses for CPU peripheral functions as Control Regis-

ters, Timer/Counters, and other I/O functions. The I/O Memory can be accessed directly, or as

the Data Space locations following those of the register file, $20 - $5F.

The AVR uses a Harvard architecture concept – with separate memories and buses for program

and data. The program memory is executed with a single-level pipelining. While one instruction

is being executed, the next instruction is pre-fetched from the program memory. This concept

enables instructions to be executed in every clock cycle. The program memory is a download-

able SRAM or a mask programmed ROM.

With the relative jump and call instructions, the whole 24K address space is directly accessed.

Most AVR instructions have a single 16-bit word format. Every program memory address con-

tains a 16- or 32-bit instruction.

During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the

stack. The stack is effectively allocated in the general data SRAM, and consequently, the stack

size is only limited by the total SRAM size and the usage of the SRAM. All user programs must

initialize the Stack Pointer (SP) in the reset routine (before subroutines or interrupts are exe-

cuted). The 10-bit SP is read/write accessible in the I/O space.

The 512-byte data SRAM can be easily accessed through the five different addressing modes

supported in the AVR architecture.

The memory spaces in the AVR architecture are all linear and regular memory maps. A flexible

interrupt module has its control registers in the I/O space with an additional global interrupt

enable bit in the status register. All interrupts have a separate interrupt vector in the interrupt

vector table at the beginning of the program memory. The interrupts have priority in accordance

with their interrupt vector position. The lower the interrupt vector address, the higher the priority.

AT43USB325 Datasheet(PDF) 8 Page - ATMEL Corporation