AT43USB325 datasheet(10/109 Pages) ATMEL

10 / 109 page

3355C–USB–4/05

AT43USB325

3.1

X-, Y- and Z- Registers

Registers R26..R31 contain some added functions to their general-purpose usage. These regis-

ters are address pointers for indirect addressing of the Data Space. The three indirect address

registers X, Y, and Z are defined as:

In the different addressing modes these address registers have functions as fixed displacement,

automatic increment and decrement (see the descriptions for the different instructions).

3.2

Arithmetic Logic Unit (ALU)

The high-performance AVR ALU operates in direct connection with all 32 general-purpose work-

ing registers. Within a single clock cycle, ALU operations between registers in the register file

are executed. The ALU operations are divided into three main categories – arithmetic, logical

and bit-functions.

3.3

Program Memory

The AT43USB325E contains 16K bytes on-chip downloadable memory for program storage

while the AT43USB325M has a masked programmable ROM. Since all instructions are 16- or

32-bit words, the program memory is organized as 8K x 16. The AT43USB325 Program Counter

(PC) is 13 bits wide, thus addressing the 8,192 program memory addresses.

Constant tables can be allocated within the entire program memory address space (see the LPM

- Load Program Memory instruction description).

The program memory of the AT43USB325E is automatically written with data stored in an exter-

nal serial EEPROM during the chip's power on reset sequence. The power on reset is the only

way the on-chip program memory of the AT43USB325E will be written or modified.

The two versions of the AT43USB325 are binary compatible. A firmware written for the

AT43USB325E will work unaltered on the AT43USB325M. The only functional difference

X-register

R27 ($1B)

R26 ($1A)

Y-register

R29 ($1D)

R28 ($1C)

Z-register

R30 ($1F)

R31 ($1E)

AT43USB325 Datasheet(PDF) 10 Page - ATMEL Corporation