ATmega165 datasheet(233/336 Pages) ATMEL

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233

ATmega165/V

2573G–AVR–07/09

Note that the user software can never read any code that is located inside the RWW

section during a Boot Loader software operation. The syntax “Read-While-Write sec-

tion” refers to which section that is being programmed (erased or written), not which

section that actually is being read during a Boot Loader software update.

RWW – Read-While-Write

Section

If a Boot Loader software update is programming a page inside the RWW section, it is

possible to read code from the Flash, but only code that is located in the NRWW sec-

tion. During an on-going programming, the software must ensure that the RWW section

never is being read. If the user software is trying to read code that is located inside the

RWW section (i.e., by a call/jmp/lpm or an interrupt) during programming, the software

might end up in an unknown state. To avoid this, the interrupts should either be disabled

or moved to the Boot Loader section. The Boot Loader section is always located in the

NRWW section. The RWW Section Busy bit (RWWSB) in the Store Program Memory

Control and Status Register (SPMCSR) will be read as logical one as long as the RWW

section is blocked for reading. After a programming is completed, the RWWSB must be

cleared by software before reading code located in the RWW section. See “Store Pro-

gram Memory Control and Status Register – SPMCSR” on page 237. for details on how

to clear RWWSB.

NRWW – No Read-While-Write

Section

The code located in the NRWW section can be read when the Boot Loader software is

updating a page in the RWW section. When the Boot Loader code updates the NRWW

section, the CPU is halted during the entire Page Erase or Page Write operation.

Table 91. Read-While-Write Features

Which Section does the Z-

pointer Address During the

Programming?

Which Section Can

be Read During

Programming?

Is the CPU

Halted?

Read-While-Write

Supported?

RWW Section

NRWW Section

Yes

NRWW Section

None

Yes

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