ATmega16U2 datasheet(232/314 Pages) ATMEL

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232

7799B–AVR–07/09

ATmega8U2/16U2/32U2

Figure 23-3. Boot Process Description

23.7

Addressing the Flash During Self-Programming

The Z-pointer is used to address the SPM commands. The Z pointer consists of the Z-registers

ZL and ZH in the register file. The number of bits actually used is implementation dependent.

Since the Flash is organized in pages (see Table 25-7 on page 249), the Program Counter can

be treated as having two different sections. One section, consisting of the least significant bits, is

addressing the words within a page, while the most significant bits are addressing the pages.

This is shown in Figure 23-4. Note that the Page Erase and Page Write operations are

addressed independently. Therefore it is of major importance that the Boot Loader software

addresses the same page in both the Page Erase and Page Write operation. Once a program-

ming operation is initiated, the address is latched and the Z-pointer can be used for other

operations.

The (E)LPM instruction use the Z-pointer to store the address. Since this instruction addresses

the Flash byte-by-byte, also bit Z0 of the Z-pointer is used.

HWBE

BOOTRST ?

Ext. Hardware

Conditions ?

Reset Vector = Application Reset

Reset Vector =Boot Lhoader Reset

RESET

PD7/HWB

SHRH

HHRH

ATmega16U2 Datasheet(PDF) 232 Page - ATMEL Corporation