ATMEGA640 datasheet(311/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

•

When erasing or writing a page located inside the NRWW section, the CPU is halted during the entire

operation

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 section” 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.

29.3.1

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 section. 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 (that is, by load program memory, call, or jump instructions 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 “SPMCSR – Store Program Memory Control

and Status Register” on page 323. for details on how to clear RWWSB.

29.3.2

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.

Figure 29-1. Read-While-Write vs. No Read-While-Write

Table 29-1.

Read-While-Write Features

Which Section does the Z-pointer

Address during the Programming?

Which Section can be

Read during Programming?

CPU Halted?

Read-While-Write

Supported?

RWW Section

NRWW Section

Yes

NRWW Section

None

Yes

Read-While-Write

(RWW) Section

No Read-While-Write

(NRWW) Section

Z-pointer

Addresses RWW

Section

Z-pointer

Addresses NRWW

Section

CPU is Halted

During the Operation

Code Located in

NRWW Section

Can be Read During

the Operation

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