AT43USB353M datasheet(7/95 Pages) ATMEL

7 / 95 page

AT43USB353M

3307B–USB–4/04

As shown in Table 2, each register is also assigned a data memory address, mapping them

directly into the first 32 locations of the user Data Space. Although not being physically imple-

mented as SRAM locations, this memory organization provides great flexibility in access of the

registers, as the X-, Y-, and Z-registers can be set to index any register in the file.

X-, Y- and Z-

Registers

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

isters 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 displace-

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

ALU – Arithmetic

Logic Unit

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

working 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, log-

ical and bit-functions.

Program Memory

The AT43USB353M contains 24K bytes on-chip masked programmable ROM program mem-

ory. Since all instructions are 16- or 32-bit words, the program memory is organized as 12K x

16. The AT43USB353M Program Counter (PC) is 14 bits wide, thus addressing the 12,288

program memory addresses.

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

LPM - Load Program Memory instruction description).

Where the functions overlap, the AT43USB353M and the AT43USB355 are binary compati-

ble. A firmware written for the AT43USB355 will work unaltered on the AT43USB353M as long

as the functions are supported by the AT43USB353M. The only functional difference between

the two devices are listed in Table 3.

X-register

R27 ($1B)

R26 ($1A)

Y-register

R29 ($1D)

R28 ($1C)

Z-register

R30 ($1F)

R31 ($1E)

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